ESTRO 2020 Abstract Book

S401 ESTRO 2020

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. OC-0711 Acute toxicity of MR-guided radiotherapy for prostate cancer at the 1.5 T MR-linac P.K. Møller 1 , J. Gornitzka 1 , U. Bernchou 2,3 , L. Dysager 1 , A.S. Bertelsen 2 , C.J. Nyborg 1 , T. Schytte 1,3 1 Odense University Hospital, Department of Oncology, Odense, Denmark ; 2 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark ; 3 University of Southern Denmark, Institute of Clinical Research, Odense, Denmark

Purpose or Objective MR-guided radiotherapy (MR-RT) on the MR-linac (MRL) is a new technology using real-time MR images for daily RT plan adaption. To evaluate the incidence and severity of acute toxicity of MR-RT for prostate cancer in the MRL, clinician-reported as well as patient-reported outcomes are collected as part of a prospective clinical study. The current study presents the clinician-reported treatment toxicity over time until the 4-week follow-up. Material and Methods Patients treated with 60 Gy/20 fractions for prostate cancer at the MRL and having a minimum of 4 weeks follow-up were included. Clinicians reported toxicity (CTCAE v.5.0) at baseline, week 2, end of treatment (EOT) and during follow-up week 2, 4 and 8. The patients were included in two different protocols; the PRISM protocol (n=9) for patients having localized prostate cancer (intermediate risk) and a protocol for patients newly diagnosed with low metastatic burden not eligible for the PRISM protocol treated ad modum PRISM (n=11). These patients were treated to 60 Gy in the prostate and the proximal part of the seminal vesicles (SV) at the clinician’s discretion. PRISM patients were treated to 60 Gy in the prostate and proximal 1 cm of the SV and 48 Gy to an additional 1 cm of the SV. The PTV covering the 60 Gy dose level was generated using a 5 mm margin, except 3 mm posteriorly. For the PRISM patients an isotropic PTV margin of 5 mm was used for the 48 Gy dose level. Results In total, 20 patients were included in this analysis. The mean age was a little lower among PRISM patients (66.7) compared to the ad modum PRISM group (68.5), however, both groups having around 90% with a WHO performance status 0 (89-91%). The proportion of androgen deprivation therapy (ADT) was lower in the PRISM group with only 44% compared to 82% of the patients in the ad modum PRISM group. No grade 3 gastrointestinal toxicities (GI) were reported. During the 12 weeks, no other grade 2 GI toxicities besides diarrhea (20%) occurred, however this was only experienced by one patient 2 weeks following treatment. No grade 3 genitourinary toxicities (GU) were reported. Acute grade 2 GU toxicity included urinary frequency (25%), urinary retention (5%), urinary urge (5%) and bladder spasm (5%) peaking at the last treatment (Figure 1). Two patients had a urinary infection treated with antibiotics 2 weeks following treatment. Dysuria was added to the CTCAE reporting as three patients (15%) experienced grade 2 dysuria at the day of their last treatment fraction and at the 2-week follow-up. Four weeks following treatment around 40% of the patients still had > grade 1 fatigue compared to 20% at baseline (Table 1).