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

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.

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 (PTV_WB) 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, PTV_WB, organs at risk, treatment efficiency (monitor units, delivery time), Paddick conformity index, and homogeneity index were compared using Wilcoxon signed-rank test.

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 PTV_WB 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).

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.