(P 315) SCALP VOLUMETRIC MODULATED ARC THERAPY USING 3D MILLED BOLUS: DOSIMITRY, TOXICITY AND OUTCOMES

Objective: Radiotherapy for scalp lesions is frequently challenging due to the competing needs for target volume coverage, sparing of numerous critical nearby OARs, and reproducible bolus setup despite the irregular and highly convex scalp geometry. Here, we evaluate the use of 3D milled bolus in treating such tumors.

Methods: A retrospective analysis of scalp patients (pts) treated between 2016-2022 using 3D milled bolus and Volumetric Modulated Arc Therapy (VMAT). All pts were treated with 6 MV photon beams, IGRT via daily CBCT, and 6-degree of Freedom (6-DOF) couch tops. Patient demographics, tumor characteristics, dose volume histograms (DVHs), toxicities and outcomes were evaluated. Regional control defined as control of the disease in the rest of the scalp/neck and local control defined as disease control within the PTV

Results: Eight pts were identified. Median age 74 (60-77), ECOG performance status was 0-1 in 7 pts (88%) and 88% were males. 38% of patients were treated with definitive intent to gross disease; the remaining 62% were treated post-operatively, 13% with microscopically positive margins and 50% with negative margins. 50% had T3/T4 and 88% had N0 disease. Median radiation dose was 66 Gy (60-69.96Gy). 50% received concurrent systemic therapy with Paclitaxel. In 63% of pts, the whole scalp was treated; in the remaining 37%, the median ratio of PTV volume to scalp volume was 60% (30-90%). 25% of patients (n=2) had neck irradiation for prophylactic (1pt)/ neck lymph node involvement (1 pt). The median homogeneity index was 1.07 (1.04-1.11). The median brain Dmax was 65 Gy (56-71.9), median brain mean dose was 22Gy (6-38.4), median eye Dmax was 11Gy (01.7-49), median cochlea mean was 8.4Gy (0.8-20.4), median lacrimal gland mean dose was 8.3Gy (0.11-38), median hippocampus Dmin was 6.5Gy (0.1-14.9) and Dmax was 12Gy (0.3-23). Most common acute side effects were grade 1-2 fatigue (100%), grade 1 pain (75%), grade 1 dysgeusia (25%), grade 1-2 dermatitis (63%). The only grade >=3 adverse event was grade 3 dermatitis in 37%. 25% developed grade 1 memory impairment. 25% had grade 1 eye dryness. None developed brain radionecrosis. At a median follow-up was 25 months (7-75), the 2-year locoregional control was 100%. 2-year DM control was 57%. 2-year OS was 70%.

Conclusion: VMAT planning with 3D milled bolus permits technically sound radiotherapy for scalp targets with an acceptable toxicity profile and relatively favorable clinical outcomes. This approach warrants further evaluation in a larger prospective study.