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How has COVID-19 impacted the provision of radiation therapy?

Stereotactic radiosurgery provides precision irradiation of intracranial and skull-base tumours. (Courtesy: the Radiosurgery Society)

“The thing that becomes overwhelming is that we are used to considering cancer care as one of the highest priorities of the healthcare system. To be in a position where we have to fight for every single aspect of cancer care is a very new position for all of us. It’s like all of a sudden, the diagnosis and treatment of cancer doesn’t matter anymore.”

Shalom Kalnicki, chairman of the radiation oncology department at Montefiore Medical Center in New York, highlighted the challenges faced by radiation oncologists, physicians and medical physicists looking to treat cancer patients in the time of COVID-19.

Kalnicki was one of a panel of experts speaking in a webinar hosted by the Radiosurgery Society last week. The webinar focused on how radiation treatments have had to adapt to the current pandemic. In particular, the participants considered stereotactic radiosurgery (SRS), which delivers precise radiation doses to brain tumours in up to five fractions, and stereotactic body radiotherapy (SBRT), a similar approach used for extracranial targets.

Hopefully, this will encourage additional studies and investigations into SBRT management of cancer patients
John Kresl, webinar co-chair

One specific concern of any radiation treatment is that it is delivered in a series of fractions over several days, requiring patients to attend repeat hospitals visits and increasing the associated risks. Kalnicki emphasized that each visit must be treated as the first, with patients screened for potential symptoms of COVID-19. He noted that screening patients and staff can add up to 50% extra time to each visit. “Everything takes much longer, nothing is simple,” he said.

So how does this new scenario impact patient scheduling? Helen Shih from Massachusetts General Hospital explained that it’s important to assess which patients really need to be seen in person for evaluation, adding that 90–95% can be assessed via telephone or video consultations. It’s also possible to delay in-person visits for patients with benign diagnoses or non-urgent surveillance cases.

“Cancer is a high priority disease, and anyone who is having active treatment, post-treatment symptoms or concerns for residual disease are still prioritized and seen, remotely or in person as needed,” Shih added.

If patients must attend the hospital, steps are taken for their protection. Kalnicki said that all patients are given surgical masks and gloves when they arrive and are screened for symptoms. At MGH, meanwhile, Shih noted that patients are phoned and asked about any symptoms before each visit.

In some cases, however, rules may be too strict. Michael Schulder, director of the Brain Tumor Center at Northwell Health, NY, described how one fairly disabled patient whom he treated was not allowed to be accompanied by her husband. “I think this is taking restriction to an extreme, common sense has to apply as well,” he said.

It’s equally essential to protect the healthcare workers. Iris Gibbs, at Stanford University Medical Center, CA, explained that the centre has separate entrances for staff and patients, with screening in place at both, as well as a screening app for workers to declare their health each day. But the most important factor in protecting healthcare workers and patients, she emphasized, is testing.

Gibbs explained that Stanford Medicine developed its own test for COVID-19, which was FDA-approved by the end of February. “On March 3, we launched widespread testing within our healthcare community and we’ve been collecting data since,” she said. “When quick 15-minute tests became available, our radiation oncology department had access to that; now all our patients are tested at the time of simulation.”

The situation in New York is less favourable. “As opposed to Stanford, who were fortunate to have their own kits, for now, we are dependent on kits available from industry,” said Schulder. “We only test patients who have a particular set of symptoms, have been exposed to someone known to be infected or are having trans-nasal or sinus surgery. We’re certainly not testing everybody, so that adds a measure of risk.”

Gibbs noted that all treatment planning and dosimetry is performed remotely at Stanford, and for treatment delivery, there’s an emphasis on physical distancing. For SRS, for example, the CyberKnife system has a mode in which the physician can view the treatment console from a different room. For SBRT cases delivered on a linac, however, the physician still needs to be close to the machine.

Site specific changes

The panel also considered how radiation treatments of specific disease sites are being adapted, starting with tumours of the central nervous system (CNS) and brain. Simon Lo from the University of Washington explained that for high-grade gliomas with poor prognosis and elderly patients, they are using more hypofractionation – in which larger dose-per-fraction is delivered over fewer treatments. At the other end of the scale, patients with low-grade gliomas will have their treatment delayed where possible.

“For post-operative SRS of surgical cavities, I usually favour treatments of 3–5 fractions,” said Lo. “But a lot of our patients come from surrounding states, so to minimize visits, I would do a single fraction treatment instead, knowing that the BED [biologically effective dose] may be lower. But we need to strike a balance between virus exposure versus BED delivered to the cavity.”

At Northwell Health, said Schulder, all benign SRS has been deferred. Patients with malignant tumours are still being treated, but with 3-week rather than 6-week fractionated radiation. He also noted that all non-COVID-19 clinical trials have been shut down.

Lung tumour — Some centres are now treating lung cancers with a single fraction. (Courtesy: the Radiosurgery Society)

Shankar Siva, who leads the SABR programme at the Peter MacCallum Cancer Centre in Australia, explained that lung cancer treatments have also changed dramatically. The centre has switched all patients with peripheral early-stage primary lung cancer to receive a single fraction of 30 Gy. “There are three randomized phase 2 studies in lung that give us some confidence that moving to single fraction schedules is the way to go,” he said, adding that fractionated SBRT is still used for centrally located lung tumours.

Jun Yang, chief medical physicist at Philadelphia CyberKnife, noted that his practice is hesitant to move to single-fraction treatments. “Even in this critical time, we pretty much still stick to 3–5 fractions,” he said. “For a lung cancer patient, we actually would stick to the SBRT plan as much as we can to speed up the treatment and lower the patient’s chance of getting the virus.”

Patients with prostate cancer face another hurdle. Radiotherapy of such tumours usually requires implantation of fiducial markers to ensure accurate targeting. But in New York, said Kalnicki, state law precludes the placement of fiducial markers as it is considered an elective procedure, which is not allowed.

Najeeb Mohideen from Northwest Community Hospital, IL, is still performing fiducial placements for prostate cancer radiotherapy, but only under local anaesthetic, which does not require anaesthesiology services. Indrin Chetty, a medical physicist at Henry Ford Health System, MI, pointed out that cone-beam CT could provide an alternative, even in the SBRT setting, if slightly larger margins and lower fractionations are used.

What does the future hold?

Finally, the panel considered whether the approaches currently in place will impact radiation treatments in the future. Gibbs suggested that we are “forever changed”, and that risk mitigation and protection measures will continue. Hypofractionated treatment could also remain. Some centres have introduced hypofractionation for the first time, and if similar clinical outcomes are seen, may adopt this in the long term, she explained.

Siva agreed that some habits are likely to stick, such as electronic rather than paper consenting. He thinks that the Peter Maccallum Cancer Centre may continue to use a single radiation fraction for treating primary lung cancer, but that this decision will be centre-dependent.

John Kresl, from the Phoenix CyberKnife and Banner Good Samaritan Medical Center, AZ, pointed out that several surgeons have contacted him to ask about non-surgical options for their patients at this time. “I’ve had referring physicians call that maybe weren’t aware of, or were resistant to, SBRT and were asking questions about having patients treated. Hopefully, this will also encourage additional studies and investigations into SBRT management of cancer patients,” said Kresl, who co-chaired the webinar with Ben Slotman from Amsterdam UMC in the Netherlands.

“Amidst all the destruction the virus will leave in its wake, inevitably there will be opportunities to do things better,” concluded Schulder. “There are things that we’ve already adjusted to and should keep going – related to patient selection, treatment planning and delivery. I’m sure we’ll be able to make some good out of all this.”

Ask me anything: Chad Orzel

What skills do you use every day in your job?

I effectively have multiple jobs – including teaching, writing, administrative work and some research – so I tap into a range of skills gained from different parts of my education. Probably the most important skill across the board, though, is being somewhat obsessive about checking and rechecking things. The worst nightmare for a research scientist is to discover that your “big result” is actually just a statistical fluke, so you need as much confirming evidence as possible, in the form of different experiments or calculations that all arrive at the same conclusion.

That general approach turns out to be useful across all sorts of fields. When teaching a class, it’s good to have multiple ways of explaining the same thing, in case some students don’t follow the details of the first one you try. When writing it’s important to make sure that all your stories fit together into a coherent whole, and also that the colourful anecdote that perfectly illustrates some point is actually true. And when doing administrative work to support academic programmes and institutions, it’s important to cross-check all the numbers that justify a particular decision. So, with all my tasks, I spend a lot of time asking, “Is there another path to this same conclusion?” as a way of making sure that it’s the right thing to do.

More specifically speaking, one of the most useful skills I picked up from physics is the idea of doing quick order-of-magnitude estimates, to check whether it’s worth doing a more detailed investigation of any task. If your back-of-the-envelope numbers say you’re a factor of 100 away from having the resources you need, you shouldn’t waste any more time refining that estimate. The other absolutely essential “soft skill” is communication: having the confidence and ability to speak up in both small and large meetings, as well as clearly explaining what you’re doing (or want to do) is invaluable. No matter what line of work you end up in, you’ll always need to be able to convince other people to help you achieve your goals.

What do you like best and least about your job?

The absolute best moments come from teaching and writing – when something I said makes an idea click into place for a student or reader. It’s incredibly gratifying to see students light up when they finally get a tricky problem, or to hear from readers saying things like “I’ve heard about this topic before, but now it finally makes sense to me!” It’s an honour to be in a line of work where I get to do that on a somewhat regular basis.

I also like the variety of the job. Even when I’m teaching the same class for the fourth year in a row, there’s always some new twist, a new explanation that occurs to me on the fly, or a question that no previous student has asked. On the writing and research side, I get to pick whatever topics I want to pursue, so if I stumble across some new quirk of physics, I can go chasing down all the details of that topic in the lab or library, and usually get something out of it that I end up being able to use later.

My absolute least favourite thing in the world is grading. It’s just miserable, and I will invent elaborate ways to procrastinate on doing it, up to and including answering interview questions from physics magazines.

What do you know today that you wish you knew when you were starting out in your career?

My first few years as a faculty member, when people asked how I found the job, I would say “It’s a lot more work than it looked like from out in the classroom.” That’s still true, and something to keep in mind for anyone on the academic track: a one-hour lecture in a class takes the same amount of prep time as a one-hour seminar talk, but you’re expected to do 3–4 new class lectures per week, every week.

The other key advice I would offer is that basically every task you need to do, either for work or in life more generally, can expand to exceed the time available for doing it. It’s critically important to set boundaries, and make sure that you’re blocking out enough time to do each of the many things you’ll need to do, and not letting any one of them encroach on the time for the others. To make this a reality can involve some unexpected changes. I remain somewhat surprised that I’ve turned into a morning person. I do most of my writing between about 7 a.m. and 9 a.m., from when the kids get up to when I need to go to campus, because that’s a block of time when I know I can work uninterrupted.

I work hard to keep that block of time free and not spend it on any of my other tasks. That’s been essential to maintaining writing productivity despite having two school-age kids and expanding responsibilities at work. I don’t necessarily recommend waking before dawn as a path for everyone, but the general idea of finding a block of time for what you really need to do, and protecting that time against everything else, is critical.

Doing physics in the time of COVID-19

Across the world, personal and professional lives have been profoundly affected during the past few months – and scientists are no exception. In this episode of the Physics World Stories podcast, we find out how physics and physicists are adapting to coronavirus-related lockdowns. Among physicists – as with many professions – there is a growing realisation that things are not about to go back to normal anytime soon.

Isolation from colleagues, facilities and important conferences bring obvious disadvantages. But as you will hear in the podcast, some physicists are also finding positive outcomes from the situation. Before the lockdown, did you ever meet a researcher who didn’t complain about being time-poor? Plenty were sick of travelling to international events because they felt they had to show their face. And you name an academic didn’t have a paper they kept meaning to write but never got around to it because of things like endless faculty meetings.

First up in the episode is the theoretical physicist and author Sabine Hossenfelder. Among other things during the pandemic, she has teamed up with climate physicist Tim Palmer to record a coronavirus-inspired reworking of the REM classic hit “It’s the End of the World as We Know It (And I Feel Fine)”. You will also hear from Bonnie Tsim and Rebecca Waters who both attended the recent Women in Graphene Career Development Day – an online event that reimagined various aspects of real-world conferences. Perhaps the success of this virtual event is a sign of what physics conferences will be like for the foreseeable future.

For more personal accounts of the impact of the COVID-19 lockdowns, take a look at the “physics in the pandemic” series on the Physics World blog.

Scientists identify gene responsible for butterfly’s dazzling structural colours

The shimmering wings of certain butterfly species are well-known examples of “structural” colour – that is, colour produced by light-scattering nanostructures rather than by reflection from coloured pigments. Many of the most striking species are, however, rare and hard to breed, hindering efforts to study these nanostructures in more detail. Now researchers in the US have struck iridescent gold by breeding blue structural colour into a more common species – a result that also helped them identify the first gene known to be associated with structural colour.

“Often conversations about butterfly structural colour focus on structures with more elaborate shapes than typical scales,” says Rachel Thayer, a PhD student at the University of California, Berkeley and lead author of a paper about the work, which is published in eLife. “We are showing that simple, normally-shaped butterfly scales are also an important source of structural colour, which suggests that many butterfly species may have the same phenomenon.”

Studies using a combination of electron microscopy and spectrophotometry had previously identified several butterfly species that produce structural colour in the lower lamina of their wing scales, which are made from a polymer called chitin. But Thayer points out that extending these spectroscopic methods to the high-throughput, live visualization studies needed to tease out the genetic and evolutionary basis of structural colour is more difficult. A further complication is that the exotic butterfly species with the most elaborate iridescent patterns are hard to keep in captivity.

Breeding beauties

The breakthrough came about, in part, because of Edith Smith. The co-founder of the Shady Oak Butterfly Farm in north-central Florida has a soft spot for the common buckeye butterfly (Junonia coenia), and one day she spotted something different. “I saw some bright blue on the top edge of a buckeye’s wings and wondered if it would become more prevalent or brighter if I bred some with blue to others with blue,” she recalls.

In just 12 months, Smith succeeded in breeding buckeyes that were far bluer and shinier than the predominantly brown wildtype. Then, when a video of Smith and the blue buckeyes caught Thayer’s eye, the Berkeley evolutionary biology student thought, “These are really blue, they look iridescent, probably a structural colour, and in one of the best species [for laboratory research] – I’ve got to check these out!”

Rows and rows of buckeye butterflies lined up in a box — Buckeye beauties: A drawer full of samples of buckeye butterflies (*Junonia coenia*) shows how their colours vary from predominantly brown to strongly blue. (Courtesy: Nipam Patel)

Thayer used helium-ion microscopy (HIM) to identify an increase in the thickness of lower lamina scales in blue patches on the specially-bred buckeyes. Her hunch about structural colour had been right. But she also wanted to know whether the rapid colour change caused by Smith’s selective breeding was relevant to natural evolution.

Adding colour the genetic way

By taking specimens from 10 closely-related buckeye and pansy species, Thayer matched the HIM structures identified in individual patches to their reflection spectra. It was exacting work. “Training my hands to dissect scales as thin as a soap-bubble wall, without breaking them – that was pretty hard,” she says.

Despite the fiddliness of the task, Thayer was able to show that, although many different pigments were present within the scale nanostructures, the greatest contributor to the range of iridescent colours across species was a structural element: film thickness. “The differences in lamina thickness between species indicate that one way structural colour has evolved is by tuning the thickness of this tiny film in each butterfly scale to produce different hues,” she says.

Finally, Thayer fitted a genetic piece into the puzzle. A group of researchers at Cornell University in New York had previously mutated a well-known butterfly pigment gene known as optix, and they were curious to know whether this gene might also play a role in creating structural colour. When they sent specimens to Thayer, she found that lamina thickness was responsible for their colour changes, too.

Nanoscale structures give some butterflies ‘ultra-black’ wings

“Optix is the first gene we know of that can change the shape of a photonic structure in butterflies,” says Thayer, adding that she is excited by the possibility of investigating how nanostructures develop, and of finding other genetic clues. Further down the line, she says that understanding these elements could inspire more efficient ways of producing photonics devices such as solar panels and displays.

Extending the work

Pete Vukusic, a biophotonics expert at the University of Exeter, UK, who was not involved in the research, calls it “a lovely piece of ‘evo-devo’ [evolutionary development] work” and says he is intrigued by some of Thayer’s spectral measurements. “Butterflies appear to use ultraviolet scattering extensively, so I’d like to see [the measurements] extended into the near ultraviolet range, as that would help to tell a more complete story with respect to functionality.”

Although Thayer was unable to measure ultraviolet scattering with her current set-up, she says that Fresnel’s classical thin-film equations for reflectance suggest that some species do indeed scatter ultraviolet light from their scales. Her next steps are to find out whether optix or other genes are responsible for the newly-gained blueness of Smith’s selectively-bred buckeyes. As for Smith, she has an eye on some other butterfly traits, so who knows what scientific gold she’ll strike in her future experiments.

Some glaciers may be moving faster than previously thought, new ‘slip law’ suggests

An equation that describes motion of glaciers over soft, deformable ground has been developed by Neal Iverson of the Iowa State University and Lucas Zoet of the University of Wisconsin-Madison in the US. Their new “slip law” was derived from lab experiments and could help remove uncertainties from existing glacial flow models. Indeed, models that include the slip law predict more rapid ice sheet discharges into the oceans in the future – leading to additional sea level rise.

Understanding how glaciers move over different types of terrain is vital to predicting how much glacial melt will contribute to changes in sea level. However, there are currently large gaps in our knowledge – particularly for fast-flowing, ocean-reaching glaciers. These can be found in Antarctica and Greenland, where they lie on soft, glacially-deposited sediment called till.

“Glacier ice is a highly viscous fluid that slips over a substrate – in this case a deformable till bed – and friction at the bed provides the drag that holds the ice back,” Iverson explains. “In the absence of friction, the weight of the ice would cause it to accelerate catastrophically like some landslides,” he adds.

Data are difficult to obtain

Obtaining data on glacial drag in the field, however, is extremely difficult. Drilling to the bottom of the ice to make measurements, for example, would inherently change the nature of the interface between the glacier and the bed.

Taking a different approach, Iverson and Zoet have been simulating their very own glaciers in the lab. In 2009, Iverson built a ring-shear device that features a ring of ice 20 cm thick and 0.9 m in diameter that can be rotated at speeds of between 0.3–3048 metres per year over a chosen substrate. This occurs within a hydraulic press that can squeeze the ice to simulate the weight of an overlying glacier that is 244 m thick.

The device is stored in a walk-in freezer, with the ring surrounded by a circulating fluid that keeps the ice just at its melting point, so it slides along a thin film of water – like all fast-flowing glaciers do. For the simulated base under the ice, the researchers used real glacial till with the correct mix of mud, sand and larger rock particles.

“We were after the mathematical relationship between the drag holding the ice back at the bottom of the glacier and how fast the glacier would slide,” Iverson says. “That included studying the effect of the difference between ice pressure on the bed and water pressure in the pores of the till – a variable called the ‘effective pressure’ that controls friction.”

Dominant slip mechanism

From their experiments, the duo found that glaciers slide atop soft sediments at slower speeds, but once they reach a certain threshold speed, they begin to deform the underlying sediment – a process which then becomes the dominant slip mechanism.

“We are able to provide a mechanical reason for when this transition would happen and also provide a more generalized equation that could be used in ice-sheet models to simulate this process,” Zoet tells Physics World, explaining that the effective pressure is what controls the strength of the sediment bed and the resulting transition to bed deformation that changes the glacier’s drag.

When combined with the equivalent equation for glacier movement over hard beds, the researchers’ findings have the potential to be used to create a general slip law that could be applied to all glacier flow models, removing previous uncertainties.

“Higher rates of sea-level rise”

“Ice sheet models using our new slip relationship would tend to predict higher ice discharges to the ocean – and higher rates of sea-level rise – than slip laws currently being used in most ice sheet models, Iverson added.

Martin Truffer, a geophysicist from the University of Alaska commended the work, noting that others models “have a notoriously difficult time with […] boundary processes and a vast majority still use rules for basal motion that are not supported by observations.”

“One of the hardest problems in glaciology is how to parametrize basal motion of large ice sheets,” says Hilmar Gudmundsson, a glaciologist from Northumbria University. “In the past a lot of arguments have focused on if basal sliding is a viscous or a plastic process. This new work suggests that both of the processes are possible: at low sliding velocities we have the viscous limit, at high velocities we reach the plastic limit.”

Gaining further insights

With their initial study complete, the researchers are now looking to gain more insight into how the rates of sediment deformation change when variables such as ice velocity and effective pressure change.

“Developing a relationship that allows us to estimate the sediment deformation rate at the glacier’s base will give us a missing piece of information in estimating how long it takes glaciers to build and destroy landforms,” such as those found over much of Europe, North America and the base of Antarctica, Zoet says. “Using the new ring shear apparatus with a transparent sample chamber we can observe this directly.”

The research is described in Science.

Alanine dosimeters line up for MRI-guided radiotherapy

MR-guided radiotherapy enables real-time imaging during radiation delivery with high soft-tissue contrast, and could ultimately enable real-time adaptive treatments. Reference dosimetry in the presence of a strong magnetic field, however, is challenging.

“A reference dosimeter is a detector that is calibrated directly by comparison to a national dosimetry standard, or indirectly through intermediate calibrations,” explains Ilias Billas from the UK’s National Physical Laboratory (NPL). “Its main task is to enable hospital physicists to measure radiotherapy doses accurately and consistent with international dosimetry standards.”

The ionization chambers used for reference dosimetry in conventional radiotherapy systems are strongly affected by magnetic fields – making it challenging to perform beam output measurements. As such, there’s a real need for a robust and stable reference dosimeter for use in MRI-guided radiotherapy. A team headed up at NPL is investigating the suitability of an alanine detector for this task (Phys. Med. Biol. 10.1088/1361-6560/ab8148).

Dosimeter characterization

Alanine is an α-amino acid that produces a stable free radical when irradiated. The concentration of these free radicals is proportional to the absorbed dose, and is measured using electron paramagnetic resonance (EPR) spectroscopy. As alanine is a solid-state detector, it should experience a lower electron return effect (ERE) than an air-filled ionization chamber.

To quantify the performance of the alanine dosimeter in the presence of a magnetic field, Billas and co-workers performed measurements and Monte Carlo (MC) simulations of alanine pellets irradiated at three photon beam energies and various magnetic flux densities. They placed pellets (roughly 2.3 mm high and 5 mm in diameter) in a waterproof polyether ether ketone (PEEK) holder shaped like a Farmer-type ionization chamber. They then placed these alanine dosimeters in an electromagnet and irradiated them with either a ⁶⁰Co source, or 6 or 8 MV linac beams, over a range of magnetic flux densities

Alanine dosimetry — Left: schematic view of the experimental setup. Right: the NPL alanine holder. (Courtesy: *Phys. Med. Biol.* 10.1088/1361-6560/ab8148)

For ⁶⁰Co irradiation, the researchers irradiated the alanine dosimeters inside a PMMA phantom, with any air gaps between the phantom and holder filled with water to avoid the ERE. They irradiated the phantom within magnetic flux densities of 0, 0.5, 1, 1.5 and 2 T. In the linac setup, they placed the alanine holder in a water phantom and irradiated the dosimeter at 0, 0.35, 0.5, 1 and 1.5 T.

The researchers validated MC models of the 6 and 8 MV linac beams by comparing simulated with experimental beam profiles and depth doses. To validate the model of the experimental set-up, they performed MC simulations and measurements, at 1.5 T, with the holder partially loaded with alanine pellets. In both cases, the MC models were successfully validated and used to support their research.

One potential issue with placing alanine pellets inside the holder is the impact on measured dose of air gaps within the holder – due to the bevelled edge of the pellets and the space between the pellets and the holder’s inner wall. Simulations performed at 1.5 T, with and without air gaps, revealed that such gaps did affect the alanine response, due to the ERE caused by the magnetic field. The maximum deviations between models with and without air gaps were 0.45% and 0.55%, for 6 and 8 MV beams, respectively. For the ⁶⁰Co beam, all data deviated by less than 0.4%.

Monte Carlo models for investigating the effect of the air gaps around the alanine pellets. The pellet is shown top-right. (Courtesy: *Phys. Med. Biol.* 10.1088/1361-6560/ab8148)

The team also investigated uncertainties due to the random positions of pellets inside the holder. MC simulations of the pellets in four different positions inside the holder showed that uncertainties increased with magnetic flux density, up to 0.52% and 0.47% for 6 and 8 MV at 1.5 T, respectively. For ⁶⁰Co, the highest uncertainty was 0.52% at 2 T. For other magnetic flux densities, uncertainties were all below 0.30%. Billas notes that this is the dominant component in their uncertainty budget, which includes the unavoidable effect on the alanine response due to the air gaps.

Correction calculations

By combining their measurements with Monte Carlo simulations of absorbed dose in water, the researchers found that the response of alanine to ionizing radiation was modified in the presence of a magnetic field. The effect was energy independent and, if uncorrected, could increase the alanine/EPR signal by 0.2% at 0.35 T and 0.7% at 1.5 T.

To determine the true absorbed dose in the presence of a magnetic field, the team calculated a correction factor. This factor – which incorporates the effects of the magnetic field on intrinsic alanine sensitivity, dose distribution in water, dose to alanine and fluence perturbation by the holder – tended to decrease with increasing magnetic flux density. Averaged over all magnetic flux densities, the calculated correction factors were: 0.9946 ± 0.0019 for ⁶⁰Co; 0.9973 ± 0.0018 for 6 MV beams; and 0.9982 ± 0.0033 for 8 MV beams.

The team concluded that, with inclusion of this small correction factor, alanine/EPR provides a suitable reference class detector for MRI-guided radiotherapy, with comparable uncertainties to a Farmer-type ionization chamber.

“The next step in this project is the development of guidelines for a new dosimetry calibration protocol and the development of methodologies for dosimetry audit,” says Billas. “This would involve the use of alanine to provide the traceability from the NPL’s primary standard of absorbed dose, a graphite calorimeter, from a conventional linac to an MRI-linac.”

Kondo cloud seen at last

The first experimental measurement of a Kondo cloud – a condensed-matter phenomenon that drastically increases the electrical resistance of certain metals at low temperatures – confirms that this long-predicted structure really exists, more than half a century after it was first hypothesized. The new measurement could improve our understanding of condensed-matter systems that contain multiple magnetic impurities, including high transition-temperature superconductors.

In the 1930s, physicists spotted a surprising trend in the electrical resistance of metals that contain magnetic impurities. Unlike metals without such impurities, electrical resistance increases rapidly once the temperature drops below a certain threshold – and then keeps increasing as the temperature drops further.

The phenomenon was not explained until 1964, when the Japanese theorist Jun Kondo showed that at low temperatures, the spin of a magnetic impurity collectively couples, or becomes “stuck”, to all the electrons in the area. The resulting cloud of spin-coupled electrons – the Kondo cloud – screens off the conducting electrons and prevents them from moving. The result is an increase in the metal’s resistance.

Isolating a Kondo cloud

Although the spins interact locally with electrons around the magnetic impurity, the Kondo cloud can, in theory, spread out over several microns. This prediction inspired a team of researchers from Japan’s RIKEN Center for Emergent Matter Science, City University of Hong Kong, Korea Advanced Institute of Science and Technology (KAIST), the University of Tokyo, and Ruhr-University Bochum in Germany to try to measure the length of a Kondo cloud in a related system: the tiny pieces of semiconducting material known as quantum dots (QDs). Here, an unpaired electron spin trapped in the dot plays the role of a magnetic impurity in a metal.

In their work, the researchers fabricated a QD and connected it to a long, one-dimensional channel that houses a Fabry-Pérot interferometer containing an electron reservoir. When the unpaired electron spins in the QD couple to the electrons in this channel, a Kondo cloud forms. “In this way, we isolate a single Kondo cloud around a single impurity and can control the size of the cloud as well,” explains study lead author Ivan Borzenets.

The researchers applied varying voltages at different points along the channel to induce weak barriers along it. They then observed how the channel’s electrical conductivity and the Kondo temperature – a quantity inversely proportional to the length of the Kondo cloud, and fairly straightforward to measure – changed as a function of the barrier strength and position.

The data collected from the experiment. Photo source: DOI: 10.1038/s41586-020-2058-6

“When we place a perturbation (a weak barrier) in our 1D channel, at a controlled length away from the magnetic impurity, and then activate this perturbation, this results in a change in the measured Kondo temperature,” Borzenets tells Physics World. “If the perturbation is at a length that is within the Kondo cloud, the Kondo temperature is strongly affected. Conversely, if it is outside the cloud, the effect is very small.”

The results showed that oscillations in conductance coincided with oscillations in the measured Kondo temperature. By plotting the amplitude of the Kondo temperature oscillation against the distance between the barrier and the impurity divided by the theoretical cloud length, the team found that all their data points fell onto a single curve, just as predicted (see graph).

Proportionality factor

The team say they have unequivocally proved the existence of the Kondo cloud by directly measuring its length. They also identified the proportionality factor that relates the size of the cloud to the Kondo temperature. They now plan to study more complicated Kondo systems containing multiple impurities.

“For example, we could place two impurities in the quantum dot at the same time and observe how they react when the clouds overlap,” Borzenets says. “The results from these experiments should provide important insights into the behaviour of multiple impurity systems, such as Kondo lattices, spin glasses and high transition-temperature superconductors.”

“It is very satisfying to have been able to obtain real space images of the Kondo cloud, as it is a real breakthrough for understanding various systems containing multiple magnetic impurities,” adds team leader Michihisa Yamamoto. “This achievement was only made possible by close collaboration with theorists.”

Heung-Sun Sim, the KAIST theorist who proposed the method for detecting the Kondo cloud, agrees: “It is remarkable from a fundamental and technical point of view that such a large quantum object can now be created, controlled, and detected.”

The research is detailed in Nature.

Going with the flow

There are many ways to navigate the transition from physics degree to the more formal world of work, but not all of them involve rigorously mapped career pathways and a laser focus on “the perfect job”. Sometimes it pays to just go with the flow – a strategy that appears to have worked out well for physicist Aidan White, who joined TÜV SÜD National Engineering Laboratory in East Kilbride, Scotland, as a project engineer in spring of last year.

After completing a five-year MPhys degree at the University of Strathclyde, White says he “kind of fell into the TÜV SÜD opportunity”, acknowledging that the move was something of a departure from the research project he’d been working on for the previous 18 months – running computer simulations to evaluate medical radioisotope production in the university’s next-generation “laser-wakefield” particle accelerator. “It’s important to have an open mind,” he says of that initial job search. “Physics is a broad subject and it can be tricky to find your niche because of all the diverse opportunities available to you.”

Fast forward a year or so and, somewhat serendipitously, it appears that White has already carved out his niche within TÜV SÜD. The organization’s East Kilbride laboratory, which manages the UK’s national standards for flow and density measurement, is one of the leading global providers of flow measurement and related equipment calibration. It also offers consultancy and R&D services to the oil and gas industry, instrumentation manufacturers and the wider energy sector. Furthermore, as a National Measurement Laboratory, the business carries out industrial R&D on behalf of the UK government, developing technologies and best practice in sectors such as alternative fuel measurement (for example, hydrogen and liquefied natural gas) and carbon capture and storage.

I’ll give something a try before I tell you I can’t do it

As White tells it, the key message for other new graduates is “to get your foot in the door and roll with it”. He should know: he applied for one job as a mathematical modeller at TÜV SÜD, only to end up being hired as a project engineer in the group’s flagship test laboratory – the new £16m Advanced Multiphase Facility (AMF) – which was being commissioned around him last summer. “Adaptability is a real asset,” White maintains. “It helps that I’m a hard worker with a can-do attitude. I’ll give something a try before I tell you I can’t do it.”

If you build it, they will come

At the operational level, White and his colleagues in the project-engineering team are currently rolling out the AMF to a global customer base. Their specific focus is the £50bn-per-annum global subsea oil and gas industry, with the AMF designed to address current and future measurement challenges through company-led R&D projects, new product development, hands-on industry training and academic research.

Specifically, the AMF is being put to work evaluating the impact of extreme subsea operating environments on multiphase flow meters from a range of manufacturers. These instruments, which typically cost hundreds of thousands of pounds per unit, are a mainstay of the oil and gas industry. They are used to measure mixed streams of oil, gas and water flowing through a well-head or distribution system on the ocean floor. Such measurements are increasingly vital as larger production wells dwindle and energy companies seek to exploit smaller, more numerous wells in deeper waters and extreme environments.

“A big portion of our work is in supplying contract test and R&D versus calibration, validation and certification of all sorts of multiphase flow meters,” says White. “Many manufacturers have their own small-scale test facilities, but none can hit the pressures and flow rates we have here in East Kilbride, which are much closer to what the meter will experience under field conditions.” Fundamental research is also on the AMF agenda. The facility’s three-phase X-ray tomography system, for example, enables high-definition imaging of complex multiphase flows and their impact on flow measurements.

Talk it up: Although he’s been with TÜV SÜD National Engineering Laboratory for less than 12 months, Aidan White (left) is already selling the lab’s capabilities out on the conference circuit. (Courtesy: TÜV SÜD)

Learning by doing

When White joined the TÜV SÜD laboratory, the AMF building was already in place with all the heavy plant installed. Next came commissioning and mechanical completion of the core equipment and instrumentation. “It was a case of the right place at the right time for me,” says White. “My first task was to get to know the AMF inside out, seeing where everything went and how it all worked together – the valves, pipework, secondary instrumentation and reference flow meters.”

Although that meant a “steep learning curve and total immersion” over those first few months, White acknowledges that the benefits were immediate and long-lasting. “In many ways, the AMF resembles a big physics experiment – a 1600 m² factory-sized one!” he notes. “Having been part of the AMF commissioning team, you can throw a valve number or transmitter number at me and I’ll pretty much be able to tell you where in the facility it is.”

Equally invaluable are the problem-solving skills White developed during his physics training – being able to look at the big picture and break that down into its component parts while working on a range of projects. “A solid mathematical background also helps, especially with respect to data mining and data analysis,” he adds. “We have to go through lots of data to figure out what’s relevant, why it’s relevant and what it all means.”

Right now, White is relishing the fact that no two days are the same and that new opportunities and responsibilities are never far away. Although he’s been with TÜV SÜD for less than 12 months, White has already been selling the AMF’s capabilities on the conference circuit, presenting the facility to industry executives and engineers at their regular Oil and Gas Focus Group in Aberdeen.

“TÜV SÜD is a prestigious place to work, with all sorts of talented people making up our cross-disciplinary teams of scientists, engineers and technicians,” White concludes. “My priorities for this year are to keep learning from all of them and to get some research formally published based on the work I’ll be doing with our AMF customers.”

High-pressure experiment sheds light on Earth’s outer core

Extreme conditions close to those found within the Earth’s outer core have been created in the lab by planetary scientists in Japan. Researchers led by Yasuhiro Kuwayama at the University of Tokyo created the temperatures and pressures needed for their experiment using a highly specialized diamond anvil. Their discoveries could lead to a better understanding of the composition and behaviour of the Earth’s outer core, and perhaps even the interiors of other planets.

The Earth’s core begins about 3000 km below the surface and much of what we know about it comes from looking at seismic waves from earthquakes that have travelled through the centre of the Earth. The core’s properties have also been studied by doing computer simulations and experiments that subject materials to extreme temperatures and pressures. Research has revealed that the centre of our planet is separated into a solid inner core composed mainly of an iron-nickel alloy and an outer core dominated by liquid iron.

Now, Kuwayama’s team has increased our knowledge of the outer core using a diamond anvil, which exploits diamond’s almost unparalleled hardness to subject samples to extremely high pressures and temperatures. In their study, they compressed a liquid iron sample to pressures of up to 116 GPa and heated it to of 4350 K. While 4350 K is believed to be a typical temperature within the outer core, 116 GPa is slightly lower than the pressure expected at the top of the outer core.

Sustained pressure

An important feature of this latest research is that this extreme pressure and temperature can be maintained indefinitely – at least in principle. This is unlike previous studies in which extreme conditions were only sustained for a few microseconds. The team squeezed a tiny liquid droplet of liquid iron to 116 GPa and then heated it to 4350 K using an infrared laser. Then the team probed their sample’s properties in detail, primarily by doing X-ray scattering experiments at RIKEN’S Spring-8 synchrotron in Hyōgo prefecture.

After combining their observations with existing data, Kuwayama and colleagues compared the measured thermodynamic properties of their high-pressure, high-temperature liquid iron to what is known about Earth’s outer core. They found that the Earth’s outer core must be around 7.5% less dense than the liquid iron, suggesting that it must contain a high abundance of lighter elements that have yet to be identified. The team also found that material in the outer core must flow around 4% more easily than liquid iron, although both materials display a similar resistance to compression.

Kuwayama’s team says that its work offers important new insights into the physical properties of Earth’s core. Their work could also inform future studies of other planetary cores – which even within the solar system, encompass a rich variety of compositions, structures, and relative sizes. As Kuwayama concludes, “we were pleasantly surprised by how effective this approach was and hope it can lead to a greater understanding of the world beneath our feet”.

The research is described in Physical Review Letters.

Advanced Microwave Topics for Quantum Physicists

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A quantum experiment is difficult. Many factors interfere with obtaining accurate results, including noise, interference, individual qubit behaviour and the effects multiple qubits can have on each other. Controlling the qubit state, its entanglement, and ensuring controlled and stable measurement requires the precise application of electromagnetic waves. In this webinar, Mark Elo covers microwave control systems, which include methods of synthesis, pulse shaping and modulation techniques used in both semiconductor- and photon-based qubits. He also covers analogue signal-generation techniques common in many systems today, plus direct to microwave digital signal generation and measurement based on new DAC and ADC technology.

Mark Elo is the US general manager for Tabor Electronics. He began his career as a design engineer in Hewlett-Packard’s Microwave Division in 1990 and has since held various senior engineering and management positions at Agilent Technologies, Anritsu, Gigatronics and Keithley Instruments in R&D, marketing and business development. Elo has almost 30 years of test and measurement experience in microwave instrumentation, especially with respect to signal simulation and spectrum analysis – specializing in product definition and product realization of RF and microwave-frequency synthesis and analysis platforms. He has also held the chair for the AXIe marketing committee, participated in wireless standards and has published multiple articles.

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