SU‐FF‐T‐373: Quality Assurance of Individual Segments of Step‐And‐Shoot IMRT

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Purpose: Absolute point dose measurements as part of the quality assurance of step‐and‐shoot intensity modulated radiation therapy (IMRT) often reveales unexpected deviations. This study investigates the feasibility of IMRT‐QA of individual segments to explain these deviations, and the efficacy of different evaluation methods. Method and Materials: The dose in a small volume around the point of interest (POI) was calculated for the individual segments of an IMRT plan for which a software script was added to our treatment planning system (Pinnacle3 7.6c, Philips). Furthermore, two input parameters for analysis of the measurements, i.e. the dose per monitor unit (D/MU) and the maximum dose gradient (δD/δr), were calculated for each segment. A software application was developed for automated data acquistion and analysis. The analysis was carried out either disregarding measurement data with a low confidence level (δD/δr or D/MU outside a critical range), or using γ‐evaluation. Results: IMRT‐QA was performed for 10 prostate and 5 head and neck patients. We found significantly higher deviations for segments at which the detector turned out to be located within the penumbra, which was consistent with 0.15 < D/MU < 0.55 cGy/MU, or δD/δr > 0.25 cGy/mm. When these data were filtered out, the total dose deviation ranged from −2% to 3% (not filtered: ‐ 5% to 5%). However, only 78% of the fraction dose was verified on average due to filtering. When γ‐evaluation was used, 93% of the fraction dose was within acceptance criteria (2.5% Dsegm; 2.5mm) on average. Datapoints outside these acceptance criteria were predominantly related to measurements behind jaws or leafs (D/MU < 0.15). Conclusion: IMRT‐QA of individual segments revealed that penumbral ionometric uncertainty is the main cause of deviations in overall IMRT‐QA. This method is feasible in daily clinical routine and provides more insight in deviations between measurements and calculations.