SU‐F‐T‐128: Dose‐Volume Constraints for Particle Therapy Treatment Planning

Abstract

Purpose: Determine equivalent Organ at Risk (OAR) tolerance dose (TD) constraints for MV x‐rays and particle therapy. Methods: Equivalent TD estimates for MV x‐rays are determined from an isoeffect, regression‐analysis of published and in‐house constraints for various fractionation schedules (n fractions). The analysis yields an estimate of (α/β) for an OAR. To determine equivalent particle therapy constraints, the MV x‐ray TD(n) values are divided by the RBE for DSB induction (RBEDSB) or cell survival (RBES). Estimates of (RBEDSB) are computed using the Monte Carlo Damage Simulation, and estimates of RBES are computed using the Repair‐Misrepair‐Fixation (RMF) model. A research build of the RayStation™ treatment planning system implementing the above model is used to estimate (RBEDSB) for OARs of interest in 16 proton therapy patient plans (head and neck, thorax, prostate and brain). Results: The analysis gives an (α/β) estimate of about 20 Gy for the trachea and heart and 2–4 Gy for the esophagus, spine, and brachial plexus. Extrapolation of MV x‐ray constraints (n = 1) to fast neutrons using RBEDSB = 2.7 are in excellent agreement with clinical experience (n = 10 to 20). When conventional (n > 30) x‐ray treatments are used as the reference radiation, fast neutron RBE increased to a maximum of 6. For comparison to a constant RBE of 1.1, the RayStation™ analysis gave estimates of proton RBEDSB from 1.03 to 1.33 for OARs of interest. Conclusion: The presented system of models is a convenient formalism to synthesize from multiple sources of information a set of self‐consistent plan constraints for MV x‐ray and hadron therapy treatments. Estimates of RBEDSB from the RayStation™ analysis differ substantially from 1.1 and vary among patients and treatment sites. A treatment planning system that incorporates patient and anatomy‐specific corrections in proton RBE would create opportunities to increase the therapeutic ratio. The research build of the RayStation used in the study was made available to the University of Washington free of charge. RaySearch Laboratories did not provide any monetary support for the reported studies.

A feasibility study of spatiotemporally integrated radiotherapy using the LQ model

“…schemes with the nominal α/β ratios or one could fit the α/β ratios of OAR to the isoeffect BED curve using two or more clinical tolerance doses used in practice with different fractionation schedules (Stewart et al 2016). Our STMRT presented here used the LQ based BED to estimate the damage to the tumor and OAR, however, it should be noted that there are other radiobiological models to describe the time-dose relationship, e.g.…”