ESTRO 36 Abstract Book

S901 ESTRO 36 _______________________________________________________________________________________________

calibrated, a CT to CBCT HU conversion table was created to obtain a reliable CBCT based dose calculation. To validate this approach, we evaluated 11 CT-CBCT registrations of the head with no visible deformations, and compared plan calculations on both scans. To assess the potential to monitor planned dose on the CBCT, 22 patients receiving postoperative head and neck irradiation with 2 or 3 dose levels were evaluated retrospectively for a total of 265 CBCT scans. 5 Patients received a new CT and a replanning during the treatment course. All dose distributions were evaluated on V95% of the PTV, mean dose on parotid glands, mandible, oral cavity, larynx, maximum dose on myelum, and low dose volume (<5Gy). Results Validation on 11 patients of the dose calculation showed an average deviation between planning CT and CBCT scans of less than 1% on all evaluated dose metrics (Figure 2a). Evaluation of 22 patients shows deviations of <5% in PTV coverage in 20 patients over the course of the treatment (Figure 2b). Two patients showed a higher deviation. Patient 14 showed anatomical variation that was not detected during treatment. Patient 18 had a relevant reduction in PTV coverage during treatment course due to weight loss and received a new plan. Four other patients received a replanning because of other considerations, e.g. a deteriorating condition or treatment side effects. In the evaluated OAR’s, variations in evaluated metrics of <5% were observed. Conclusion The automated evaluation tool in this study provides a reliable prediction of delivered dose for the daily patient anatomy. Evaluation of a series of fractions shows that it is can detect dose deviations and trigger plan adaptation, with an action level of approximately 5% deviation in V95%. Inclusion of deformable image registration is expected to further increase the reliability of the DVH predictions.

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