ESTRO 2020 Abstract book

S440 ESTRO 2020

N. Andratschke 1 1 UniversitätsSpital Zürich, Radiation Therapy, Zürich, Switzerland Purpose or Objective With the MRIdian system (Viewray) it is now possible to image radiotherapy patients using MRI technology not only for initial positioning but also during beam delivery. Additionally, it is possible to adapt the treatment plan to daily variations in patient anatomy online, although this process prolongs the time in treatment position. This study aimed at evaluating the time efficiency of a fully adaptive workflow. Material and Methods 50 patients with various fractionations were treated with the MRIdian in the past 6 months. Daily procedure includes initial patient set up, low-resolution followed by high- resolution MR imaging, re-contouring, plan re- optimization, QA procedure, re-imaging, a check of gating feasibility if beam delivery is to be gated and finally beam delivery. Patients were scheduled for a 90 minutes appointment for their first fraction and 60 minutes subsequently. The time required for each step of the treatment process was recorded and analyzed. Beam time was defined as time from first beam on to end of treatment delivery. Total treatment time was defined as the time from setup until the time when the patient was taken of the couch. Results Median total treatment time after 6 month of machine use, was 1 hour and 5 minutes with a range from 38 minutes to 1 hour and 44 minutes. Beam time requires the most time in the process with a median of 11 minutes (6-44), treatment time is dependent on the dose, gating use and the number of segments. Median beam time was 10 minutes for the pelvis and 14 for the lung, as lung patients are treated in inspiration breath hold whereas pelvis patients are treated in free breathing. Re-contouring takes a median time of 10 minutes (2-37). Re-contouring time is heavily dependent on treatment site (6 minutes in lung, 12 in abdomen). Lung patients have less organs at risk in need of contouring and benefit more from the MRI tracking provided by the system. After 6 months, median time was 6 (3-20) minutes for in room patient set up, 7 (2-37) for total image registration (acquisition and match time for both low and high-resolution images), 7 (1-30) for plan re- optimization and 4 (1-5) and 9 (2-30), for QA and gating feasibility check respectively. After implementing a faster QA process we could reduce the time required for this step by 25%. Furthermore, in the past month imaging time has been further decreased to 5 minutes, as staff-members are getting more comfortable with registration. Conclusion With our current workflow, we have a median treatment time of 65 minutes. We could change some of our processes to increase speed, such as decreasing our imaging and registration time by choosing faster sequences or making faster matching a priority. We could have the same physician that originally contoured to perform the adaptation or compromise the level of contouring accuracy required for the adaptations. To reduce our median beam time we would need to limit the number of MLC segments per plan.

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 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 >15⁰ (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.

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 ,