ESTRO 38 Abstract book

S330 ESTRO 38

registration of the images to the planning CT and correcting the setup with an automatic couch shift. In our department this entire procedure is performed by RTTs. Traffic light protocols guide fast decision making in case of anatomical deviations and prevent the necessity of a physician or physicist. With the clinical introduction of the MR-linac (Unity, Elekta AB), a linear accelerator with an integrated MRI scanner, workflow has changed. At the Unity MR-linac, set-up errors are always corrected by a plan adaptation based on the pre-beam MRI. Therefore, a new radiation plan is computed for each fraction, which has to be evaluated and approved online, which necessitates the presence of a physician and physicist. Our aim is to implement similar traffic light protocols on the MR-linac for decision making by RTTs only, which will alleviate the need for a physician and physicist to be present for all fractions. Material and Methods In the pre-clinical phase, MRI scans were acquired at the MR-linac of patients with different tumor sites after informed consent. Based on a maximum of 5 MRI scans per patient, an MR-linac workflow, which included plan adaptation, was simulated. For anatomical changes we used our existing anatomical traffic light protocol, in which it is indicated when a physicist or physician should be contacted offline. Furthermore, isocenter shifts in LR, AP and CC direction were simulated within Monaco (v5.19.03) to analyze the differences between the reference plan and the adapted plans and to define limits in which the differences would be clinically acceptable. This resulted in a dosimetric traffic light protocol to judge whether the adaptation is better (green), worse but acceptable (orange) or unacceptable (red). ‘Orange’ plans can be approved by the RTTs, where ‘red’ plans need to be judged by a physician and/or physicist (an example is presented in table 1). An independent check of the adapted plan was performed for Monitor Units (MU), MU- fluence, largest aperture and area weighted MUs. Results Currently, four prostate cancer patients and one rectum cancer patient started their treatment on the MR-linac, with 60 fractions given. For one patient, the anatomical traffic light protocol resulted in an off-line replanning, which is part of our routine adaptive procedure. In two fractions the dosimetric criteria for the PTV coverage were not met, which resulted in a red traffic light. In these two cases the physician and physicist decided to continue treatment. Limits were only exceeded by 7 and 14 cGy. In one case the independent MU check was not within the boundaries, but also accepted by the physicist.

performed to assess the impact on workload for both RTT’s and RO’s, the burden of the responsibility for the RTT’s and the involvement of the RO’s by comparing one for the standard TLP and one for the TAP.

Results In 16 out of 22 bladder and 18 out of 56 prostate patients anatomical changes were seen on CBCT, that required a decision of the RTT’s (see table 1). For the TAP, in only 2 bladder and 6 prostate patients further decision making of an RO was required (compared to all 34 for the TLP). 99% of the CBCT’s were reviewed accurately by the RTT’s. In 5 out of 34 patients additional instruction in the decision making by an IGRT specialist was required. The results of the surveys showed that for both the RTT’s and the RO’s, the TAP provided a better balance between workload and efficiency in relation to the clinical relevance of acting on anatomical changes. The perceived involvement of the RO’s and burden of the responsibility for the RTT’s was comparable between the two protocols. Also acting appropriately on clinical relevant anatomical changes, was found to be improved by applying the TAP. Conclusion The Take Action Protocol for anatomical changes provides an accurate method to act on anatomical changes, and gives RTT’s more responsibility in the decision making, with an improved balance in efficiency and workload in relation to the clinical relevance of adapting on these anatomical changes. OC-0621 Changing responsibilities for RTTs on the MR- linac A. Betgen 1 , J. Bilderbeek 1 , T. Janssen 1 , J. Kaas 1 , M. Nowee 1 , T. Vijlbrief 1 , L. Wiersema 1 , U. Van der Heide 1 1 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The current set-up of patients for irradiation on a conventional linac consists of positioning the patient, acquiring verification images with cone-beam CT, Table 1

Conclusion After a learning period and multidisciplinary discussions, the expectation is that RTTs at the MR-linac can perform online plan adaptation without the presence of a physician or physicist.

Poster Viewing : Poster viewing 12: GI and Urological Cancers

PV-0622 NCTP model for postoperative pulmonary complications after trimodality therapy in esophageal cancer