SU‐E‐T‐417: Experimental Verification of Robustness Optimized Intensity Modulated Proton Therapy Plans for Craniospinal Irradiation

Abstract

Purpose:The robustness‐optimized intensity modulated proton therapy (RO‐IMPT) plans takes into account the setup errors to eliminate the hot/cold spots caused by setup errors at the junction for craniospinal (CSI) irradiation fields. The RO‐IMPT requires a large area for the junction of two fields. This work is to verify the treatment planning system calculated dose with introduction of setup errors by two‐dimensional (2D) dose measurement. Two fields of total lengths of 45 cm with junctions of 21 cm were measured.Methods:2D dose distributions in the coronal plane were measured by a commercial 2D ion‐chamber‐array detector in a cubic solid water phantom. A total spine field consisting of an upper and a lower spine field was measured at the depths of 5.4 and 6.9 cm. The active area for the measurement was 24×24 cm22. The junction and the area outside the junction of the spine fields were measured separately to complete the verification for the total field. Six different amounts of set‐up errors along the superior‐inferior direction up to 1.4 cm were intentionally applied. The measured dose with and without set‐up errors were compared with the calculated dose with and without set‐up errors using Gamma‐index analysis. 3%/3mm and 2%/2mm dose/distance agreement criteria were used. The point dose at the center of the junction was also compared.Results:For all fields with various introduced setup errors, 100 % of pixels passed the 3%/3mm criteria and on average 96% of pixels passed 2%/2mm criteria. Differences in the measured and calculated doses were within 1.1%.Conclusion:The efficacy of the robustness for the CSI RO‐IMPT plans were verified by measurement. The measurement method can be used for patient treatment field specific QA for CSI RO‐IMPT plans in our institution in the near future.