Clinical Commissioning of a Compact Proton Therapy System

Abstract

The purpose of this thesis is to present a commissioning work for a compact pencil beam scanning (PBS) proton therapy system with a gantry-mounted accelerator and to establish a clinically accurate beam model in the treatment planning system. Due to its unique mechanical and beamline configurations, the clinical commissioning process shows some distinct dosimetric and geometric characteristics.Commissioning measurements were performed to characterize depth-dose behavior, lateral beam optics, and absolute output. Integral depth dose (IDD) curves were acquired in a motorized water tank for 19 nominal proton energies spanning 30–227 MeV using two models of Bragg peak chambers (PTW 34070 and PTW 34089) to assess measurement consistency and detector dependence. IDD data were collected using both trigger-gated and reference-normalized acquisition strategies to account output fluctuations. Lateral spot characteristics were measured at the highest energy (227 MeV) using a two-dimensional amorphous silicon detector (IBA Phoenix) and radiochromic film (EBT4). Gaussian fitting of in-plane profiles provided spot-size parameters in orthogonal directions, and a virtual-source model was applied to quantify effective beam divergence through estimation of the virtual source-to-axis distance (VSAD). Absolute dose calibration was established by following the IAEA TRS-398 protocol. The complete commissioning dataset was incorporated into the treatment planning system to configure the proton beam model. The model was evaluated through comparisons between measured and calculated dose distributions in both depth and lateral directions, including validation under heterogeneous conditions using gamma analysis. The measured IDD and spot-profile datasets demonstrated stable and physically consistent energy dependence across the clinical range, and results from the two Bragg peak chambers showed close agreement, supporting the reliability of the acquired commissioning data. VSAD estimates provided meaningful geometric information for beam optics modeling. Absolute output calibration exhibited good reproducibility, and the calibrated output agreed with TPS-calculated values within 1.0%. Overall model validation showed excellent agreement between measurement and calculation, with gamma passing rates exceeding 95% under 3%/3 mm criteria for heterogeneous test scenarios, while homogeneous depth and lateral profiles were closely matched. Collectively, these results confirm that the compact proton therapy system can be commissioned with high dosimetric accuracy and that the resulting beam model is robust and suitable for clinical application. This work provides a practical and reproducible commissioning framework and establishes essential baseline data to support safe clinical implementation and future optimization of PBS proton therapy delivery.