TU‐D‐M100F‐05: Pre‐Treatment Verification of Large‐Field IMRT Dose Painting Plans for Head and Neck Cancer Using a Commercial QA Device

Objective: To develop a practical procedure for routine pre‐treatment verification of large dose painting fields using a commercial IMRT QA device, MapCheck. Materials and Methods: Optimized dose painting intensity maps were applied to a 40×40×40 cm 3 digital phantom. With a voxel size of 1 mm 3 , the dose distributions were re‐calculated for a coronal plane at a depth of 5 cm. The DMLC files were then transferred to a Varian Clinac for plan verification. We first delivered a beam with MapCheck at isocenter. The isocenter was then shifted superiorly and inferiorly by 4 cm, respectively. Two more exposures were made. The isocenter was shifted to the left and right by 4 cm, respectively. Two more exposures were made. To detect accurately the steep dose gradient regions, we doubled the effective detector surface density by shifting the isocenter by 5 mm along the X and Y axes. Two more exposures were made. Finally, we merged all these files to yield a desirable dose distribution. We used three different criteria for data analysis: percent difference, distance to agreement (DTA), and γ index. We set the percent difference threshold to 3% and the DTA to 3 mm. Results: Using these criteria, 99% ∼ 99.5% dose points passed the test for most of our IMRT dose painting plans. For a ten‐field plan, it took about 35 minutes to complete the data acquisition and post‐analysis. Conclusions: The proposed procedure not only improves the efficiency, but also enhances the accuracy of measured dose distribution in the steep dose gradient regions. MapCheck is a handy, fast, and practical tool for routine pretreatment verification of large‐field dose painting plans.