ESTRO 36 Abstract Book

S187 ESTRO 36 2017 _______________________________________________________________________________________________

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

Results Photon plans were highly robust against interfractional anatomical changes. The difference between planned and accumulated DVH parameters for the photon plans was ≤0.5% for the target and OARs. For proton therapy, coverage of the iCTV was considerably reduced for the accumulated compared to the planned dose: the mean near-minimum dose (D98%) of the iCTV reduced from 98.1% to 90.3% [79.4%–95.3%](Figure). For carbon ion therapy it was even worse; D98% was reduced with 10%, from 98.6% to 88.6% [80.7%–92.7%]. The DVH parameters of the OARs differed ≤3% between both particle modalities. For all modalities the near-maximum dose (D2%) did not differ. Conclusion Photon therapy is highly robust against interfractional anatomical changes and setup errors in pancreatic cancer patients. However, in particle therapy with either protons or carbon ions, severe reductions in target dose coverage were observed. Implementation of particle therapy for pancreaticcancer patients should be done with great care and interfractional anatomical changes must be accounted for. OC-0355 Which anatomical changes in Head&Neck cancer lead to Repeat CT/planning? S. Van Beek 1 , O. Hamming-Vrieze 1 , A. Al Mamgani 1 , A. Navran 1 , J. Van de Kamer 1 , P. Remeijer 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands

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