Ultrahigh dose rate radiation therapy (FLASH) is currently one of the hottest topics in radiotherapy research. Several animal studies (and recently, a first-in-human case report) have shown that the probability of radiotherapy side effects can be greatly reduced when the radiation dose rate is ramped up from the standard 0.1 Gy/s to 40 Gy/s or more.
In order to use such high dose rate treatments safely, it is imperative to continuously monitor where and how much dose is really being deposited in the patient. This is currently only possible for tumours on or close to the skin surface where dosimeters can be placed easily. Researchers at the University of Michigan have now proposed a new method to measure dose, even deep within a patient, while simultaneously obtaining images of the radiation target and surrounding tissue. They have published the details of their study in Medical Physics.
Radiation dose can be “heard”
The basis for the proposed imaging method is the thermoacoustic effect. As ionizing radiation deposits energy in the patient, the temperature increases in the tissue receiving this radiation dose. This causes thermal expansion, leading to a pressure wave that propagates outward. In conventional radiotherapy, these waves are extremely weak: each radiation pulse delivered by the accelerator leads to a pressure wave on the order of 10 mPa (about ten million times smaller than standard atmospheric pressure).
But with FLASH dose rates of 40 Gy/s or more, the pressure of the generated sound waves can be as high as several pascals per pulse. At this level, the signals are easily detected by conventional ultrasound probes placed on the patient’s skin. The team call this technique ionizing radiation acoustic imaging.
Overcoming equipment limitations
Since regular medical linear accelerators (linacs) are not built to deliver FLASH treatments, first author Ibrahim Oraiqat, senior researchers Issam El Naqa and Xueding Wang, and colleagues first had to modify a regular linac to enable the delivery of 6 MeV electrons at FLASH dose rates. Their experimental setup consisted of a gelatin phantom (to simulate human tissue) and ultrasound probes, both immersed in water. Since the same ultrasound probes were used for both acoustic dosimetry and standard ultrasound imaging, the group needed a way to prompt fast switching between the two probe modes. They accomplished this by using Cerenkov radiation, emitted by the high-energy electrons as they pass through the water, as a trigger to signal to the probes that the beam was on.
The group’s experiments showed that the acoustic imaging signal increased linearly with the dose-per-pulse – a highly desirable property in a dosimeter – and that acoustic dose measurements at different depths agreed with benchmark measurements using commercial film dosimeters. Finally, the group successfully performed simultaneous acoustic dosimetry and ultrasound imaging in a moving tissue-mimicking phantom of a rabbit liver. They could accurately pinpoint where the radiation dose was deposited as the phantom moved and monitor it in real-time, as shown in a supplementary video.
Towards standard clinical use
Since FLASH is still an experimental treatment method, Oraiqat and colleagues are working to also make acoustic imaging accessible for standard radiotherapy. The group’s next steps will include incorporating ultrasound information on tissue properties in the acoustic image reconstruction. This could make it possible to measure how much dose is delivered even if the target is not a homogeneous gel but, say, a patient consisting of various tissue types. This would be another large step towards the first real-time acoustic dose measurement in a patient.
