ESTRO 2023 - Abstract Book

S392

Sunday 14 May 2023

ESTRO 2023

tattoos. The Monaco treatment planning system allows a cranial-caudal (CC) offset of 5 cm and 3 cm left-right and dorsal ventral between pre-treatment and treatment isocenter position. Patients therefore need to be positioned daily to within this margin on the couch. The palpable most cranial part of the pelvic bone and a fixed knee support (Macromedics Holding B.V.) at a certain index were chosen for longitudinal setup without tattoos. For lateral setup, alignment of body midline (belly button-nose) with laser was chosen, together with the fixed knee support. The location of the knee support and the origin marking were set during pre-treatment MR simulation (using a MR only preparation). Pelvic rotations in both patient groups were measured in the offline Monaco planning system, using a volume of interest (VOI) around the pelvic bones (figure 1).

Results Both patient groups had successful treatments with non-significant patient setup time differences (total setup time around 4 minutes). The involved 28 radiotherapy technicians got used to the new workflow within 10 patients, both during simulation and treatment. Two index points (knee support and pelvic bones) were necessary to reproduce the setup within the 5cm longitudinal setup margin, because patients have difficulty to feel the exact position of the knee fix. The obtained rotations are summarized in table 1. The X-axis rotation spread is somewhat larger without skin markers (SD = 0.5 vs 1.3 degrees, p < 0.05) but the 95% confidence interval remained within our clinical acceptance range of 3 degrees. The spread for the Y- and Z-axis rotations were larger with tattoos but the difference was not statistically significant.

Conclusion Even though we found slightly larger X-axis pelvic rotations without skin markers, confidence intervals without tattoos remained within 3 degrees and we consider removing tattoos for all prostate patients treated on the MR-linac. PD-0484 Bowel prep for prostate radiotherapy: A comparison of daily enemas versus first 10 fractions only. A. Mckenna 1 , G. Bestwick 1 1 Gloucestershire Hospitals NHS Foundation Trust, Oncology, Cheltenham, United Kingdom Purpose or Objective Micro enemas are used to provide a consistently empty rectum to optimise prostate radiotherapy. They are routinely used for all 20 fractions of prostate radiotherapy at our department. This study aimed to evaluate the impact of using micro enemas for the 1st 10 fractions of radiotherapy compared to using them for all fractions in terms of rectal toxicity and daily plan reproducibility. Materials and Methods Two cohorts of 21 patients (selected consecutively) received 20 fractions of radiotherapy to the prostate using daily CBCT image guidance (n=42). The cohorts undertook different bowel preparation. Group 1 used enemas before all 20 fractions. Group 2 used enemas before the first 10 fractions only. The number of repeat CBCT scans for bowel issues was compared between the 2 groups to check for treatment plan reproducibility. Rectal toxicities graded during week 4 of radiotherapy according to the Common Terminology Criteria for Adverse Events (CTCAE) were also compared between the 2 groups. Results Using enemas for the first 10 fractions only did not cause an increase in the number of repeated CBCT scans fractions 11 20 for bowel issues (Group 1 repeat scans: mean = 0.86, standard deviation 1.35, range 0-4; Group 2 repeat scans: mean