ESTRO 2020 Abstract Book

S267 ESTRO 2020

treatment plan quality, providing a more comprehensive approach to treatment plan evaluation in multicenter and multi-platform studies. 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.

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 MR defined case, the point dose measurement was taken in a target volume at the centre of the brain. In the multiple metastases treatment case, the point dose measurement was taken in a target volume approx. 5cm off central axis of the brain. Results The point dose results of the initial audit measurements are shown in Figure 2. For the classic SRS case, all plans were single isocentre and showed a maximum local dose discrepancy of 3.5%. For the MR defined multiple metastases case, plans included were both single and multiple isocentre, with a maximum local dose discrepancy of 1.5%. For the five target multiple metastases audit case, plans included were both single and multiple isocentre, with a large difference between the point dose results observed. The multiple isocentre plans showed closer agreement to measurement, with a maximum local dose discrepancy of -1.4%, compared to - 12.9% for the single isocentre plans.

Analysis of EBT3 and XD film for 2D delivery accuracy is ongoing. Conclusion The ACDS has developed a comprehensive SRS dosimetry audit for a range of clinical scenarios. Preliminary results