Room: Exhibit Hall | Forum 7
Purpose: Proton therapy is impacted by range uncertainty. CT calibration (CC) among other sources contributes to systematic errors in treatment delivery. A range probing method is proposed for evaluation of range errors and optimization of CC.
Methods: The stoichiometric method was used to establish the initial CC. Afterwards three types of animal tissue phantoms (a pigâ€™s head, â€œthoraxâ€? and femoral bone) were prepared, scanned and transferred to the TPS to calculate pencil beams directed through the phantoms. A water slab was appended to the scan to simulate the detector that was used for integral depth dose curve measurements. Tissue phantoms were positioned in the planned position at the treatment room isocenter. A set of shot-through pencil beams was delivered and depth dose profiles were measured using a multi-layer ionization chamber. Measured depth dose curves were compared to calculated ones and the range error per spot was defined. Based on the WEPL of every spot, the range error margin based on recipe of 2.4% + 1mm was defined. Ratios between measured rang errors and theoretical margins per spot were calculated. The CC optimization was performed by minimizing the ratios between range errors and uncertainty margins.
Results: When performing the analysis using the initial CC, increased range errors were observed for the femur. The slope of the CC segment representing bone-like tissues was adjusted to improve agreement between measurements and calculations. After adjustment for a full data set (1600 spots) the ratio of the range error and the uncertainty margin for 1.5Ïƒ did not exceed 0.75.The experimental error analysis shows that positioning variability was within 0.5mm.
Conclusion: The use of range probing is feasible for assess,ent of the range errors and the use of the optimized CC resulted in a 25% reduction of the published uncertainty margins.
Funding Support, Disclosures, and Conflict of Interest: University of Groningen - University Medical Centre Groningen, Department of Radiation Oncology has research collaborations with IBA, RaySearch, Siemens, VisionRT and Mirada