Proton and Proton-Photon Hybrid Partial Stereotactic Ablative Boost Radiotherapy (P-SABR) for Unresectable Large Tumors

Overview

This prospective, multicenter, observational cohort study will enroll 60 adults with unresectable larege tumors (longest diameter \> 5 cm) who receive one of two treatment modes: (A) proton P-SABR, in which both the proton partial-SABR boost (course 1) and the subsequent proton CFRT (course 2) are delivered at the proton center; or (B) proton-photon hybrid P-SABR, in which course 1 is delivered with protons at the proton center and course 2 CFRT is delivered with photons. The primary objective is to describe, in each cohort, the incidence of treatment-related grade ≥ 3 acute and late adverse events (CTCAE v5.0). Secondary objectives are dosimetric and the 1-year local control rate (RECIST 1.1, in-field). Participants are followed for up to 5 years.

BACKGROUND Large tumors (longest diameter \> 5 cm) are often unresectable and respond poorly to conventionally fractionated radiotherapy (CFRT) because of their size and hypoxic core, while normal-tissue tolerance prevents delivering stereotactic ablative radiotherapy (SABR) to the whole tumor. Partial stereotactic ablative boost radiotherapy (P-SABR) delivers a SABR boost only to the central part of a large tumor, followed by CFRT to the whole target. Proton beams, with their Bragg-peak dose fall-off, may sharpen this "onion-skin" dose distribution and further increase the biologically effective dose (BED) within the tumor core. DESIGN This prospective, multicenter, observational cohort study will enroll 60 adults with unresectable bulky tumors (longest diameter \> 5 cm) who, as part of routine care, receive one of two treatment modes: (A) proton P-SABR, in which both the proton partial-SABR boost (course 1) and the subsequent proton CFRT (course 2) are delivered at the proton center; or (B) proton-photon hybrid P-SABR, in which course 1 is delivered with protons at the proton center and course 2 CFRT is delivered with photons (IMRT/VMAT) either at the same center or at Peking University First Hospital or one of six photon sub-centers, after central review of the composite plan. Cohort membership follows the patient's own choice and care pathway; the study does not assign treatment or intervene in clinical decisions, and no formal between-cohort hypothesis testing is planned. OUTCOMES The primary objective is to describe, in each cohort, the incidence of treatment-related grade ≥ 3 acute and late adverse events (CTCAE v5.0). Secondary objectives are dosimetric (percentage of the GTV receiving BED10 ≥ 100 Gy, PTV V95% and V100%, organ-at-risk doses) and the 1-year local control rate (RECIST 1.1, in-field). Participants are followed for up to 5 years.