Author: Laura Buchanan, Samantha G. Hedrick, Stephen L. Mahan, Isabella Pfeiffer, Chester R. Ramsey π¨βπ¬
Affiliation: Thompson Proton Center π
Purpose: Linear energy transfer (LET) and variable relative biological effectiveness (RBE) evaluation are important steps in the future of proton therapy, moving beyond RBE=1.1 for all treatment planning. In this work, we used an in-house python script (βSpotDeleteβ) to preferentially remove proton Bragg peaks (spots) out of organs-at-risk (OAR) to reduce variable RBE in OARs for 3 treatment sites. The impact of planning technique, single-field vs multi-field optimization, was also evaluated for effectiveness of SpotDelete
Methods: SpotDelete was implemented in RayStation version 23B for breast, head and neck (H&N), and prostate patients. Breast planning was performed with 3 increasing levels of βMFO-nessβ, allowing more inter-field heterogeneity. For all sites, spots were deleted in select OARs, not including target voxels, and optimization was continued with remaining spots. Plans were evaluated using Monte Carlo dose calculations and a variable RBE model (McNamara model with Ξ±/Ξ² = 3 Gy). The impact on dose distribution, LET, and variable RBE was assessed.
Results: SpotDelete effectively reduced high LET and variable RBE dose in OARs without compromising target coverage or plan robustness for all treatment sites. In breast cancer patients, applying SpotDelete with a hybrid multi-field optimization (MFO) technique (120MFO) reduced acute skin toxicity, decreasing CTCAE Grade 2 radiation dermatitis from 36% to 33% and Grade 3 from 7% to 0%. In Prostate and H&N cases, SpotDelete demonstrated reduced variable RBE dose to OARs.
Conclusion: The SpotDelete technique allows for control over the placement of high LET proton spots, reducing variable RBE dose to OARs. This method can be safely and efficiently implemented in clinical practice without additional costs and has the potential to reduce side effects and improve therapeutic ratios in proton therapy.