Initial Experience With Special Medical Physics Consultations for Proton Reirradiation

To reduce the volume of tissues receiving additional radiation dose, proton therapy is often used in patients receiving reirradiation (reRT). Standardized workflows to systematically assess cumulative dose and potential risks of reRT are limited. We present our institutional implementation of a proton reRT SMPC, characterizing its workflow and initial clinical impact.

Using published literature, physicians agreed upon institutional organs-at-risk (OARs) equivalent dose in 2-Gy fraction (EQD2) goals; constant relative biological effectiveness of 1.1 was applied to physical doses before EQD2 conversion. The SMPC used a deformable registration-based analysis of cumulative dose to OARs for all patients with available DICOM-RT files. Physicists assessed registration quality and suggested mitigation strategies. Using OAR-specific alpha/beta (α/β) ratios, cumulative EQD2 distributions were created. Cumulative EQD2 volumes, prior dose contributions, and time interval between courses were recorded for each OAR. SMPC-induced repeat plan optimizations ("reoptimization") and resulting dosimetric changes were recorded.

Proton reRT SMPC with voxelwise EQD2 accumulation was performed for 192 reRT courses after center-wide implementation, 26% of which were hyper- or hypofractionated. Common disease sites were thorax (21%), brain/Craniospinal irradiation (20%), head and neck (19%), and pelvis (19%). Median reRT interval was 33 months (interquartile range, 16-63 months) from recent prior RT and the shortest for thorax (16 months). Overall percentage of consulted organs meeting institutional cumulative EQD2 goals was 75%. Physicists designated 153/249 (61.4%) registrations as acceptable for clinical use and addressed 66/96 (68.7%) of issues before dose accumulation. After SMPC, 25.5% of patients underwent reoptimization, most commonly for thoracic (32.6%) and abdominal/liver (45.5%) and least often for brain/Craniospinal irradiation (16.2%) reRT. Carotid artery was the most common OAR prompting reoptimization.

Our experience demonstrates that implementation of a proton reRT SMPC frequently prompts clinically meaningful reoptimization and mitigation strategies for poor registration. Our methods provide a standardized comprehensive assessment of cumulative dose to OARs to inform risk assessment.