Initial Experience with Special Medical Physics Consultations for Proton Re-Irradiation

To reduce the volume of tissues receiving additional radiation dose, proton therapy is often used in patients receiving re-irradiation (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 (OAR) equivalent dose in 2Gy 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 voxel-wise 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/CSI (20%), head and neck (19%), and pelvis (19%). Median reRT interval was 33 months (16-63 months interquartile range) 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/CSI (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.