Image guidance for FLASH radiotherapy

Abstract

FLASH radiotherapy (FLASH-RT) is an emerging ultra-high dose (>40 Gy/s) delivery that promises to improve the therapeutic potential by limiting toxicities compared to conventional RT while maintaining similar tumor eradication efficacy. Image guidance is an essential component of modern RT that should be harnessed to meet the special emerging needs of FLASH-RT and its associated high risks in planning and delivering of such ultra-high doses in short period of times. Hence, this contribution will elaborate on the imaging requirements and possible solutions in the entire chain of FLASH-RT treatment, from the planning, through the setup and delivery with online in vivo imaging and dosimetry, up to the assessment of biological mechanisms and treatment response. In patient setup and delivery, higher temporal sampling than in conventional RT should ensure that the short treatment is delivered precisely to the targeted region. Additionally, conventional imaging tools such as cone-beam computed tomography will continue to play an important role in improving patient setup prior to delivery, while techniques based on magnetic resonance imaging or positron emission tomography may be extremely valuable for either linear accelerator (Linac) or particle FLASH therapy, to monitor and track anatomical changes during delivery. In either planning or assessing outcomes, quantitative functional imaging could supplement conventional imaging for more accurate utilization of the biological window of the FLASH effect, selecting for or verifying things such as tissue oxygen and existing or transient hypoxia on the relevant timescales of FLASH-RT delivery. Perhaps most importantly at this time, these tools might help improve the understanding of the biological mechanisms of FLASH-RT response in tumor and normal tissues. The high dose deposition of FLASH provides an opportunity to utilize pulse-to-pulse imaging tools such as Cherenkov or radiation acoustic emission imaging. These could provide individual pulse mapping or assessing the 3D dose delivery superficially or at tissue depth, respectively. In summary, the most promising components of modern RT should be used for safer application of FLASH-RT, and new promising developments could be advanced to cope with its novel demands but also exploit new opportunities in connection with the unique nature of pulsed delivery at unprecedented dose rates, opening a new era of biological image guidance and ultrafast, pulsebased in vivo dosimetry.

FLASH radiotherapy and the associated dosimetric challenges

“…However, Cherenkov based detector systems are expected to be energy dependent and determining the relation between light yield and absorbed dose can be complicated under experimental conditions. Intensive research is ongoing [20,31].…”

“…For superior soft tissue contrast, MR-images would be ideal, however this is not yet available in combination with FLASH-RT. El Naqa et al 2022 argues that future multiparametric on-board MRI protocols also could include functional information that could be explored in combination with FLASH-RT [31]. As for today, the widely accessible on-board imaging system is cone beam computed tomography (CBCT), which provides 3D volumetric kilovoltage imaging of the patient and has shown to reduce localization errors in radiotherapy by approximately 50% [38].…”

“…Motion management systems using optical surface imaging, so called SGRT, are used clinically for patient positioning and real time motion monitoring and have been suggested to be a potential candidate for motion management system at FLASH-RT [31,40].…”

Contemporary Applications of Cherenkov Imaging in Radiation Therapy

“…Given the nature of instant emission, CR imaging via fast electronics is suitable for ultrafast monitoring and recording of FLASH-RT delivery which typically takes less than tenth of a second. Figure 5 demonstrates imaging CR in real time of a UHDR electron FLASH delivery to a minipig's skin with optically clear bolus resolving the treated field [4,5].…”

Real-time, volumetric imaging of radiation dose delivery deep into the liver during cancer treatment

“…With precalibration of the Grüneisen parameter, medium density, pulse time profile and sensor sensitivity, the linear relationship between the absorbed dose and deposited dose could enable iRAI to both localize and quantify the absolute dose deposition during RT [32][33][34][35][36][37] . Most recently, the feasibility of iRAI for real-time monitoring of misalignment between the targeted tumor and the delivered beam has been presented for conventional as well as ultra-high dose rate (FLASH) radiation treatments 32,34,38 .…”