ESTRO 2020 Abstract Book

S400 ESTRO 2020

and MP by safely delegating all routine decisions to radiotherapy technologists (RTTs). Material and Methods On the Unity we distinguish 3 levels of adaptation (see figure). ‘Adapt to position’ (ATP) is a virtual couch shift. With ‘adapt to rotation’ (ATR), rigid target rotations and OAR deformations are corrected by the RTTs. ‘Adapt to shape’ (ATS) involves also re- contouring of the target by a RO. Prostate cancer patients with a prescribed dose of either 20x3Gy or 5x7.25Gy to the prostate and seminal vesicles were treated on the Unity. ATP was used as standard. ATR was performed for CTV rotations >15 ⁰ (given 5mm PTV margins). ATS could be considered when after rotation, the prostate and seminal vesicles didn’t fit in the PTV. Plan acceptance and QA were performed using a traffic light protocol based on pre-defined criteria for the differences between the adapted and the reference plan. In case of a traffic light violation a MP had to be consulted. To assess the impact of this workflow on clinical practice, we determined the number of fractions receiving ATP, ATR or ATS in 63 fractions in 30 consecutive patients. Within a larger cohort of 142 fractions in 41 consecutive patients, we summarize the number of traffic light violations and from the Unity logfiles we determined the time difference between the workflows.

Conclusion With the developed phantom and organ model, an organ shift induced by the diaphragm could be detected. Furthermore, the displacement of the organ model is within the range of human capabilities. In addition, the displacement in the cranio-caudal direction should be further strengthened, this could be achieved by loosely attaching the organ model in cranio-caudal direction and by integrating a flexible element on the lower side of the phantom to allow a larger longitudinal movement.

Proffered Papers: Proffered papers 38: Current status of MR guided RT

Results 9/63 (14%) fractions had rotations requiring ATR, while ATS was never necessary. The mean total time for registration and adaptation was 1093 ± 343 seconds for ATP and 1293 ± 527 seconds (+18%) for ATR. Only in 3 (2%) cases with ATP workflow there was a red QA traffic light, due to a change in segment area, which in all cases was approved by a MP. Conclusion We present a workflow for prostate patients, that efficiently uses the possibilities of MR-guided online adaptation. In 84% of all fractions all decisions were made by the RTTs. Optimizing efficiency by balancing ATP, ATR and ATS workflows remains necessary when MR guidance is pushed further to achieve margin reductions or tumor dose escalation. OC-0710 Initial clinical experience with the MR-Linac System – Treatment workflow management J. Day 1 , A. Moreira 1 , C. Weber 1 , N. Weitkamp 1 , L. Wilke 1 , M. Guckenberger 1 , H. Garcia Schüler 1 , S. Tanadini-Lang 1 , N. Andratschke 1 1 UniversitätsSpital Zürich, Radiation Therapy, Zürich, Switzerland

OC-0709 How to be efficient with online adaptation on a MR-linac T. Janssen 1 , A. Betgen 1 , L. Wiersema 1 , R. Verhage 1 , E. Van der Bijl 1 , F. Pos 1 , M. Donker 1 , P. Remeijer 1 , M. Nowee 1 , U.A. Van der Heide 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Online plan adaptation is slowly becoming clinical practice on dedicated hardware. In a typical workflow, a radiation oncologist (RO) is present to re-contour the target and a new plan will be optimized and delivered. Quality Assurance (QA) needs to be performed online, requiring the presence of a medical physicist (MP). Therefore, online adaptation is a burden on efficiency, logistics and cost. However, full online adaptation may not be necessary for each fraction, allowing for a simpler treatment with no RO or MP required. Thus for online adaptation to become efficient, workflows need to be optimized that meet the clinical needs. As an example, we propose a workflow for prostate cancer patients treated on the Elekta Unity MR-linac where we exploit the advantages of online MR-based adaptation, while minimizing treatment time and the presence of RO