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Dose calculations reveal how patient motion impacts proton minibeam therapy

Calculated dose distributions — **Dose calculations** Left: dose distributions for thoracic pMBRT without motion (3D), with respiratory motion (4D) and with respiratory motion at high dose rate (4D HDR). Right: dose distributions for a brain treatment with zero shift, a 1 mm shift and a 2 mm shift. (Courtesy: *Phys. Med. Biol.* 10.1088/1361-6560/ae8ef8)

Proton minibeam radiotherapy (pMBRT) is an innovative new cancer treatment that uses an array of narrow proton beams to create alternating regions of high and low dose, before converging to a homogeneous dose distribution within the target volume. Numerous studies in small animals have shown that this spatially modulated dose distribution can destroy tumours while sparing healthy tissue and reducing side effects.

To implement pMBRT in the clinic, it’s important to first assess the impact of organ motion during treatment, which could lead to overlapping of the dose peaks and valleys, and potentially diminish the sparing effect. With this aim, a team at Institut Curie in France has developed a Monte Carlo (MC)-based framework to quantify the dose distribution for realistic clinical treatments, reporting their findings in Physics in Medicine & Biology.

To deliver pMBRT, a multi-slit collimator is used to create narrow beams, typically 0.3 to 1 mm wide, spaced 2 to 6 mm apart. As this collimator blocks much of the initial proton beam, longer treatments are needed to deliver the prescribed dose, increasing the likelihood of organ motion.

“Given the longer irradiation times, possible patient movement could deteriorate the heterogeneous spatial dose distribution of pMBRT,” explains senior author Ludovic De Marzi. “This is all the more problematic because it is this heterogeneous distribution that is believed to be responsible for the technique’s biological sparing effect.”

The dose reconstruction workflow

De Marzi and colleagues developed a 4D dose calculation workflow based on time-resolved MC simulations to determine how motion affects pMBRT dose distribution. The tool requires two main inputs: a 4D CT dataset capturing the patient’s anatomy and breathing motion over one respiratory cycle; and a proton pencil-beam scanning (PBS) treatment plan created on a reference CT.

To validate their workflow, the researchers irradiated a thoracic motion phantom with a conventional PBS plan and compared the measured dose with the simulations. For a 3%/3 mm gamma index (a standard quality check tool), the mean pass rate for nine phantom irradiations was 97.3%, confirming that the calculation performs reasonably well for 4D dose modelling in conventional PBS.

They then used the tool to assess the impact of motion in representative clinical cases, first examining a thoracic treatment with large breathing movements. They calculated 4D dose distributions for three scenarios: 3D pMBRT without motion; 4D pMBRT with 20 mm breathing motion; and 4D high-dose rate pMBRT, with the instantaneous dose rate increased tenfold to reduce beam-on time by a factor of ten.

The calculated dose distributions showed that breathing motion introduced significant variations in the patterns of peaks and valleys, impacting a crucial parameter: the peak-to-valley dose ratio (PVDR). At a depth of 5 mm, for example, the mean PVDR dropped from 11.9 for the 3D scenario to 6.7 in 4D. The high-dose rate scheme slightly mitigated this degradation, but not significantly.

“The PVDR is an index that takes into account both doses in the valleys (which are likely correlated with the biological sparing effect on healthy tissue) and the maximum delivered doses (which are likely responsible for the anti-tumour effect),” De Marzi explains. “It can also be used to help optimize the 3D dose distribution of the treatment plan.”

Breathing motion also degraded the dose delivered to the tumour, with the mean target dose and D_95% (the minimum dose received by 95% of the target volume) reduced by 30% and 35%, respectively, for 4D pMBRT. Nearby organs-at-risk (OARs) were also impacted, with a decrease in dose to the stomach and a significant increase in mean dose to the spleen for the 4D cases.

The researchers next examined an intracranial treatment. While there is no intra-organ motion in this case, the patient’s head undergoes small rigid translations – generally limited to less than 1 mm using thermoplastic masks. As such, they modelled patient motion as a simple continuous 1 or 2 mm translation.

Motion did not significantly impact the mean dose or D_95%for the target or OARs. A 1 mm shift was, however, enough to degrade the peak–valley patterns, reducing the mean PVDR by 10% from the static case (at 15 mm depth). For a 2 mm shift, this increased to a 24% reduction in mean PVDR.

A valuable tool

The researchers conclude that their 4D dose reconstruction workflow provides a useful tool for assessing interplay effects caused by organ motion during pMBRT. Without a specific motion management strategy, only intracranial treatment with movements below 1 mm was robust to interplay effects. A similar workflow could prove valuable for estimating the sparing effect of pMBRT while accounting for patient motion and adapting treatment plans accordingly.

Proton minibeams could improve treatment of cancer metastases

Looking forward, it may be possible to minimize delivery times by combining pMBRT with the ultrahigh-dose rate delivery used for FLASH treatments.

“This idea is appealing because it could both solve the problem of reduced dose rate in pMBRT and add an additional biological effect (FLASH) to further spare healthy tissue,” says De Marzi. “However, its feasibility remains to be demonstrated, as does the value of developing a technique that combines such considerable complexities. A simple increase in dose rate – without going as far as FLASH – would already be of interest.”

In their next step towards clinical application, De Marzi and colleagues are working to validate calculation tools and measurement procedures tailored to this pMBRT technique. “Ones that are fast, accurate and usable by clinicians,” he says.

From student to scientist – how to build a career with physics

Whether it’s cosmology, quantum mechanics or particle theory, most of us go into physics because we’re fascinated by the universe and its mysteries – and not because of the career options it might lead to. In fact, by the time you’re at university, you’re so snowed under with lectures, labs, coursework and tutorials – not to mention having fun – that there’s almost no time to think about what to do after graduating.

It doesn’t help either that most university staff are only familiar with careers in research, and so aren’t the best career mentors for physics students exploring the many options outside academia. That’s why Physics World has published countless careers articles over the years looking at what physicists do after they graduate – not just in research but also in industry, finance, teaching, IT and engineering. Each year we also gather many of these into our annual Physics World Careers guide – which includes a directory of employers seeking physics graduates.

But when the time comes to look for a job or plan your next steps, you won’t want to plough through every guide we’ve published. So to help you through the maze of options, this article brings together all our tips, information and help in one place. In today’s tough job market, a physics degree is an absolutely fantastic thing to have, giving you a deep technical knowledge and an ability to think rigorously. But before even starting to apply for jobs or courses, you first need to sit down and think hard about what it is that makes you happy and fulfilled in life.

Stop and think

As Crystal Bailey from the American Physical Society advised in a 2025 Physics World Live discussion “Not sure what to do with your physics degree? Our expert panel give their careers advice”, first ask yourself what it is you want for your career and your life. Do you prefer theoretical questions or practical problems? Do you like working alone or with people? Do you want a good work–life balance, a flexible schedule or just to make pots of money? “You need to understand what will be a good match for you,” she advises.

We suggest you do this kind of self-reflection long before you expect to graduate – don’t leave it to the last minute when you’re short of time. It also helps to make an audit of all the “soft skills” you have as a physicist, of which there are 16 different kinds. From being able to manage your time, knowing how to communicate or sort out conflict, these attributes are just as important to employers as your technical prowess.

Young man looking at a collection of sticky notes on a noteboard — **Think for a minute** Before even starting to apply for jobs, first reflect on what it is you want from life. (Courtesy: iStock/Layla Bird)

The “hard” skills you gained while studying – such as knowing how to solve a differential equation or line up mirrors on an optics bench – are easy to provide evidence for, from the modules you took, or the lab work you did. But the other more nuanced and practical skills – such as presenting in front of your peers, writing reports or keeping to deadlines – are just as crucial.

While you’re still at university, it also helps to take advantage of the huge amount of talented people you’re surrounded by – or as Katie Perry, a physicist who works at the Daphne Jackson Trust, once put it, “network like crazy”. Scientific meetings, talks and conferences are a great place to do this, where our tip is to devise an “elevator pitch” – a short and pithy statement of who you are. If someone asks you about yourself, you can then confidently reel off a few well-considered words.

Networking doesn’t necessarily come easy so if you find it awkward, you just need to get out there and practise. As science communicator Claire Malone believes, networking doesn’t have to be scary and something that only super confident people excel at. Learn to ask questions, be a good listener, take an interest in other people – and remember to ask for their business card so you can contact them later.

And our final piece of advice before even brushing up your CV or creating a LinkedIn page is to get a sense of the current job market for physics. It always goes through ups and downs, with the rise of artificial intelligence (AI) causing huge uncertainty right now. But overall there are some constant characteristics to the physics jobs market, as you can find out in a 2023 Institute of Physics report, which is packed with information about the UK physics jobs market by size, location and salary.

From reflection to action

So now you’ve done all that self-reflection, analysed the job market, audited your soft skills, and had a go at networking, it’s time to start applying for things to do after you graduate.

A number of you might be considering a PhD and, if so, we have some specific advice just for you (see “The PhD pathway” box below). But more than half of today’s graduates move into the private sector and pursue industry careers instead. If that’s your target, plan ahead by spending a summer interning at a company and try to find the right mentor for you. It’s also a good idea to do some extracurricular activities beyond your core academic aims, which will help you develop a range of transferable skills that can help you switch into a different area beyond physics.

The PhD pathway

Cartoon illustration of academia, with books, pens, coffee, caps and gowns and students — (Courtesy: iStock/elenabs)

For graduates who choose to remain in academia, and have found a PhD position, resilience and critical thinking are key skills to have. While it can be exciting and fulfilling to do your own research for the first time, completing a PhD is challenging. Most people have tough moments during a PhD, but people don’t often share their difficulties and it’s easy to think you’re the only one struggling.

Creativity is the backbone of scientific success so, if you’re doing a PhD, take the time to have a break and let new ideas come to light. Things can go wrong – from funding cuts to global pandemics – and it’s important to make sure that you value your mental health and don’t burn out. If you are one of the many neurodivergent people in science, it’s a good idea to ensure you have the accommodations necessary for success, and to embrace your diversity and the unique perspective you bring to your work.

Once you have completed all your research and gathered your data, the final step is to write your thesis – which can be a particularly stressful task, so its important to find different ways to motivate yourself and complete that final hurdle. Even if you don’t stay in academia, a PhD will teach you a lot about how to get ahead under your own steam and complete a major piece of work on your own.

A degree in physics opens doors into many different fields, from nuclear energy to quantum and beyond (see “Across the physics careers spectrum” box below). You might also be keen to use your physics skills in a more lateral way, whether in scientific patent law or science communication. You may choose to be an entrepreneur and set up your own business or decide to teach future generations of physicists. Remember, if your original plan doesn’t pan out as you imagined, it’s never too late to take a risk and make a change even at a later stage of your career.

Across the physics careers spectrum

Six images illustrating renewable energy, quantitative finance, a chessboard, the human circulatory system, quantum computing and nuclear fission — **Take your pick** From medical physics to green energy, and everything in between; your physics degree offers you many options. (Top row: Shutterstock/hrui; iStock/peshkov; Shutterstock/shaneinsweden. Bottom row: Shutterstock/Yevhen Vitte; iStock/chayanan; iStock/Peter Hermes Furian)

Green energy and sustainability: Physics graduates are ideally placed to tackle the world’s environmental challenges, with their technical knowledge and problem-solving skills. You can find out more about the physicists who are doing their bit to build a greener, more sustainable future in the field of decarbonizing energy sources.

Quantitative finance: With strong mathematical and analytic skills, physicists are a great fit for the financial world, and are often in high demand in quantitative finance in particular. Meet five physicists who pivoted to a career in finance, and explore what physics skills they use day to day.

Intelligence and cyber security: If a challenging but rewarding career is what you are looking for, you may want to consider a role in security. Here you can find out more about the careers of two leaders from GCHQ – the UK’s intelligence, security and cyber agency – with a background in physics.

Medical physics: Medical physicists have a wide variety of roles across academia and industry. From clinical research and medical imaging to computational simulations and testing new scanners, many job options for medical physicists around the world are included here.

Quantum: The quantum sector is thriving and there are numerous roles across the four main pillars of quantum computing; quantum simulation; quantum communication; and quantum sensing and metrology. From industry to academia, government and policy, and education; there are many ways to join the revolution.

Nuclear energy: With the push away from fossil fuels, there is a shortage of skills in the nuclear power sector. Here we speak to six physicists working across the nuclear energy industry – from design and construction, to safety and waste and beyond – highlighting how a background in physics can open many doors in this expanding sector.

The overall picture for all of you graduating with a degree in physics is positive. Physicists all over the world are keen to help solve some of humanity’s biggest challenges and help further our understanding of the universe at large. We hope that all of the resources above – alongside the annual Physics World Careers guide, the Physics World Jobs Hub and the Physics World LinkedIn page – will give you the headstart you need in planning your path and building a fulfilling career.

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No show again for mirror neutrons

Could a mirror world – in which every type of elementary particle has a corresponding mirror particle – exist in parallel to our ordinary world? For now, the answer is probably no, but in exploring this question, physicists at the Ultracold Neutron (UCN) Physics Group at the Paul Scherrer Institute (PSI) in Switzerland, say they have refuted this hypothesis with the best precision yet. Their work, which involved looking for so-called neutron-to-mirror-neutron transitions, also excludes mirror neutrons as potential components of dark matter, the mysterious substance thought to make up over 25% of the universe’s mass. Dark matter has never been detected directly, but its existence is inferred through observed gravitational interactions.

Theoretical physicists put forward the idea of a mirror world as long ago as the 1950s to address problems related to parity symmetry conservation in particle physics. Here, a set of mirror particles would exist as exact duplicates of their ordinary particle counterparts except that their so-called handedness would be reversed. In this picture, particles like the electron e, proton p and neutron n have mirror partners denoted by e′, p′ and n′. These mirror particles would have almost the same mass as the ordinary particles.

Not easy to detect

Finding proof for the existence of this mirror matter is important as it could account for dark matter – provided that during the evolution of the universe mirror matter was cooler than ordinary matter. Mirror matter is not easy to detect, however, because mirror particles are predicted to interact with ordinary matter in only two ways. The first is via the force of gravity and the second is through the rare oscillation of neutral particles such as neutrons that “mix” with their mass-degenerate mirror partners. This, explains Bernhard Lauss, one of the physicists at the PSI’s Center for Neutron and Muon Sciences who led this new study, means that ordinary neutrons might, on very rare occasions, turn into their mirror particles and simply “vanish” from our world. The time period up to this event is known as the n−n′ oscillation time, and afterwards a neutron could even reappear as if out of nowhere.

Neutron-to-mirror-neutron mixing has been predicted to have an n−n′ oscillation time as small as few tens of seconds, but it is not easily detectable because it is suppressed by environmental factors such as the presence of ordinary and/or mirror matter or of ordinary and/or mirror magnetic fields.

In recent years, researchers have been trying to reduce these environmental effects by studying neutrons cooled to extremely low temperatures. There are only a few laboratories around the world that can produce such ultracold neutrons at high regular flux. One is the Institut Laue–Langevin (ILL) in Grenoble, France, and another is the PSI.

Unexplored regions

The results from a series of experiments at the ILL in the past 20 years conducted at various magnetic fields were interpreted as being anomalous signals in the range of magnetic fields at which n−n′ oscillations could take place, thereby rekindling interest in the existence of mirror neutrons. More recently, the nEDM collaboration at PSI excluded a big part of this magnetic field range, but some magnetic field regions remain unexplored.

In the latest work, Lauss together with his colleague Géza Zsigmond and co-workers from PSI, the ETH Zurich and the Jagiellonian University in Krakow used ultracold neutrons from the PSI UCN source produced via a technique called superthermal moderation in a deuterium crystal cooled down to a temperature of 5 K. The researchers trapped these neutrons in a non-magnetic vessel made of stainless steel under vacuum. They were able to trap an unprecedented number of neutrons – on the order of 25 billion in all – something that allowed them to significantly improve the sensitivity of such experiments. A set of eight rectangular coils surrounding the storage vessel was used to generate a magnetic field that could be precisely controlled and scanned over two orders of magnitude, from 5–109 µT.

The team released around 1.5 million neutrons from the storage vessel into a gas-electron-multiplier based UCN-detector every six minutes to count how many neutrons remained in the tank after being stored there for 200 s. The researchers repeated the process over a period of several months, gradually varying the strength and flipping the direction of the magnetic field to scan all the relevant fields over which oscillations between neutrons and mirror neutrons are expected to occur. Finally, they performed complex computational simulations on the EULER cluster at the ETH Zurich to predict theoretical probabilities of n-n’ transitions that they then compared with the neutron losses they measured.

New limits set

The result? The researchers say they saw no evidence for such oscillations under the tested conditions. This finding, explains Lauss, means that there is a “very high probability” that previous speculations about the transformation of neutron particles into their mirror counterparts can be ruled out and that new limits for the n−n′ oscillation time constant have now been set. “Indeed, we have excluded the parameter space previously claimed for potential signals to 99.98 %,” he explains.

“The result therefore also excludes mirror neutrons as relevant dark matter particles,” Lauss tells Physics World.

Despite the existing limits already being very tight, Lauss, Zsigmond and their colleagues say they now plan to study the n−n′ transitions theoretically expected at zero mirror-magnetic fields to establish even more stringent limits. “Improving sensitivity further will be very challenging though, and perhaps even impossible,” admits Zsigmond. “A twofold increase, for example, will require 16 times more neutrons because the sensitivity of the experiment scales with the fourth root of the neutron counts.”

Quiz of the week: how will the new ‘AstroRad’ vest be of benefit to astronauts?

Fancy some more? Check out our puzzles page.

Magnetic field first kills superconductor, then brings it back to life

Unconventional superconductors are poorly understood materials with exceptionally useful properties. One example is their response to magnetic fields. In conventional superconductors, applied magnetic fields suppress superconductivity by either breaking up the paired electrons that facilitate it (Cooper pairs) or introducing magnetic flux which introduce resistance when it moves through the material. However, in a few unconventional materials, competition between ferromagnetic ordering, where the magnetic moments align to point in the same direction, and superconducting ordering, where paired electron moments point in opposite directions, can lead to a phenomenon known as re-entrant superconductivity (RSC). This is where, in an applied field, the magnetic ordering will at first destroy superconductivity, but when the field increases further, the superconductivity reappears.

An international team of physicists led by Denis Maryenko of Japan’s RIKEN Center for Emergent Matter Science has now observed this phenomenon for the first time in a two-dimensional (2D) unconventional superconductor. “We did not look for the RSC, and it was extremely surprising that we saw that,” Maryenko tells Physics World.

2D or not 2D

RSC has previously been observed in three-dimensional ferromagnetic materials such as CeRh ₂ As ₂ , organic superconductors and possibly in so-called heavy fermion com pounds such as UTe ₂ . However, 2D materials have several advantages over 3D ones when it comes to superconductivity. For example, in the conductor MoS ₂ , the material’s electronic band structure and spin-orbit coupling (SOC) combine to keep the electron spins pointing in a direction perpendicular to the sample’s surface. This locking greatly increases the value of the upper critical field, which is the maximum magnetic field at which superconductivity can persist.

Since 2D superconductivity had previously been observed at interfaces between KTaO ₃ and other materials, the RIKEN team took this as a starting point. “Our approach was to grow LaTiO₃, [since] that has good lattice matching [to KTaO₃], which is beneficial for epitaxial growth,” Maryenko explains.

Writing in Science Advances, the physicists describe how they performed magnetotransport measurements on their LaTiO₃/KTaO₃heterostructures to understand how these samples behaved in the presence of increasingly large applied magnetic fields. “We just wanted to see how the critical magnetic field behaves as a function of temperature, but suddenly we started seeing something else emerging at a rather low field: a resistive peak that seemed to separate two superconducting regions,” Maryenko says.

Rainbow colour plot of the interface resistance as a function of magnetic field and gate voltage. Higher gate voltages and magnetic fields are generally coloured red, but at a field of 0.9 T, there's a bright streak that appears to bleed down from the red area and stand out against the blue background. — Reentrant superconducting response: A plot of the interface resistance as a function of magnetic field and gate voltage, showing reentrant superconductivity at a magnetic field of around 0.9 T. (Courtesy: D Maryenko *et al. Science Advances* 12 26 DOI: 10.1126/sciadv.aeg0460)

This cusp, he explains, appears at a magnetic field B^∗ of 0.9 T with zero resistance measured at both lower and higher applied fields. In this experiment, B^∗ is independent of temperature and the concentration of charge carriers, which Maryenko and colleagues controlled experimentally by tuning a gate voltage across the interface.

Explaining re-entrant superconductivity

The researchers considered several possible explanations for the RSC, including Zeeman splitting, orbital effects and something called the Jaccarino-Peter effect, where internal and external magnetic fields compensate for each other and cancel out.

To identify the correct explanation, Igor Maznichenko and Sergey Ostanin from Martin Luther University in Germany and Arthur Ernst from Johannes Kepler University in Austria performed ab initio calculations to determine the electron band structure for the interface. These calculations identified the presence of a so-called Van Hove singularity, where for certain momenta, the electrons will have very high density of states.

Then, Vitalii Dugaev from Rzeszów University of Technology in Poland and Evgeny Ya Sherman at the University of the Basque Country in Spain developed a SOC-based model, taking inspiration from symmetry arguments and the ab initio band structure calculations. In the absence of an applied field, they found that there is a p ↔−p symmetry in the bands. This means that electrons with opposite momenta will have the same energy, so two electrons that form a Cooper pair, and have the same energy, have a total momentum equal to zero. In other words, the system favours spin-singlet Cooper pairs which form easily.

Applying a field breaks this symmetry, thereby reducing the Cooper pairing efficiency and decreasing the critical temperature T_c for the onset of superconductivity. On the other hand, the applied field also shifts the electrons closer towards the Van Hove singularity, increases the density of states and increases T_c. The combination of these opposing effects results in a minima appearing in T_c(B), hence RSC. The discovery of RSC in these materials establishes the system as a robust platform for studying unconventional superconductivity in 2D systems.

With e-MERLIN facing the axe, what next for Jodrell Bank and UK astronomy?

The UK’s Science and Technology Facilities Council (STFC) recently announced funding cuts to a range of physics and astronomy projects as it seeks to reduce costs. One of the most high-profile casualties is e-MERLIN, a network of seven radio telescopes spread across the UK, including the iconic Lovell Telescope at Jodrell Bank.

To discover what the cuts could mean for UK science and the livelihoods of those affected, podcast host Andrew Glester caught up with two astronomers at Jodrell Bank: Simon Garrington, e-MERLIN director, and David Williams-Baldwin, an e-MERLIN research support scientist.

They discuss why the cuts have caused such alarm among scientists, how the UK astronomy community is mobilizing in opposition, and why the consequences could extend far beyond Jodrell Bank.

e-MERLIN combines signals from its telescopes to produce sharp radio images of the Universe – with a resolution comparable to that of the Hubble Space Telescope. As things stand, funding for the telescope array will run out in March 2028, putting the future of the network and scientific operations at Jodrell Bank in doubt.

It’s an uncertain time for an observatory that was granted UNESCO World Heritage status in July 2019 for its historical contributions and its technical and scientific ingenuity since the 1940s.

An STFC spokesperson emphasises that Jodrell Bank is “not closing” and “will continue to play an important role in UK science”. This includes the Square Kilometre Array Observatory that has its headquarters at Jodrell Bank as well as other radio astronomy research projects in the UK and elsewhere.

“We recognise the strength of feeling about Jodrell Bank and the e-MERLIN network, and the contribution they have made to UK science. The achievements of the scientists, engineers and staff who have supported e-MERLIN over many years are widely respected across the research community,” they add. “We are focusing on the next generation of radio astronomy to ensure UK researchers continue to be at the forefront of the field. Rising cost pressures mean that we have had to make difficult choices about STFC funding to make sure we are able to fund world class research sustainably into the future.”

Earthquake fault irregularities control rupture speed and influence ground motion

Earthquakes that propagate at higher speeds make the ground move more than it would in slower but otherwise comparable earthquakes, say geophysicists. Using physics-based models, Mohamed Abdelmeguid and colleagues at the California Institute of Technology in the US showed that so-called “supershear” ruptures produce stronger and longer-duration ground motion than equivalent subshear ruptures. Because models of ground motion that underpin infrastructure standards do not account for this effect, these models may need to be updated to accurately describe hazards such as the 2025 Myanmar earthquake, which produced supershear ruptures in both directions.

Geophysicists have identified two categories of earthquake based on how fast their ruptures propagate through the Earth’s crust. Shear waves, or S waves, travel at certain expected velocities (the shear-wave velocity) depending on the stiffness and density of the ground. When a rupture travels faster than the shear-wave velocity of the surrounding ground, it is termed a supershear earthquake. Otherwise, it is subshear.

Although Abdelmeguid notes that supershear ruptures were first predicted theoretically in the 1970s, and have subsequently been observed in both laboratory experiments and the real world, most previous studies focused on their underlying mechanisms and the transition from subshear to supershear. The new study, which appears in the Bulletin of the Seismological Society of America, goes a step further by examining how the ground motion in a supershear quake differs from that of an otherwise comparable subshear one. The question is relevant, Abdelmeguid says, because nearly 36% of strike-slip earthquakes during the past 15 years contained supershear rupture, yet existing models do not factor it in.

Supershear simulations

To address this gap, Abdelmeguid and colleagues developed a dynamic rupture model that generates simulated ground motion data and used it to identify the characteristics of supershear earthquakes. They found that prolonged supershear propagation produces what they term “a distinct spatial pattern of stronger and longer-duration shaking”. However, episodic supershear segments, where rupture speed intermittently changes between supershear and subshear, produce ground motion similar to subshear ruptures.

Simulations like these are important because some supershear rupture scenarios are poorly represented in the observational record. This record is used to develop ground motion models, so simulations can “reveal physical dependencies that may not be apparent from existing observations,” Abdelmeguid says.

Characterizing the 2025 Myanmar earthquake

The 2025 Myanmar earthquake – a supershear quake in which many regions experienced higher than expected ground shaking – is a case in point. It occurred on the 1200-kilometre-long Sagaing Fault, which lies at the boundary of the Burma microplate and the Sunda plate and is not far from Mandalay, the country’s second-largest city. At magnitude 7.7, it was the largest earthquake in Myanmar in over a century, and the long rupture meant that large parts of Myanmar and Thailand were affected, with thousands of deaths and injuries as well as severe damage to infrastructure.

Initial studies were inconclusive about whether this earthquake’s rupture exhibited intermittent or sustained supershear speed. This lack of consensus is partly because recording intermittent supershear rupture is challenging when near-fault sensors are sparse, as is the case in Myanmar (and indeed most countries, with notable exceptions such as Japan, Taiwan, New Zealand and parts of the US).

A new study published in PNAS addresses this question. Using satellite earth observations, teleseismic data and CCTV footage as well as the available near-fault seismic sensor data, researchers led by Lingling Ye at the Southern University of Science and Technology (SUSTech) in China modelled the rupture process of the Myanmar quake. The result, Ye says, is a “well-resolved rupture process” that “reveals previously unrecognized dynamic rupture features”. Notably, the team’s simulations showed that the rupture transitioned between supershear and subshear as it tore through around 460 kilometres of the country.

Influence of geometry and stress distribution

To understand the cause of these variations, the researchers examined the geometry and stress distribution along the fault. Based on an existing fault model, they calculated a parameter called fault misalignment, which acts as an indicator for irregularities in the fault geometry. Higher fault misalignment, for example, indicates higher bends and branches in the fault.

When the researchers overlaid fault misalignment with the observed rupture speed, they noticed that supershear speeds coincided with fault stretches that had lower misalignment. For fault stretches with higher misalignment, subshear speed prevailed, meaning that the rupture decelerates at locations with high fault misalignment, transitioning from supershear to subshear. The researchers also found that the rupture speed, whether supershear or subshear, responded to the varying levels of local stress, with higher stress levels inducing faster rupture speed.

Although not unique in exhibiting supershear behaviour, the 2025 Myanmar earthquake rupture propagated away from the epicentre on both sides, travelling around 80 kilometres to the north and 380 kilometres to the south. Initial studies debated whether the ruptures on both sides were supershear. The researchers found that the northern rupture propagation was not supershear and claim that its shorter rupture length is caused by a combination of the fault’s complex structure and low accumulated stress resulting from the magnitude 6.8 earthquake that struck in 2012.

“Every earthquake has unique behaviour”

In future work, Ye is eager to revisit the 2023 Turkey doublet earthquakes, which she says have many more near-fault strong motion observations than the 2025 Myanmar quake, as well as a now well characterized complex fault system. “Every earthquake has unique behaviour,” she says. “Determining the key features that control their rupture processes is still underway.”

For Abdelmeguid, who recently started his own research group at the University of Houston, US, understanding these complex rupture processes is important because it will ultimately improve our understanding of geophysical processes and the ground motion models that are used for engineering applications. “We are currently extending this work to examine ground motions generated by ruptures on more complex faults, including cases in which fault geometry and heterogeneity influence both ground motion and the dynamic process, as observed during the multiple earthquakes,” he tells Physics World.

This article was amended on 2 September 2026 to clarify the nature of the rupture propagation in the 2025 Myanmar earthquake. The researchers found that the northern rupture was subshear, not supershear as originally stated.

Encapsulated islet transplants restore blood sugar control in diabetic mice

Uncoated and coated cell clusters — **An “invisibility cloak” for cells** The top row of photos, taken with a scanning electron microscope, shows uncoated cell clusters (islets). The bottom row shows a series of islets covered in a thin film that hides the donor cells from the body’s immune response, potentially offering a novel approach to treating diabetes. Scale bar: 0.05 mm. (Courtesy: Provided by Kyungsene Lee)

Islet transplantation, in which insulin-producing islet cells from a donor are implanted into a patient’s liver, is a promising therapy for type 1 diabetes. To prevent rejection of the donated cells, however, recipients need to continuously take immunosuppression drugs. Using a new type of cell encapsulation, researchers at The Pennsylvania State University demonstrated that diabetic mice could maintain normal blood glucose levels for 100 days, without requiring immunosuppressants. This represents a substantially longer time period than traditional cell therapies for diabetes.

The Penn State researchers developed their cell encapsulation process based on the molecular characteristics of the zona pellucida, a thin membrane that coats mammalian eggs. Once an egg is fertilized, biochemical reactions lead to hardening of the zona pellucida, which acts as a protective shield during embryo development. Here, biomimetic zona pellucida (BZP) encapsulation serves to hide the donor islets from the body’s immune system.

Pancreatic islets are clusters of five types of cells that work in tandem to regulate insulin and sugar levels in the body and prevent severe insulin deficiency. Islets harvested from healthy pancreases of deceased organ donors were first transplanted into Type 1 diabetes mellitus patients in 1974.

And in 2000, a landmark procedure increasing the mass of islets transferred and using steroid-free immunosuppression regimens was introduced by the University of Alberta in Edmonton, offering the hope of curing diabetes. But when implemented clinically, only 24% of recipients achieved insulin independence at 28 months with the Edmonton protocol.

Research on islet implantation continued unabated, particularly initiatives in islet encapsulation, using semi-permeable membranes to allow the entry of oxygen and nutrients and prevent the entrance of immune response cells. Most current work in this area uses large, 1.5 mm capsules. The Penn State team took an opposite tack: creating a thin 20-µm hydrogel capsule that spontaneously generates on the cell surface with 100% encapsulation efficiency.

Protection from immune attack

Principal investigator Yong Wang and colleagues spent eight years developing an ultrathin hydrogel layer that effectively lays against the curved edge of living cells or cell clusters without impacting their functionality. The islet encapsulation process takes place spontaneously in aqueous solutions under physiological conditions, without exposure to harsh physical, chemical or biological factors, ensuring that BZP development on the cell membrane does not cause any loss of cellular viability or function.

Writing in Nature Biomedical Engineering, the researchers explain that they emulated the biological processes that harden the zona pellucida on a fertilized egg (calcium ion-induced glycoprotein cleavage, assembly and crosslinking on the egg surface). In spite of its thinness, the BZP protects the cell as effectively as the much larger 1.5 mm diameter capsules.

The small capsule size significantly reduces the potential transplantation volume, improving an organ’s ability to receive and accommodate the large number of islets needed to be effective. When coating a 100-µm islet, for example, a 20-µm-thick BZP uses only 1/71 of the polymer volume of a 500-µm microcapsule. The researchers also suggest that the reduced polymer volume used for cell protection by the thin capsule may be a safety advantage of BZP compared to larger capsules.

In vivo assessment

The researchers first examined whether BZP could reduce the level of foreign body response to polystyrene microparticles injected into the peritoneal cavities of mice. Compared with uncoated microparticles, the BZP-coated particles induced a lower immune system response. They examined BZP synthesized using two types of alginate (involved in crosslinking reactions during BZP formation and hardening): chemically modified ultrapure alginate (BZP-U) and regular alginate (BZP-R). Tests in mice verified that BZP-U significantly mitigated the foreign body response compared with BZP-R encapsulation.

Immunocompetent diabetic mice treated with BZP-U-coated islets had their blood sugar restored to healthy levels within seven days. Additionally, the majority stayed diabetes free for over 100 days without continuously needing systemic immunosuppression. By comparison, mice treated with uncoated islets could not maintain healthy blood sugar levels for more than seven days.

In addition to effectively regulating blood glucose levels, one of the biggest advantages of the BZP approach is that it eliminates the need for long-term systemic immunosuppression. Lantidra, the only islet transplantation treatment for Type 1 diabetes cleared by the US Food and Drug Administration, requires continuous immunosuppressants to stop immune system attacks. Such long-term immunosuppressant use can lead to increased risk of infection and other serious health issues, including cancer.

Wang and colleagues plan to further study the BZP approach to better understand the specific duration of resistance each islet transplant may offer, as well as its effect in large animals.

“Large animal studies will be necessary to fully demonstrate their potential in cell transplantation,” the researchers write. “When large animals are used, one needs to examine not only glucose levels but also other key biomarkers. Moreover, the long-term immune responses to BZP and BZP-encapsulated cells warrant further investigation to yield deeper insights into BZP optimization and improvement.”

Word wave puzzle no.8

Here’s how the game works:

1. Enter a word guess – in this game the word has six letters.
2. After submitting your guess, each letter in the guessed word is coloured to provide feedback:
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3. Using this colour feedback, refine your next guess.
4. Continue guessing until you correctly identify the hidden word(s) or run out of attempts.

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Gamma Knife radiosurgery brings precision treatment to benign brain conditions

This episode of the Physics World Weekly podcast features Katharine Hunt, Head of Radiosurgery Physics at Sheffield Teaching Hospitals NHS Foundation Trust in the UK. She explains how Gamma Knife radiosurgery is being used to treat arteriovenous malformations (AVMs) and other benign brain conditions.

Sheffield is home to one of the world’s longest-running Gamma Knife centres, and Hunt shares the facility’s insights into treatment planning, dose precision, paediatric care, and long-term outcomes.

This podcast is sponsored by Elekta, which produces the Gamma Knife radiosurgery system.

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