Dosimetric comparison of TomoTherapy and non-coplanar VMAT for hippocampal-avoidance prophylactic cranial irradiation

Purpose To compare the dosimetric characteristics of non-coplanar volumetric modulated arc therapy (VMAT) and TomoTherapy for hippocampal-avoidance prophylactic cranial irradiation (HA-PCI) in small cell lung cancer (SCLC) patients. Methods Ten SCLC patients who received HA-PCI were retrospectively analyzed. Two plans were generated as TrueBeam non-coplanar VMAT with four arcs and helical TomoTherapy. The hippocampal avoidance zone used a 3-mm margin, reduced from RTOG 0933’s 5-mm specification. Planning target volume of whole brain (PTVWB) was prescribed 25 Gy in 10 fractions, normalized to D 95% =2500 cGy. RTOG 0933 hippocampal constraints (D max ≤ 1600 cGy, D 100% ≤900 cGy) were applied. Dosimetric parameters for hippocampus, PTVWB, organs at risk, treatment efficiency (monitor units, delivery time), Paddick conformity index, and homogeneity index were compared using Wilcoxon signed-rank test. Results Non-coplanar VMAT achieved significantly lower hippocampal D max than TomoTherapy (1353 cGy vs 1638 cGy, p=0. 004), meeting RTOG 0933 constraints, while TomoTherapy exceeded the per-protocol constraint by 38 cGy but remained within the acceptable deviation threshold, indicating clinically acceptable dosimetric outcomes. Non-coplanar VMAT demonstrated superior PTVWB coverage: V 98% (96. 68% vs 95. 77%), V 95% (97. 66% vs 96. 48%), D 98% (2320 cGy vs 2095 cGy) (all p=0. 004). Paddick conformity index was higher (0. 91 vs 0. 84, p=0. 012) and homogeneity index lower (0. 20 vs 0. 27, p=0. 004). Non-coplanar VMAT reduced monitor units by 88. 5% (748 vs 6528 MU, p=0. 004) and treatment time by 25. 2% (287 vs 384 seconds, p=0. 004). Bilateral eye D max was 21-27% lower (all p=0. 004) and bilateral cochlear D max approximately 15% lower (p ≤ 0. 008). Conclusions Non-coplanar VMAT demonstrated superior dosimetric characteristics compared to TomoTherapy for HA-PCI with 3-mm margin, meeting hippocampal constraints while improving target coverage and treatment efficiency. Prospective validation with neurocognitive outcomes is needed.