TU‐F‐12A‐04: Differential Radiation Avoidance of Functional Liver Regions Defined by 99mTc‐Sulfur Colloid SPECT/CT with Proton Therapy

Purpose: Radiotherapy for hepatocellular carcinoma patients is conventionally planned without consideration of spatial heterogeneity in hepatic function, which may increase risk of radiation‐induced liver disease. Pencil beam scanning (PBS) proton radiotherapy (pRT) plans were generated to differentially decrease dose to functional liver volumes (FLV) defined on [99mTc]sulfur colloid (SC) SPECT/CT images (functional avoidance plans) and compared against conventional pRT plans. Methods: Three HCC patients underwent SC SPECT/CT scans for pRT planning acquired 15 min post injection over 24 min. Images were reconstructed with OSEM following scatter, collimator, and exhale CT attenuation correction. Functional liver volumes (FLV) were defined by liver:spleen uptake ratio thresholds (43% to 90% maximum). Planning objectives to FLV were based on mean SC SPECT uptake ratio relative to GTV‐subtracted liver and inversely scaled to mean liver dose of 20 Gy. PTV target coverage (V95) was matched between conventional and functional avoidance plans. PBS pRT plans were optimized in RayStation for single field uniform dose (SFUD) and systematically perturbed to verify robustness to uncertainty in range, setup, and motion. Relative differences in FLV DVH and target dose heterogeneity (D2‐D98)/D50 were assessed. Results: For similar liver dose between functional avoidance and conventional PBS pRT plans (Dmean≤5% difference, V18Gy≤1% difference), dose to functional liver volumes were lower in avoidance plans but varied in magnitude across patients (FLV70%max Dmean≤26% difference, V18Gy≤8% difference). Higher PTV dose heterogeneity in avoidance plans was associated with lower functional liver dose, particularly for the largest lesion [(D2‐D98)/D50=13%, FLV90%max=50% difference]. Conclusion: Differential avoidance of functional liver regions defined on sulfur colloid SPECT/CT is feasible with proton therapy. The magnitude of benefit appears to be patient specific and dependent on tumor location, size, and proximity to functional volumes. Further investigation in a larger cohort of patients may validate the clinical utility of functional avoidance planning of HCC radiotherapy.