ESTRO 36 Abstract Book

S189 ESTRO 36 _______________________________________________________________________________________________

During a course of radiotherapy for head and neck (H&N) cancer, non-rigid anatomical changes can occur. For example, changes in volume of the target, changes in neck diameter (contour) due to edema or weight loss, shifts of hyoid or thyroid bone or other localized soft tissue deformations. These anatomical changes cannot be corrected for by a couch shift, but they can be observed on daily Cone Beam CT (CBCT) and are scored digitally by RTTs according to a traffic light protocol (TLP)(green: no action, orange: evaluation of dose consequences before the next fraction, red: immediate evaluation of dose consequences). Orange and red scores can lead to a new radiation plan, either on the original planning CT scans (O- pCT) with local adjustment of target volumes or on a new pCT scans (N-pCT) with complete re-delineation. In this work, we evaluated how often re-planning was done for non-rigid anatomical changes and which anatomical changes lead to which new plan actions during the 7 weeks of treatment. Material and Methods A consecutive series of H&N cancer patients (416) treated from January 2015 until September 2016 were retrospectively selected using the digital log of CBCT scans (10862 H&N logs). These digital logs were analysed for the number of new treatment plans on an O-pCT or a N-pCT. Reasons for re-planning were categorized into: target volume increase, target volume decrease, contour decrease, contour increase and shift of target volume. To evaluate the timing of re-planning, the week in which delivery of the new plan started was scored as well. Results In 9% (37/416) of the H&N patients included in this analyses, the treatment plan was adapted due to anatomical changes detected during radiation treatment on CBCT. Re-planning on a N-pCT with complete re- delineation was done 22 times. In fifteen cases a new plan was created after adjustment of contours on the O-pCT. For 4 patients, two actions were taken, first a new plan on the O-pCT and secondly (further in the treatment) a new plan on a N-pCT. Figure 1 shows the anatomical changes observed at the time of re-planning, as well as the time of occurrence during treatment. In the early weeks of treatment, the most observed reason for re- planning was a target volume increase, both on a N-pCT as well as on the O-pCT. In the last part of treatment, re- planning on a N-pCT was mainly done because of contour decrease, while re-planning on the O-pCT was chosen in the event of local shifts of target volume. The majority of adaptive treatment plans were made in the second, third and fourth week of treatment for relatively 10, 9 and 10

re-plans.

Conclusion Visual detection of anatomical changes on CBCT during treatment of head and neck cancer, without pre-defined adaptive radiotherapy protocol, results in re-planning in 1 out of 11 patients. OC-0356 Adaptive strategy for rectal cancer: evaluation of plan selection of the first 20 clinical patients R. De Jong 1 , N. Van Wieringen 1 , J. Visser 1 , J. Wiersma 1 , K. Crama 1 , D. Geijsen 1 , L. Lutkenhaus 1 , A. Bel 1 1 Academic Medical Center, Department of radiation oncology, Amsterdam, The Netherlands Purpose or Objective For rectal cancer, sparing the organs at risk with the use of state-of-the-art planning techniques (IMRT/VMAT) is compromised by the large margins that are necessary to compensate for daily shape changes. In our clinic we implemented a plan selection strategy with multiple plans made prior to treatment. For each fraction, the best fitting plan is selected based on daily cone beam CT (CBCT) scans. The aim of this study is to assess the plan selection strategy for the first 20 clinical patients with respect to available plans, selected plans and safety. Material and Methods Multiple plans for plan selection were created for each patient based on a single CT scan. For 20 patients, 3 PTVs were created with different anterior margins for the upper mesorectum. Margins could be either 25 mm, 15 mm, 0 mm, or -15 mm, with choice of margins based on the anatomy as captured on the CT scan (fig. 1). Patients were treated with either a long or short treatment schedule (25x2 Gy, and 5x5 Gy, respectively). All plans were delivered with VMAT. Plan selection was based on daily CBCT. Selection was performed by 1 trained radiotherapist (RTT), a physician and a physicist for all fractions of the first week, and from the second week onwards by 2 RTTs, one of whom trained in plan selection. Once a week a post-treatment CBCT scan was acquired to assess the validation of the selected plan at the end of treatment. An expert IGRT RTT performed a weekly review, inspecting all plan selections retrospectively, as well as consistency between selected plans in the imaging system