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

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.

Adjuvant treatment of locally-advanced head and neck tumours.

Every year about 40,000 new patients are diagnosed as having squamous cell carcinoma of the head and neck (HNSCC) and nearly 60% of this population present with locally-advanced, but non-metastatic, disease. For many years, radiation therapy (RT) alone has been the standard non-surgical treatment for locally-advanced disease. Nevertheless, even the most effective RT regimens (the once-daily, hyperfractionation, or accelerated fractionation) show an improvement in local control rates of 50%-70% and disease-free survival rates of 30%-40%. These modest results have stimulated the search for novel strategies combining RT and chemotherapy. Several chemotherapeutic agents have been tested in combination with RT and cisplatin appears to be the most powerful agent in the treatment of locally-advanced head and neck tumours; as has been demonstrated in a large meta-analysis and, recently, in two multi-centred randomised trials. Based on these results, concurrent chemo-radiotherapy may be considered the standard adjuvant treatment for locally-advanced head and neck tumours.