The PRISM Trial- First UK Experience of MRI-Guided Adaptive Radiotherapy

Abstract

Magnetic resonance imaging (MRI) guided systems integrate a linear accelerator and MRI scanner enabling adaptive radiotherapy (RT) and real-time imaging. We report our experience of the first five patients treated with MRI-guided RT (MRIgRT) in the UK, within the Prostate RT Integrated with Simultaneous MRI (PRISM) trial (NCT03658525) designed to assess the feasibility, safety, and tolerability of prostate MRIgRT. Five patients with localized prostate cancer were recruited. One week following insertion of three fiducial markers, reference computed tomography (CT) and MRI scans were acquired and fused using markers. Clinical target volume 1 (CTV1) consisted of prostate plus proximal 1 cm of seminal vesicles (SV), CTV2 was prostate plus proximal 2 cm of SV. Planning target volume 1 (PTV1) was created from CTV1 by addition of a 5 mm margin, except 3 mm posteriorly. PTV2 was generated by expanding CTV2 by 5 mm isotropically. Dose fractionation of 60 Gy in 20 fractions was delivered to PTV1 (PTV2 received 48.6 Gy). Reference CT plan was generated using inverse planned step and shoot intensity modulated RT with 7 co-planar non-opposing beams using a treatment planning system. At every fraction, after acquisition of a 2 minute 3D T2-weighted MRI, a clinician amended CTVs according to daily anatomy. Organs at risk were amended when required. A new daily plan was created by optimizing the segment shapes from the reference plan, also known as ‘adapt to shape’ workflow. Following checking of the plan with a secondary dose calculation, a second MRI was acquired to determine whether further adaptation of the new plan was required due to intrafraction motion. RT delivery was carried out with real-time cine MRI. Acute toxicity was assessed prospectively by RTOG and CTCAE during weeks 2 and 4 of RT then 4, 8 and 12 weeks following RT. Table 1 summarizes mean (standard deviation in brackets) patient characteristics, timings and toxicity. Patients received all fractions on the MR-Linac with a new online plan in 99/100 fractions. Mean treatment time was 42 minutes, 97/100 fractions were delivered within 60 minutes. Treatment was well tolerated with no unexpected toxicity. Daily prostate MRIgRT with online replanning is feasible, safe and delivered within a reasonable time. PRISM recruitment is ongoing with formal safety analysis after 10 patients and total recruitment of 30 patients planned.Abstract 2690; Table 1Patient characteristicsMean age (years)69.6 (3.9)Mean presenting PSA (ug/l)8.8 (5.3)Gleason score3+3 (1 patient), 3+4 (4 patients)Mean CTV2 volume (cm3)55.3 (15.4)Treatment timings (min)CTV amending9.1 (2.3)Plan re-optimization5.2 (1.1)RT delivery4.7 (0.7)Total time to end of RT delivery42.0 (4.9)Acute toxicity to dateHighest gastrointestinalCTCAE- Grade (Gr) 2 proctitis RTOG- Gr 2 proctitis and diarrheaHighest genitourinaryCTCAE- Gr 2 frequency, cystitis, urgency RTOG- Gr 2 cystitis Open table in a new tab