ESTRO 38 Abstract book

S224 ESTRO 38

clinicians’ discretion, after review of the former treatment plan. Using RayStation (RaySearch Laboratories, Stockholm, Sweden), cumulative doses were calculated in 3 ways: 1. Rigid image registration (RIR) of former and reRT plans with physical dose summation (current clinical standard) e summation (i.e. each dose distribution converted to equivalent dose in 2Gy fractions (EQD2), then summated, using α/β = 3Gy for all tissues, except nerves, where α/β = 2Gy) Deformable image registration (DIR) with dose mapping and radiobiological dose summation as per 2. above (Fig. 1). Registrations were assessed by two clinicians who considered all DIR images of greater clinical relevance than RIR images. 4. 2. 3. RIR with full radiobiological dose

Conclusion Evaluation of cumulative doses in the reRT setting has traditionally been crude. Physical dose summation is radiobiologically meaningless, thus clinically unhelpful. An approach that incorporates radiobiology has revealed cases where OARs received higher doses than intended. DIR substantially influenced dosimetry in 70% of cases. We have developed a clinically implementable solution for radiobiological dose summation. This can be applied in many situations, including reRT and adaptive radiotherapy. PV-0428 A multi-centre study for implementation of MRI-only prostate planning P. Greer 1,2 , P. Pichler 1 , T. Young 3 , J. Martin 1,4 , P. Hunter 1 , C. Wratten 1,4 , J. Denham 1,4 , L. Holloway 3,5 , M. Sidhom 3,5 , J. Dowling 6 1 Calvary Mater Newcastle Hospital, Department of Radiation Oncology, Newcastle, Australia ; 2 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle, Australia ; 3 Liverpool Hospital, South Western Sydney Radiation Oncology, Sydney, Australia ; 4 University of Newcastle, School of Medicine and Public Health, Newcastle, Australia ; 5 University of New South Wales, South Western Sydney Clinical School, Sydney, Australia ; 6 CSIRO, E-Health Research Centre, Brisbane, Australia Purpose or Objective MRI-only planning is a departure from conventional CT based planning practice where dose calculations are performed on artificially created pseudo-CT (pCT) scans. It is paramount that procedures and processes are developed to ensure the safe implementation of this new technology. This project aims to investigate whether MRI- only prostate planning can be implemented effectively and safely in a prospective multi-centre study. Material and Methods The protocol was a 2 phase design, where for phase 1 centres that had not previously validated MRI-only plans recruited 5 patients for retrospective analysis. Following successful completion of this phase centres then recruited to the prospective MRI-only treatment phase. To date 22 patients have been recruited and analysed across two treatment centres with 17 prospective. Whole-pelvic MRI scans were converted to pCT using an established atlas- based method. Dose plans were generated using MRI-based anatomy and pCT dose calculations. A conventional QA CT scan was acquired subsequent to MRI-only plan

Results Table 1 shows cumulative OAR doses based on each strategy. Summated physical doses (Strategy 1) should not be evaluated against constraints given the different dose per fraction from each treatment course, potentially masking OAR overdose. For example, incorporating radiobiology (Strategy 2) revealed that one patient received cumulative maximum EQD2 of 94.8/106.9/119.5Gy to colon/small bowel/sacral plexus, not appreciated at the time of clinical plan prescription, or identified by physical dose summation. By incorporating DIR and radiobiology (Strategy 3), for 11/114 threshold/maximum doses, there was ≥20% difference in cumulative EQD2 (range -33 to +43%) compared to RIR with radiobiology, affecting at least one OAR in 7/10 cases: cumulative doses were lower based on DIR in 5/7.