ESTRO 2021 Abstract Book

S1614

ESTRO 2021

more than 95% of the PD and pV 150

to receive less than 50% of the PD. The IOP dose constraints for OAR for rectum were

rV 100 was lower than 15%. The differences between the dose distributions obtained in IOP and PID were compared. Paired sample T test and two related samples test-wilcoxon tests were used for statistical analysis. Results Comparison of IOP and PID revealed that the differences for pV 90 , pV 100 , pD 90 , uD 10 , uD 50 , uV 100 , uV 150 and rectum rV 100 parameters were statistically significant, but there were no significant differences for pV 150 , uD 30 , rV 50 , pV 150 . Both prostate and OAR doses were evaluated to determine whether the objective criteria was met in PID. Prostate median D 90 , pV 150 values met the objective criteria while the median pV 100 value was slightly lower than the determined constraint. The median D 10 and D 30 doses for the urethra and the uV 150 failed to meet the objective criteria. The rectum V 100 and V 150 volumes were within the constraints. For all dosimetric parameters median (min-max) doses and p values are presented in table 1. <1cc, rV 150 <0.1cc and urethra uD 10 was less than 150% of the PD, uD 30 was lower than 130% of the PD, and uV 150

Conclusion Although the aimed dose distributions were obtained during IOP, there were significant dosimetric differences between IOP and PID plans. This was most probably secondary to patient position differences and seed shifts. Clinical follow-up results will be reported in another publication PO-1895 Is the traditional dose prescription concept still relevant in modern SABR? M. Robinson 1 , B. George 2 , A. Nash 2 , S. Teoh 1 , A. Gaya 2 , P. Camilleri 2 , S. Mukherjee 1 , J. Good 2 1 University of Oxford, MRC Oxford Institute for Radiation Oncology, Oxford, United Kingdom; 2 GenesisCare, Radiotherapy, Oxford, United Kingdom Purpose or Objective There is now widespread use of SABR for both primary and oligometastatic disease. SABR has been facilitated firstly by modern IGRT and IMRT and now increasingly by online adaptive replanning combined with MR imaging. Modern SABR is often delivered with risk-adapted PTV dose coverage; dose constraints to nearby or overlapping OARs are met through compromise of dose coverage to sub regions of PTV without dose de-escalation to other regions of PTV. This is a significant departure from standard radiotherapy dose prescription. In light of this, is the simple reporting of dose prescription sufficient or meaningful in the era of modern SABR? Materials and Methods We audited all patients treated with SABR to pelvic/abdominal nodes and either primary or metastatic liver/pancreatic targets at a local institution on Viewray MRLinac. Patient dose prescription, PTV volume, volume of PTV receiving 25-60Gy in 5Gy increments and PTV max dose (0.1cc) was recorded on baseline (non-adapted) treatment plans. Dose prescription was subject to coverage criteria of 60% PTV receiving prescription dose and PTV max dose (0.1cc) >110% and <140% of prescription at planning. Using volume of PTV receiving 25-60Gy and PTV max dose we evaluated the number of cases where dose prescription could be nominally increased or decreased by 5Gy without any change to treatment plan whilst still meeting coverage criteria. Results 79 patients were identified (13 abdominal nodes, 10 pelvic nodes, 22 liver and 34 pancreas). Patient dose prescriptions and PTV coverage varied depending on the challenges of each individual case. In 5 cases clinical prescription dose did not achieve 60% coverage criteria- one liver case and 4 pancreas cases. Table one shows dose prescriptions by clinical site, median PTV volume and median PTV coverage criteria. 62% of abdominal nodes could have had dose prescription nominally either increased or decreased by 5Gy and met coverage criteria, as could 20% of pelvic nodes, 64% of liver and 24% of pancreas cases.