SU‐GG‐T‐156: Patient Specific QA Results for Step and Shoot and Sliding Window Deliveries of IMRT Beams as Implemented by Two Treatment Planning Systems

Abstract

Purpose: To present the patient specific QA results for step and shoot (SS) and sliding window (SW) deliveries of IMRT beams as implemented by two treatment planning systems (TPS). Method and Materials: From a sample of our clinical practice 648 (108 cases) Eclipse AAA_7_5_‐18‐SW and 464 (66 cases) Pinnacle 7.4f ‐SS IMRT beams were evaluated using the MapCheck diode array system. The planar dose measurements at 5 or 10 cm depth in water were compared with the predictions from both TPS. A 3 % −3mm distance to agreement (DTA) criteria was used to score the percent of passing points (PPP) for each individual field. The samples for both TPS were composed of 75 % of Head and Neck cases, 15% prostates and 10 % other sites. Results: The distribution of the PPP showed that 83% and 85% of the evaluated beams were above the 95 % PPP for Eclipse and Pinnacle respectively. Between 90 and 95 % the PPP were 15% and 14% and below 90% the PPP were 1.9 % and 1.7 % for Eclipse and Pinnacle respectively. The passing rates for Eclipse‐SW are significantly improved when the first 20 Eclipse‐SW cases (130 beams) are not considered for this comparison. The Eclipse‐SW PPP above 95% increases to 91.1 %. The patterns of failure observed for the two TPS were distinctively different with Pinnacle‐SS exhibiting clusters of hot points (measurements higher than calculations) while Eclipse‐SW showed cold points aligned in the direction of the MLC leaves movement. Conclusion: The results from our patient specific IMRT QA using the Eclipse‐SW delivery system exhibited a better agreement when compared with our previous experience using Pinnacle‐SS. A detail analysis is required in order to assess the clinical significance of the different failure patterns observed between these two TPS.