Proton pencil beam scanning lattice radiotherapy: Technical implementation and comparison against photon-based delivery.

OBJECTIVES: Spatially fractionated radiotherapy in the form of a lattice of high-dose regions within the tumour has potential to improve the therapeutic response compared to standard radiotherapy. We aimed [i] to develop and dosimetrically validate a proton pencil-beam scanning lattice radiotherapy (LRT) planning technique, [ii] to compare against a photon VMAT LRT technique, and [iii] to assess which clinical sites are feasible for treatment. METHODS: 10 cases were selected, covering sites in the brain, neck, pelvis, abdomen and shoulder with gross tumour volumes greater than 100 cm3. LRT plans were developed for a Varian ProBeam system, aiming to deliver 20 Gy to the lattice vertices while minimising dose to surrounding tissues. Plan-specific quality assurance (PSQA) was performed by independent Monte-Carlo calculation and physical dosimetric verification. End-to-end tests were performed using a CIRS head phantom containing EBT4 film. Plan quality was evaluated against VMAT LRT. RESULTS: Clinically acceptable proton LRT plans were produced, delivering 16-20 Gy to the lattice vertices, a mean PTV dose of 2 Gy and satisfying organ-at-risk dose constraints. PSQA and end-to-end tests met clinical tolerances for non-LRT plans. For large target volumes, proton LRT produced lattices with a greater number of vertices than VMAT LRT. CONCLUSIONS: Proton LRT is technically viable using currently available delivery systems. Large tumours such as sarcoma are the most likely candidates for future clinical studies, but proton LRT is feasible for target volumes as small as 100 cm3. ADVANCES IN KNOWLEDGE: The study demonstrates the feasibility of proton LRT for early phase clinical trials.