ESTRO 36 Abstract Book

S889 ESTRO 36 2017 _______________________________________________________________________________________________

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.

EP-1660 Improvement in tumour control probability by adapting dose to daily OAR DVCs D. Foley 1 , B. McClean 1 , P. McBride 1 1 St Luke's Reseach Oncology Network, Physics, Dublin, Ireland Purpose or Objective A technique using analysis of on-board CBCT images to adapt the dose to the target on a fraction-by-fraction basis was developed. This new approach involves using the upper limit of dose volume constraints (DVCs) as the objective to be met at each fraction by tracking and accumulating dose voxels. The aim was to adapt the dose per fraction such that it was optimised each day without any organ at risk (OAR) DVCs being exceeded. The impact on tumour control probability (TCP) and normal tissue complication probability (NTCP) was evaluated. Material and Methods 31 patients who underwent prostate treatment were retrospectively investigated for this study. Initial VMAT plans consisting of 2 arcs were designed to deliver 74 Gy in 37 fractions of 2 Gy each to the target. The patients had on-board CBCT scans taken prior to treatment for between 9 and 33 fractions (436 in total). An in-house registration algorithm based on phase correlation[1] was used to retrospectively register CBCT images to the planning CT to determine the transformations and deformations in patients’ anatomy. This allowed the original plan to be recalculated on the registered CT image that provided the position of the target and OARs for that fraction. By tracking individual voxels throughout treatment, the dose was accumulated and the DVHs and DVC values were determined for each fraction.