SU-E-J-74: Dosimetric Advantages of Adaptive Radiotherapy for Head and Neck Cancer Are Confirmed with Weekly CBCT Images

Purpose: Our previous study showed that weekly dose monitoring using cone-beam CT (CBCT) images can guide the timing and need for adaptive re-planning during the treatment of head and neck (HN) cancer. Here we aim to confirm the dosimetric improvement of adaptive radiotherapy (ART) using weekly CBCTs. Methods: We randomly selected seven HN patients treated with ART due to noticeable anatomic changes. Twenty weekly images acquired during the second treatment course were included. These CBCTs were aligned with both the initial and re-planning simulation CTs according to the clinical shifts. Daily doses were re-calculated for both the initial and adaptive plans. Contours of the tumor and organs-at-risk (OARs) were manually delineated by a physician on the re-planning CT and then were transferred to the CBCTs for plan evaluation. Contour modifications were made based on the daily anatomic changes observed on CBCTs. All patients were treated with 70Gy to the primary tumor and 56Gy to the elective lymph nodes. Results: Volumetric changes of the tumor (range — 43.9%∼+15.9%) were observed. The average D99 to the primary tumor was (70.1±2.0)Gy (range 62.2∼72.5Gy) for the adaptive plan and (66.0±5.5)Gy (range 50.9∼70.7Gy) for the initial plan(p<