Purpose: A 1.5 T MR Linac (MRL) has recently become available. MRL treatment workflows (WF) include online plan adaptation based on daily MR images (MRI). This study reports initial clinical experiences after five months of use in terms of patient compliance, cases, WF timings, and dosimetric accuracy. Method and materials: Two different WF were used dependent on the clinical situation of the day; Adapt To Position WF (ATP) where the reference plan position is adjusted rigidly to match the position of the targets and the OARs, and Adapt To Shape WF (ATS), where a new plan is created to match the anatomy of the day, using deformable image registration. Both WFs included three 3D MRI scans for plan adaptation, verification before beam on, and validation during IMRT delivery. Patient compliance and WF timings were recorded. Accuracy in dose delivery was assessed using a cylindrical diode phantom. Results: 19 patients have completed their treatment receiving a total of 176 fractions. Cases vary from prostate treatments (60 Gy/20F) to SBRT treatments of lymph nodes (45 Gy/3F) and castration by ovarian irradiation (15 Gy/3F). The median session time (patient in to patient out) for 127 ATPs was 26[21-78] min, four fractions lasted more than 45 minutes due to additional plan adaptation. For the 49 ATSs a median time of 12[1-24] min was used for contouring resulting in a total median session time of 42[29-91] min. Three SBRT fractions lasted more than an hour. The time on the MRL couch was well tolerated by the patients. The median gamma pass rate (2mm,2% global max) for the adapted plans was 99.2[93.4-100]%, showing good agreement between planned and delivered dose. Conclusion: MRL treatments, including daily MRIs, plan adaptation and accurate dose delivery is possible within a clinically acceptable timeframe and is well tolerated by the patients.

Background and purpose: With daily, MR-guided online adapted radiotherapy (MRgART) it may be possible to reduce the PTV in pelvic RT. This study investigated the potential reduction in normal tissue complication probability (NTCP) of MRgART compared to standard radiotherapy for high-risk prostate cancer. Materials and methods: Twenty patients treated with 78 Gy to the prostate and 56 Gy to elective pelvic lymph nodes were included. VMAT plans were generated with standard clinical PTV margins. Additionally to the planning MR, patients had three MRI scans during treatment to simulate an MRgART. A reference plan with PTV margins determined for MRgART was created per patient and adapted to each of the following MRs. Adapted plans were warped to the planning MR for dose accumulation. The standard plan was rigidly registered to each adaptation MR before it was warped to the planning MR for dose accumulation. Dosimetric impact was compared by DVH analysis and potential clinical effects were assessed by NTCP modeling. Results: MRgART yielded statistically significant lower doses for the bladder wall, rectum and peritoneal cavity, compared to the standard RT, which translated into reduced median risks of urine incontinence (DNTCP 2.8%), urine voiding pain (DNTCP 2.8%) and acute gastrointestinal toxicity (DNTCP 17.4%). Mean population accumulated doses were as good or better for all investigated OAR when planned for MRgART as standard RT. Conclusion: Online adapted radiotherapy may reduce the dose to organs at risk in high-risk prostate cancer patients, due to reduced PTV margins. This potentially translates to significant reductions in the risks of acute and late adverse effects.

Background In this study we have evaluated the accuracy of automatic, deformable structure propagation from planning CT and MR scans for daily online plan adaptation for MR linac (MRL) treatment, which is an important element to minimize re-planning time and reduce the risk of misrepresenting the target due to this time pressure. Methods For 12 high-risk prostate cancer patients treated to the prostate and pelvic lymph nodes, target structures and organs at risk were delineated on both planning MR and CT scans and propagated using deformable registration to three T2 weighted MR scans acquired during the treatment course. Generated structures were evaluated against manual delineations on the repeated scans using intra-observer variation obtained on the planning MR as ground truth. Results MR-to-MR propagated structures had significant less median surface distance and larger Dice similarity index compared to CT-MR propagation. The MR-MR propagation uncertainty was similar in magnitude to the intra-observer variation. Visual inspection of the deformed structures revealed that small anatomical differences between organs in source and destination image sets were generally well accounted for while large differences were not. Conclusion Both CT and MR based propagations require manual editing, but the current results show that MR-to-MR propagated structures require fewer corrections for high risk prostate cancer patients treated at a high-field MRL.

The aim was to investigate if oncologic treatment decision based on G8 screening followed by comprehensive geriatric assessment (CGA) and a multidisciplinary team conference in patients with G8 ≤ 14 was better than treatment decision based on standard assessment. ClinicalTrials.gov Identifier: NCT02671994. Materials and Methods: From January 2016 to June 2018, 96 patients with cancer, aged ≥70 years, were included. Patients were randomized to treatment decision based on the oncologist's clinical judgement (control) or based on screening with G8. If G8 N 14 treatment decision was made as in the control group and if G8 ≤ 14, patients were referred to CGA including intervention as needed and treatment decision after a multidisciplinary team conference (MDT). Results: The study was closed early. 47 patients were randomized to the control group and 49 to the intervention group; 28 had a G8 ≤ 14, 24 of whom attended CGA. In the intervention group 48% completed treatment as planned compared to 54% in the control group (p = .208). Thirty-eight percent experienced grade 3-4 toxicity in the control group compared with only 20% in the intervention group (p = .055). Median overall survival (OS) was 14.2 months in the control group and 19.1 months in the intervention group (p = .911). Median progression-free survival (PFS) was 9.0 months in the control group and 7.8 months for the intervention group (p = .838). Conclusion: Treatment decision based on G8 screening followed by CGA had no impact on completion rate of planned oncologic treatment, OS or PFS, but resulted in a borderline significant lower incidence of grade 3-4 toxicity.

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