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FLASH radiotherapy can reduce normal tissue damage while maintaining the tumor response. FLASH with proton beams may be obtained with high energy transmission beams at the cost of poorer dose conformality. Here, we propose FLASH painting as a method to deliver proton FLASH therapy with uncompromised dose distributions. FLASH painting only delivers FLASH dose rates in smaller volumes, where the FLASH sparing effect is mostly needed. It makes it easier to obtain FLASH dose rates. This study shows how proton FLASH painting may improve proton therapy of tumors near the brain stem.

Proton FLASH painting plans were made for two patients previously treated with proton therapy for a meningioma (Patient 1) and an anaplastic oligodendroglioma (Patient 2). The prescribed dose was 59.4Gy in 33 fractions. Since the clinical target volume (CTV) was either overlapping with or adjacent to the brain stem the CTV dose coverage was compromised in the clinical treatment plans. The FLASH painting plans had a FLASH painting field with a single energy layer (~234MeV) that delivered most of the dose in a FLASH painting volume in or near the brain stem (Figs 1a,2a) and three non-FLASH proton fields that ensured a uniform CTV dose (Figs 1b,2b). The FLASH painting fields had ~26cm water equivalent beam degrader material in the entrance path to adjust the Bragg peak to the FLASH painting volume. Delivery of the FLASH painting fields at a clinical proton facility (ProBeam, Varian Medical Systems) was simulated by assuming 800nA cyclotron current and the same beam transmission efficiency as measured for the FLASH painting beam energies at the facility (~5%). The simulated spot durations were combined with spot specific dose distributions to determine the dose rate as function of time in each voxel. The FLASH fraction was then calculated as the fraction of the dose in each voxel that was delivered continuously with at least 40Gy/s mean dose rate (Figs 1c,2c). Finally, the FLASH weighted dose to normal tissue was calculated as the dose delivered with FLASH weighted by 80% plus the dose delivered without FLASH weighted by 100% (i.e by assuming a FLASH sparing effect of 20%) (Figs 1d,2d). The brain stem dose with and without FLASH weighting was reported to illustrate the potential brain stem sparing effect of FLASH painting. Plan 1 was made without robust optimization while Plan 2 was optimized and evaluated robustly (±2mm setup, ±3.5% range uncertainties).

Both FLASH painting plans had full CTV dose coverage (Figs 1b,2b). FLASH painting reduced the brain stem V54Gy to clinically acceptable levels: from 1.54cm³ to 0.0 cm³ for Plan 1 and from 0.33cm³ to 0.02cm³ for Plan 2 (worst case robustness scenario) (Figs 1d-e,2d-e).

Proton FLASH painting was proposed as a clinically realizable strategy for proton FLASH treatments with uncompromised dose distributions. Its ability to cover the CTV while sparing the brain stem was demonstrated in simulated deliveries of two FLASH painting plans.