ESTRO 2021 Abstract Book

S423

ESTRO 2021

interfractional setup variation significantly compared to a laser-based setup in both the CC and AP direction ( p <0.0001).

Conclusion A surface-based setup significantly improved the interfractional setup variation in both the CC and AP direction. Therefore, our data endorse the use of a surface-based setup over a laser-based setup for breast cancer RT. OC-0536 A feasibility trial of buscopan to improve cone beam CT image quality during abdominal/pelvic SABR F. Slevin 1 , M. Beasley 1 , J. Zhong 2 , E. Hudson 3 , R. Speight 1 , J. Lilley 1 , L. Murray 4 , A. Henry 4 1 Leeds Teaching Hospitals NHS Trust, Leeds Cancer Centre, Leeds, United Kingdom; 2 Leeds Teaching Hospitals NHS Trust, Department of Radiology, Leeds, United Kingdom; 3 University of Leeds, Leeds Institute of Clinical Trials Research, Leeds, United Kingdom; 4 University of Leeds, Leeds Cancer Centre, Leeds, United Kingdom Purpose or Objective Stereotactic Ablative Radiotherapy (SABR) is increasingly used to treat oligorecurrence in the abdomen/pelvis. Cone beam computed tomography (CBCT) is commonly used for image guidance of SABR, but it is susceptible to bowel motion artefacts. This feasibility trial evaluated the impact of hyoscine butylbromide (buscopan) on CBCT image quality and its feasibility within an abdominal/pelvic SABR workflow. Materials and Methods A single-centre feasibility trial was performed in patients treated with 3 or 5-fraction SABR for abdominal/pelvic oligorecurrence. Buscopan was administered to separate cohorts by either intramuscular (IM) or intravenous (IV) injection on alternate fractions, providing within-patient control data without buscopan. Four-point Likert scales were used to assess overall image quality (ranging from excellent to impossible to use) and bowel motion artefact (ranging from none to severe) of CBCT. Feasibility of administering buscopan within a radiotherapy workflow was determined by patient/radiographer questionnaires and toxicity assessment. This feasibility study was not designed to demonstrate statistical significance; therefore, descriptive statistics are presented. Results Sixteen patients were treated (8 by IM and 8 by IV buscopan). In Figure A-B, CBCTs with IM buscopan (A) and without IM buscopan (B) for a patient with a right external iliac node are shown. In Figure C-D, CBCTs with IV buscopan (C) and without IV buscopan (D) for a patient with a left external iliac node are shown. Gross tumour volume (GTV) node (red) and planning target volume (PTV) node (blue) can be seen for each case. In both patients, reduced bowel motion artefact is apparent when buscopan is given (A and C) compared with when it is not (B and D). A summary of image quality results with/without buscopan is shown in Table. The percentage of images of excellent overall image quality with/without buscopan was 47% versus 29% and 65% versus 40% for IM and IV buscopan respectively. The percentage of images with no bowel motion artefact with/without buscopan was 24.6% versus 8.9% and 25.8% versus 7% for IM and IV buscopan respectively. Four patients (25%) experienced transient dry mouth after buscopan. No ≥grade 3 acute toxicities were observed. Fourteen of fifteen patients (93%) who completed questionnaires would accept buscopan prior to routine SABR treatment. Eleven of twelve radiographers (92%) who completed questionnaires reported no delay in patients’ treatments as a result of buscopan.