Gamma passing rates of daily EPID transit images correlate to PTV coverage for breast cancer IMRT treatment plans

Breast; Transit dosimetry Pit; Dosimetria de trànsit Mama; Dosimetría de tránsito Purpose The use of the transit image obtained with the electronic portal-imaging device (EPID) is becoming an extended method to perform in-vivo dosimetry. The transit images acquired during each fraction can be compared with a predicted image, if available, or with a baseline image, usually the obtained in the first fraction. This work aims to study the dosimetric impact of the failing fractions and to evaluate the appropriateness of using a baseline image in breast plans. Material and methods Twenty breast patients treated in a Halcyon were retrospectively selected. For each patient and fraction, the treatment plan was calculated over the daily CBCT image. For each fraction, the differences respect to the treatment plan values of OARs and PTV dosimetric parameters were analyzed: ΔDmean, ΔD95%, ΔD98%, ΔD2%, ΔV36Gy, ΔV38.5Gy, and ΔV43.5Gy. Daily fractions were ranked according to the differences found in the dosimetric parameters between the treatment plan and the daily CBCT to establish the best fraction. The daily transit images acquired in every fraction were compared to the first fraction using the global gamma index with the Portal Dosimetry tool. The comparison was repeated using the best fraction image as a baseline. We assessed the correlation of the dosimetric differences obtained from the CBCT images-based treatment plans with the gamma index passing rates obtained using first fraction and best fraction as baseline. Results Average values of -11.6% [-21.4%, -3.3%] and -3.2% [-1.0%, -10.3%] for the ∆PTVD98% and ∆PTVD95% per every 10% decrease in the passing rate were found, respectively. When using the best fraction as baseline patients were detected with failing fractions that were not detected with the first fraction as baseline. Conclusion The gamma passing rates of daily transit images correlate with the coverage loss parameters in breast IMRT plans. Using first fraction image as baseline can lead to the non-detectability of failing fractions.