Article ; Online: The effect of spill change on reliable absolute dosimetry in a synchrotron proton spot scanning system.
2023 Volume 50, Issue 7, Page(s) 4067–4078
Abstract: Background: Absolute dosimetry measurement is an integral part of Treatment Planning System (TPS) commissioning and it involves measuring the integrated absorbed dose to water for all energies in a pencil beam scanning delivery system. During the ... ...
| Abstract | Background: Absolute dosimetry measurement is an integral part of Treatment Planning System (TPS) commissioning and it involves measuring the integrated absorbed dose to water for all energies in a pencil beam scanning delivery system. During the commissioning of Singapore's first proton therapy center, a uniform scanned field with an Advanced Markus chamber method was employed for this measurement, and a large dose fluctuation of at least 5% was observed for 10% of the energy layers during repeated measurements. Purpose: This study aims to understand the root cause of this fluctuation by relating the actual delivered spot information in the log file with the charge measurement by the ion chambers. Methods: A dedicated pencil beam dose algorithm was developed, taking into account the log file parameters, to calculate the dose for a single energy layer in a homogeneous water phantom. Three energies, 70.2, 182.7, and 228.7 MeV were used in this study, with the 182.7 MeV energy exhibiting large dose fluctuation. The dose fluctuation was investigated as a function of detector's sizes (pinpoint 3D, Advanced Markus, PTW 34070, and PTW 34089) and water depth (2 , 6, and 20 cm). Twelve ion chambers measurements were performed for each chamber and depth. The comparison of the theoretically predicted integrated dose and the charge measurement served as a validation of the algorithm. Results: About 5.9% and 9.6% dose fluctuation were observed in Advanced Markus and pinpoint 3D measurements at 2 cm depth for 182.7 MeV, while fluctuation of 1.6% and 1.1% were observed in Advanced Markus with 228.7 and 70.2 MeV at similar depth. Fluctuation of less than 0.1% was observed for PTW34070 and PTW 34089 for all energies. The fluctuation was found to diminish with larger spot size at 20 cm depth to 1.3% for 182.7 MeV. The theoretical and measured charge comparison showed a high linear correlation of Conclusion: This is the first report of a dose fluctuation greater than 5% in absolute dosimetry measurement with a uniform scanned field and the cause of the fluctuation has been conclusively determined. It is important to choose the MU and scanning pattern carefully to avoid spill change happening when the spot delivery is near the detector. |
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| MeSH term(s) | Protons ; Synchrotrons ; Radiometry/methods ; Proton Therapy/methods ; Water ; Radiotherapy Dosage ; Radiotherapy Planning, Computer-Assisted/methods |
| Chemical Substances | Protons ; Water (059QF0KO0R) |
| Language | English |
| Publishing date | 2023-06-04 |
| Publishing country | United States |
| Document type | Journal Article |
| ZDB-ID | 188780-4 |
| ISSN | 2473-4209 ; 0094-2405 |
| ISSN (online) | 2473-4209 |
| ISSN | 0094-2405 |
| DOI | 10.1002/mp.16531 |
| Database | MEDical Literature Analysis and Retrieval System OnLINE |
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