ESTRO 36 Abstract Book

S188 ESTRO 36 2017 _______________________________________________________________________________________________

re-plans.

and the radiotherapy management system, as the two systems are not linked.

Results In total, 10 patients were treated with the long treatment schedule, and 10 with the short treatment schedule, resulting in 300 plan selections. Margin sets of 25 mm, 15 mm, 0 mm were created for 6 patients, and margin sets of 15 mm, 0 mm, -15 mm for 13 patients. One patient had a set of only two margins available (0 mm, 15 mm), due to insufficient time at treatment planning. Overall, the -15 mm, 0 mm, 15 mm and 25 mm plans were selected in 2%, 45%, 39% and 14% of fractions, respectively. For distributions per patient, see figure 2. The largest available margin was always sufficient. Treatment was delayed a total of 7 times (of which 5 times in 1 patient) to obtain a more favorable anatomy in case of a very full rectum, usually caused by gas pockets. Evaluation of the post-treatment CBCT scans showed for 1 fraction the selected plan was no longer suitable due to a moving gas pocket. The weekly review showed that a plan with a smaller margin could have been selected in 20% of fractions, and a larger margin in 2% of fractions. No inconsistencies were found in selected plans between the imaging system and radiotherapy management system.

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

Conclusion A plan selection strategy for rectum cancer patients was successfully and safely implemented. Next we will quantify the dosimetric impact of plan selection to the dose of the organs at risk in this dataset.

Proffered Papers: Physics Dosimetry