ESTRO 2020 Abstract book

S441 ESTRO 2020

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).

Conclusion No severe acute GI or GU symptoms occurred during or four weeks after MR-RT for prostate cancer. Urinary frequency was the most severe GU symptom at the last treatment fraction. At the 4-week follow-up the severity of GU symptoms were considerably reduced.