The Impact of Dose Calibration Point Differences Between the Treatment Plan and Patient-specific Quality Assurance in Passive Scattering Proton Beam Therapy

Yutaro Mori; Tomonori Isobe; Hideyuki Takei; Satoshi Kamizawa; Tetsuya Tomita; Daisuke Kobayashi; Hideyuki Sakurai; Takeji Sakae

doi:10.21873/invivo.13718

The Impact of Dose Calibration Point Differences Between the Treatment Plan and Patient-specific Quality Assurance in Passive Scattering Proton Beam Therapy

In Vivo. 2024 Sep-Oct;38(5):2478-2483. doi: 10.21873/invivo.13718.

Authors

Yutaro Mori^{1

2}, Tomonori Isobe^{3

2}, Hideyuki Takei⁴, Satoshi Kamizawa², Tetsuya Tomita⁵, Daisuke Kobayashi⁵, Hideyuki Sakurai^{1

2}, Takeji Sakae^{1

2}

Affiliations

¹ Institute of Medicine, University of Tsukuba, Tsukuba, Japan.
² Proton Medical Research Center, University of Tsukuba Hospital, Tsukuba, Japan.
³ Institute of Medicine, University of Tsukuba, Tsukuba, Japan; tiso@md.tsukuba.ac.jp.
⁴ Quantum Life and Medical Science Directorate, National Institute for Quantum Science and Technology, Inage, Japan.
⁵ Department of Radiology, University of Tsukuba Hospital, Tsukuba, Japan.

Abstract

Background/aim: Passive scattering proton beam therapy (PSPT) is performed by taking actual measurements of all pre-designated fields in a treatment plan followed by appropriate adjustments to the prescribed dose. For this reason, it is necessary to ensure precision management of the measurements (patient-specific calibration) in the administration of a planned dose. Therefore, this study investigated the impact on dose distribution in treatment planning when the patient calibration point differs from the normalized point in a treatment plan.

Patients and methods: A total of 16 cases were selected, where the patient calibration point and normalized point did not match, and the normalized point used in the treatment plan was changed to the patient calibration point using a treatment planning system (VQA ver. 2.01, HITACHI). At this point, the displacement of the relative dose at the isocenter was estimated as an error owing to the difference compared to the patient calibration point.

Results: Overall, the error was within the range of ±1.5%, with the exception of orbit cases. Calibrated points also tended to be lower than the normalized points in the treatment plan. In terms of treatment sites, a greater deviation was observed for head cases. Cases with a large deviation in sites other than the head were attributed to poor flatness within the radiation field owing to a narrower opening of the patient collimator.

Conclusion: Dose measurement errors in PSPT due to differing calibration points were generally within ±1.5%, with higher deviations observed in head treatments because of complex structures and narrow collimator openings. A γ analysis for significant deviations showed a 98.7% passing rate, suggesting limited overall impact. It is important to select stable calibration points in dosimetry to ensure high precision in dose administration, particularly in complex treatment areas.

Keywords: Proton beam therapy; dosimetry; passive scattering; patient-specific quality assurance.

MeSH terms

Calibration
Humans
Neoplasms / radiotherapy
Proton Therapy* / methods
Quality Assurance, Health Care*
Radiometry / methods
Radiotherapy Dosage*
Radiotherapy Planning, Computer-Assisted* / methods