ESTRO 36 Abstract Book

S888 ESTRO 36 2017 _______________________________________________________________________________________________

verification of VMAT techniques of various treatment sites.

Electronic Poster: Physics track: Adaptive radiotherapy for inter-fraction motion management

EP-1658 The effect of weight loss in head and neck patients in the presence of a magnetic field R. Chuter 1 , P. Whitehurst 1 , M. Van Herk 2 , A. McWilliam 2 1 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering CMPE, Manchester, United Kingdom 2 University of Manchester, Manchester Academic Health Science Centre MAHSC, Manchester, United Kingdom Purpose or Objective Head and neck patients tend to experience weight loss during treatment in a predictable pattern loosing between 5-15% of their initial weight over the first two weeks. Adaptive radiotherapy for these patients focuses on an offline protocol where the patient is re-scanned and re- planned two-to-three weeks through treatment. The MR- Linac (Elekta, AB, Stockholm, Sweden) will provide excellent soft tissue contrast which may be desirable for this group of patients. However the electron return effect, caused by the Lorentz force may potentially result in an increased dose to superficial tissues, for example the parotid glands. This effect can be controlled in plan optimisation, however it is unknown whether the presence of a magnetic field makes it necessary to adapt the plan at an earlier stage or more frequently during treatment. The purpose of this abstract is to assess the suitability of the current off-line adaptive radiotherapy workflow for head and neck patients in the presence of a magnetic field. Material and Methods Ten patients treated with either 66Gy or 60Gy in 30 fractions, were selected from the clinical archive that had shown significant weight loss during treatment which required a repeat CT. Both the initial planning CT (pCT) and the repeat CT (rCT) were fully contoured by an oncologist specialising in head and neck cancer. Two plans were optimised, at 0T and 1.5T using Monaco v5.09 (Elekta AB Stockholm, Sweden) which met the departmental constraints for Target and OAR doses. These plans were copied to the rCT and re-calculated with a 1% statistical uncertainty, allowing the segmentation and delivered MU to remain constant. The magnitude of the change in dose to the target and OARs due to weight loss was compared between the 0T and 1.5T plans. The difference between the dose distribution on the pCT was compared to the distribution on the rCT and how this was affected by the presence of the magnetic field. Results The percentage difference between 0T and 1.5T plans for the targets showed statistical differences for the D95% for PTV1, PTV2 and PTV3, D50% and mean dose for PTV2, and mean dose and 2cc min for PTV3. The only statistical difference for the OARs was the 2cc max dose for skin which increased by 1.1% for 1.5T plans. However differences between the 0T and 1.5T plans were on average all within 2%, so were considered clinically acceptable.

Conclusion This work shows that the dosimetric effect of weight loss does not cause any clinically significant changes in the presence of a magnetic field, as the difference between pCT and rCT for 0T and 1.5T are similar. Therefore, current off-line strategies for adaptive planning for head and neck patients are valid for use on the MR-Linac. EP-1659 Quantitative triggering of plan adaptation: monitoring plan quality by recalculation on CBCT scans R. Canters 1 , M. Wendling 1 , M. Kusters 1 , R. Monshouwer 1 1 Radboud University Medical Center, Radiation oncology, Nijmegen, The Netherlands Purpose or Objective Since the introduction of 3D imaging on the linac, anatomical changes observed on CBCT scans regularly lead to plan adaptation. However, adaptation is often triggered by qualitatively assessing anatomical changes between CBCT and planning CT. This regularly leads to unnecessary replanning, disrupting the regular workflow in the clinic. In this study, we created an automated evaluation tool, that recalculates the treatment plan on recorded CBCT scans to indicate if a replanning may be necessary. The aim of this work is to assess its potential for regular clinical use. Material and Methods The recalculation tool imports planning CT a nd CBCT scan, after which the treatment plan is transferred to the CBCT scan. Subsequently, the plan is recalculated on the CBCT using Pinnacle, and DVH’s are compared (Figure 1). The CT-CBCT match is derived from the CBCT match at the linac. Since Hounsfield units (HU) of the CBCT are not calibrated, a CT to CBCT HU conversion table was created