Purpose: A quality assurance (QA) program for pencil beam scanning (PBS) proton treatment planning system (TPS) was developed and implemented along with monthly CT QA.
Methods: A program consisted of a periodic test procedure that was designed for PBS TPS quality assurance (QA). The program includes verification of the CT calibration for proton dose calculation algorithm, CT images transferred to TPS, and software functions in TPS. The consistency of the dose calculation algorithm included dose comparison between standard SFO/MFO plan and forwardly re-calculated plans. The consistency of CT images of Catphan transferred to TPS was examined, i.e patient orientation, image geometry, tissue density, and contour dimensions. The aspects of software evaluated include proton layer energy, monitor unit, dose calculation, conversion between HU number to electron density, and proton Relative Stopping Power ratio (RSP).
Results: The maximum Energy layer for standard SOBP created on randomly CT images from one of our 3 CT simulators kept the same as 117.167MeV, overall the MU was 39993±23 MU, within the selected range 40000±800 MU, the geometry range is (9.62 ±0.1) cm, within selected range (9.65±0.2)cm, WED is (10.08±0.07), within preselected (10.0±0.2)cm. The maximum dose of the SOBP plan is (96.8±0.6) which is within (97±2)%, and the mean dose is (95.9±0.2)%, within (96±1)%.
Conclusion: Based on TG53, TG185, and TG224, we are the first to develop the TPS QA program for our PBS proton planning. The program is efficient and effective to check the integrity of the TPS. Test results for the dose calculation algorithm are satisfactory.The implementation of the PBS TPS QA programs verified the consistency of TPS performance which can be traced back to the commission and verification of initial TPS. Periodic testing would have to be followed to ensure the TPS consistently meets the desired treatment quality.