LAUSR

Prompt gamma (PG) imaging is investigated for spot-by-spot in-vivo range verification for proton therapy. Previous studies pointed out that the accuracy of PG is affected by the statistics of the beams and the conformity between PG and dose distribution (PG-dose correlation). Recently an approach to re-optimize treatment plans (TPs) by boosting selected pencil beams with good PG-dose correlation above the statistics limit was proposed. However, only PG-dose correlation on the planning computed tomography (CT) was considered regardless that the robustness of the PG-dose correlation is not guaranteed in the future. In this work, this approach is further explored in the case of interfractional anatomical changes. A Matlab-based Monte Carlo (MC) treatment planning system CERR is used. Geant4 is used to simulate the dose delivery and PG generation for all pencil beams in the patient anatomy using multiple CT images for representative patient cases. First, a MC TP is created using CERR. Thereby the PG emission and dose distribution for each spot is obtained. Second, PG-dose correlation is quantified using the originally proposed approach as well as a new indicator, which accounts for the sensitivity of individual spots to heterogeneities in the 3D dose distribution. This is accomplished by using a 2D distal surface (dose surcace) derived from the 3D dose distribution for each spot. A few pencil beams are selected, based on their PG-dose correlation and dose surface, and then boosted in the new re-optimized TP. All TPs are then re-calculated with MC on the CT scans of the corresponding patient at three different time points. The result shows that all TPs are comparable in terms of dose and dose averaged LET distributions. The spots recommended maintain good PG-dose correlation on the future CTs, thus ensuring reliable range verification. Comparison to another proposed spots aggregation approach is discussed.