Purpose: The Mevion S250i Hypersccan proton system employs a mini-MLC (Adaptive Aperture (AA)) to trim lateral spots for sharpened penumbra, which can be achieved through a static single layer trimming (SA) or dynamic layer-by-layer trimming (MLA). This study investigates potential dosimetric benefits that maybe offered by MLA in pancreatic cancer treatment.
Methods: Six clinical hypofractionated pancreatic treatment plans were retrospectively re-planned using these two different spot trimming techniques. Raystation TPS (V.10a) was used for planning, which models the AA either as a single static collimator in SA or multiple collimators of varying sizes in MLA. Treatment plans, which consists of 2 to 3 beams, were optimized to meet the clinical goals at the minimum and further optimized for additional improvements possible. Robust optimization incorporating a 3.5% range uncertainty and 5 mm position uncertainty in all directions were performed. Isodose distributions and DVHs for targets and OARs were compared. Conformity index (CI) of the 50% isodose line and a dose gradient index (V20%-V80%) were calculated and compared.
Results: While both SA and MLA plans meet the clinical goals for similar CTV coverage, on average, plans with MLA achieved better sparing of GI OARs (e.g., 20% reduction of V15Gy for small bowel). No statistically significant difference was observed for the CI. The SA plans yielded slightly better (4%) gradient index.
Conclusion: For pancreatic cancer treatments using the Mevion Hyperscan proton system, the benefits of MLA were demonstrated in reduced bowel dose. Benefits in other anatomical sites will need further evaluation and are likely site dependent.