ESTRO 2020 Abstract book

S287 ESTRO 2020

OC-0475 Cranial SRS dosimetry audits of complex treatments of multiple brain metastases M. Shaw 1,2 , J. Kenny 3 , A. Alves 1 , C. Davey 1 , S. Keehan 1,4 , J. Supple 1 , R. Brown 1 , A. Cole 1 , F. Kadeer 1 , J. Lye 1 1 ARPANSA, Australian Clinical Dosimetry Service, Yallambie, Australia ; 2 RMIT University, School of Health and, Melbourne, Australia ; 3 Health Stem Solutions, Health Stem Solutions, Yallambie, Australia ; 4 The Alfred Hospital, Radiation Oncology, Melbourne, Australia Purpose or Objective Treatment of multiple brain metastases in SRS regimes is becoming increasingly popular in modern radiotherapy, as reflected in current radiotherapy clinical trials such as TROG Local HER-O and OUTRUN. The risk to patients is increased with SRS techniques due to the increased dose per fraction, and as such specialised planning, treatment and QA practices are needed to ensure patient safety. Material and Methods The Australian Clinical Dosimetry Service (ACDS) began SRS audits in 2019. An end-to-end dosimetry audit was developed using a customised IMT MAX-HD TM (IMT, New York) anthropomorphic cranial phantom. The audit was designed to test three clinical cases; a single, “classic” SRS target volume, the MR imaging accuracy through delineation of four targets visible on MR imaging only, and a complex delivery of five metastases in a single session. Target volumes were defined in a range of sizes, dose fractionation schedules and distances from central axis. For the multiple metastases audit cases, single or multiple isocentres were chosen for the treatment plan as per audited facility protocol. Gafchromic EBT3 and XD film (Ashland Inc., Bridgewater NJ, USA) were used for absolute dosimetry in targets, brain stem and for integral brain dose. Figure 1 shows an example audit plan (a) with a SRS target abutting the brainstem, and a full head slice EBT3 film (b) for concurrent measurement of target, brainstem and integral brain dose. Film was analysed using FilmQA Pro (Ashland Inc. Bridgewater, USA) and in-house Matlab (Mathworks, Natick, MA, USA) softwarefor localisation to the facility CT scan.

Conclusion Treatment planning for a prostate SBRT with SIB to DIL was feasible using various technologies with no major difference compared to classical prescription. Representation of crowd-DVHs as described in this study can be useful in clinical trial settings to facilitate analysis of submitted cases that are automatically benchmarked against prior submissions as the cohort of participants grows. This can help to reduce variability and harmonize treatment plan quality, providing a more comprehensive approach to treatment plan evaluation in multicenter and multi-platform studies.

PTW microdiamond detector (PTW Freiburg, Germany) was used for point dose measurements in target volumes >1.5cc. In the classic SRS case and the multiple metastases