ESTRO 2021 Abstract Book

S1670

ESTRO 2021

Conclusion This study showed that the positioning accuracy using vaclok seemed more accurate than using a breast bracket for breast cancer radiotherapy. The use of Vac-Lok has high accuracy in rotation for all patients and in LNG direction for PMRT patients. It may be that surgery affects the lifting function of these patients’ arm. It is recommended to use this system for positioning. PO-1963 Impact of treatment delivery time on rectal volume and toxicity during SBRT prostate radiotherapy. J. mitchell 1 , J. Wright 1 , S. Adamson 1 , D. McLaren 2 , B. Nailon 1 1 Edinburgh Cancer Centre , Radiotherapy , Edinburgh , United Kingdom; 2 Edinburgh Cancer Centre , Radiotherapy , Edinburgh , United Kingdom Purpose or Objective The aim of this study was to investigate any relationship between patient reported gastrointestinal (GI) toxicity scores and the time it takes to deliver individual radiotherapy fractions during a course of prostate stereotactic radiotherapy (SBRT). Materials and Methods A total of 90 on treatment Cone beam CTs (CBCTS) were analysed from 9 SBRT prostate patients treated as part of a local clinical trial. All rectal volumes were outlined, by one individual, for all 9 patients using the original planning CT image and each pre and post CBCT taken as part of the treatment verification and delivery process. Treatment delivery time was recorded, as the time in minutes, between each pre and post treatment CBCT. Intra- fraction rectal volume changes were noted with all on treatment volumes compared against the planned volume to again correlate any link with on treatment time. Radiation Therapy Oncology Group (RTOG) patient reported GI toxicity scores , recorded as per routine patient follow up, were used to assess any relationship between the on treatment observed variations and overall treatment related toxicity . Results Overall treatment was delivered between 11 and 39 mins. Of the 45 treatment sessions analysed a delivery time between 15-16 mins was the most common, occurring in 13 sessions, with the majority of sessions delivered between 14-18 mins overall (34 sessions). 2 sessions took over 30 mins to deliver, further investigation highlighted acute GU toxicity at these time points to be the cause. Figure 1 highlights the various treatment delivery times for each patient . Rectal volumes all varied from the initial planned volume, with rectal gas observed as the cause. No trends could be reported, concerning this volume change, with random variations recorded both intra and inter-fractionally. Patient reported outcome data reported toxicity that could be well tolerated. With the exception of one patient reporting a grade 3 acute toxicity, all other follow up time points recorded scores no higher than 1. Reassuringly all outcomes reported from 18 months on wards have been reported as 0.