Purpose: Inspired by the beam-matching concept in medical linear accelerators, the feasibility of delivering exchanged pencil beam scanning treatment plans without replanning and recalculations in gantries of IBA ProteusPLUS proton system through analyzing the commissioning data and quality assurance (QA) results, is studied in this work.
Methods: Proton beam characteristics including integral depth dose (IDD) and spot profiles for 33 energy levels (70-226.09MeV) of two gantries (GTR2/GTR4 in our hospital) were measured and established beam models. 36 cases of nasopharyngeal-carcinoma, lung cancer and prostate cancer were retrospectively selected to generate patient-specific QA plans based on the beam model of gantries. Each QA plan was respectively delivered in both gantries, meanwhile, planar dose measurements were done, and were compared and statistically analyzed.
Results: Through comparison between two gantries’ models, their IDD were analyzed. It is found that the spot profiles at each energy level had slight differences. Independent of delivering the same QA plans in GTR2 or GTR4, the average passing rates of measured doses had not changed much, reaching a high level of 95% when using a 3mm/3% criterion. While the paired sample t-test shows that these differences were not statistically significant, the Pearson correlation analysis proved a certain correlation between the passing rates of two gantries. Few abnormal cases could be seen only when the stringent criterion of 2mm/3% was used, but when using the less rigorous 3mm/3% criterion which was widely used in clinical practice, there was no exception.
Conclusion: The beam dosimetric characteristics in different rooms showed high consistency and the tolerance was kept within a reasonable range while the difference of the same treatment plan delivered on two gantries met the requirement of clinical treatment. It was recognized that the same treatment plan could be delivered directly in different treatment rooms without affecting the treatment efficacy.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by the grant from the National Key R&D Program of China (No.2017YFC0112105). The authors have no competing interest to report.