Dosimetric and temporal evaluation of dynamic collimation with multi-leaf collimator in scanned proton therapy: A planning study

The MELTHEA V proton therapy system incorporates a beam nozzle capable of both passive scattering and active scanning, featuring a multi-leaf collimator (MLC). This study evaluates the dosimetric and temporal efficiency of dynamic collimation using the universal MLC with the scanning method. Three MLC configurations were defined: (1) nMLC - no MLC use; (2) sMLC - static leaf positioning across all energy layers; and (3) dMLC - dynamic leaf positioning changing with each energy layer. Since the treatment planning system does not support dMLC natively, it was implemented via scripting. First, various MLC margins were evaluated through phantom simulations. Then, using a representative margin, treatment plans for nine liver cancer cases were assessed for each MLC configuration. Irradiation delay time was simulated considering monitor units, dose rates, and the time characteristics of the irradiation system. Phantom simulations demonstrated that an MLC margin of 0.5σ (with σ representing the in-medium spot size) effectively reduced the surrounding dose while preserving target coverage. For liver cancer cases, the average conformity index at the 50 % isodose line for nMLC, sMLC, and dMLC was 3.27 ± 0.60, 2.61 ± 0.28, and 2.28 ± 0.24, respectively. The mean doses in a 10-mm ring surrounding the target were 54.9 ± 2.5, 48.3 ± 3.9, and 41.4 ± 3.8 Gy, respectively. The average irradiation delay time of the dMLC was 8.4 %. The dMLC demonstrated enhanced dosimetric performance compared to nMLC and sMLC, with minimal irradiation time delays, making it suitable for clinical liver cancer treatment.