ESTRO 2022 - Abstract Book

S289

Abstract book

ESTRO 2022

Stereotactic ablative body radiotherapy (SABR) is an effective treatment for patients with prostate cancer. SABR prostate treatment has been carefully implemented using standard linear accelerators however, the advanced imaging, recontouring and plan re-optimisation workflow of the MR-Linac lends itself to SABR prostate treatment. This study assessed the feasibility of magnetic-resonance guided adaptive radiotherapy (MRgART) for prostate cancer. Materials and Methods Fifteen patients with localised prostate cancer were treated with stereotactic prostate MRgART. Plans were prescribed to 36.25Gy to Planning Target Volume (PTV) and Clinical Target Volume (CTV) boosted to 38-40Gy in 5-6 fractions with the goal of 95% PTV to receive 100% of the prescription. OAR constraints were as per published guidelines. All fractions were delivered using an adapt to shape (ATS) workflow where the MRI is recountoured to create a new plan daily. Treatment times were recorded daily. Acute toxicity with a focus on genitourinary (GU) and gastrointestinal (GI) symptoms were measured for up to 3 months after treatment, using Common Terminology Criteria for Adverse Events (CTCAE) v.5.0. Results A total of 76 fractions (adapted plans) were successfully delivered to 15 patients. Of these patients, one had SpaceOar inserted and received 6 fractions. The median patient age was 73 years (range: 61-87 years). Median total treatment time including patient set-up, imaging, contouring, plan adaptation and beam-on was 50.2 minutes (range 43.2-63.7 minutes), and median beam-on only time was 14.1 minutes (range 7.3-16.8 minutes). 3-month follow-up data were complete for 13 patients. Four patients reported grade 1 GU toxicity, and no grade 2 or worse early GU toxicity was reported. The vast majority of GU toxicity was reported for pain, frequency and urgency. One patient reported grade 1 GI toxicity (diarrhoea) and another patient reported grade 3 GI toxicity (rectal pain). Conclusion Our experience with SABR prostate using MRgART has demonstrated safety and feasibility for this group of patients. The preliminary results gained from assessing the low incidence of GI and GU toxicity indicates that it is possible to safely deliver a stereotactic highly conformal dose to the prostate with confidence. We continue to collect follow-up data to assess late toxicity and patient reported outcome. 1 The Royal Marsden NHS Foundation Trust, Radiotherapy, Sutton, United Kingdom; 2 The Royal Marsden NHS Foundation Trust, Radiotherapy , Sutton, United Kingdom; 3 The Institute of Cancer Research, Division of Radiotherapy & Imaging, Sutton, United Kingdom Purpose or Objective Craniospinal irradiation (CSI) is used to treat medulloblastoma and other central nervous system tumours. The target volume includes the circulating cerebrospinal fluid and meninges, making this one of the most complex radiotherapy techniques delivered. An unpublished Trust audit (RT2021_187) established that local CSI CTV-PTV margins could not be reduced without improvement to patient immobilisation. With little contemporary published guidance available on optimal immobilisation and treatment technique, the team undertook to elicit current CSI practice in the UK. Materials and Methods A 24-question survey was developed to capture CSI patient preparation, immobilisation, verification, and treatment planning techniques. The survey, on Microsoft forms, included yes/no, multiple-choice and open-ended questions. A survey link was e-mailed to radiotherapy service managers from National Health Service (NHS) centres, with a request to forward to an appropriate individual within their centre. In addition, the survey was publicised at SPRIGG, a national meeting for paediatric specialist radiographers and doctors. The survey was open from 15 th Feb–19 th March 2021. Participation in the survey was voluntary without remuneration. Results Twenty-six responses were received, 8 did not deliver CSI and 3 were duplicates. Duplicate answers were clarified, resulting in CSI data presented from 15 UK radiotherapy centres. Most centres (13/15) treat paediatric and adult patients. Treatment under general anaesthetic is typically reserved for children ≤ 5-years (10/13), however, the child’s mental capacity and treatment technique used influences this decision. For patients requiring surgery prior to radiotherapy 5/15 centres allow 1-2 weeks before CT planning, however, this time ranges up to 6 weeks. Five-point immobilisation shells are favoured for both adult (11/15) and paediatric (9/13) patients. For paediatric patients 8 centres routinely add a customisable head and neck cushion to personalise immobilisation. Painting paediatric shells is popular, 11 centres ‘always or sometimes’ decorate shells. Torso immobilisation was used by 6/15 centres, utilising vac-bags. Ten centres ‘always or sometimes’ immobilised the patient’s pelvis, 7 with a knee rest and 3 with a vac-bag. All centres tattoo patients, most using 3-4 tattoos, 1 centre sometimes used surface-guided radiotherapy as an alternative. Variable verification protocols presented; 6/15 centres perform daily imaging with the others favouring an extended no action level approach. Five centres utilise two different imaging methods, kV-CBCT and 2-dimentional kV imaging were most popular. Five centres use VMAT, 4 TomoTherapy, 3 IMRT, 2 3D-conformal and 1 proton therapy. A wide range in CTV- PTV margins presented ( figure 1). PD-0326 UK National Survey of Craniospinal Irradiation: Immobilisation and treatment techniques S. Alexander 1 , K. Robinson 2 , L. Sellman 1 , S. Mowat 1 , H. Mandeville 1,3