MO-F-213AB-04: Biological Effect of Dose Shadowing by Fiducial Markers in Spot Scanning Proton Therapy with a Limited Number of Fields

Medical Physics
Taeko MatsuuraHiroki Shirato

Abstract

In spot scanning proton therapy, accurate patient positioning before and during treatment is essential. A small gold ball marker is suitable as a fiducial for prostate treatment. However, it has been pointed out that the marker causes dose shadowing because the protons are scattered with their energy quickly diminished. In this research we explore the possibility that the biological effect of dose shadowing can be mitigated with a limited number of fields. The proton dose distribution in prostate was simulated using Geant4. The simulations include the Hokkaido University spot scanning nozzle and a water phantom positioned isocentrically. The PTV was delineated at the center of the phantom and a gold ball of 2 mm in diameter was placed at the middle of the PTV. The plan was created by single-field optimization and each of the following beam arrangements was investigated; (1) single lateral field (2) two lateral fields (3) two lateral + one anterior fields (4) four-field box. The dose prescription was D95 = 74 GyE (37 fr). The minimum dose and tumor control probability (TCP) were compared for the four beam arrangements. For (1)-(4), the minimum dose values were 55%, 77%, 78%, and 84% of the prescribed dose, respectively. The redu...Continue Reading

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