ESTRO 2020 Abstract book

S791 ESTRO 2020

normalisation, 10% threshold), the D95% and the mean dose difference to the PTVs were used as metrics to ensure the evaluation between the treatment planning system and the independent patient verification system was acceptable. Results The PDDs generated by the DoseCHECK and Eclipse TPS showed good agreement with percentage differences of <0.3% at depths beyond D max, differences are more pronounced in the build-up region. The results of gamma analysis demonstrated an overall good agreement for the dose distributions calculated by DoseCHECK and PerFRACTION when compared against the Eclipse generated dose distributions. The mean gamma pass rate for the plans calculated by DoseCHECK and PerFRACTION were 97.5% and 97.4% respectively. The mean dose difference within the PTV and D95% for the DoseCHECK were 0.9% and 0.8% and similarly for PerFRACTION were 1.0% and 0.9% respectively. Conclusion DoseCHECK and PerFRACTION have shown good dose distribution agreement with Eclipse TPS. The result shows that DoseCHECK and PerFRACTION are both viable systems for independent dose calculations for patients being treated on the Halcyon platform in our clinic. PO‐1399 Evaluation of SRS MapCHECK™ for SABR patient‐specific QA Y. Miao 1 , G. Kidane 1 , A. Ifthaker 1 , L. Crees 1 , E. Almond 1 1 Queen's Hospital - Barking Havering and Redbridge University Hospitals NHS Trust, Radiotherapy Physics, Romford, United Kingdom Purpose or Objective Patient-specific quality assurance (PSQA) for stereotactic ablative radiotherapy (SABR) is currently performed using an ion-chamber and GAFchromic™ films. The development and analysis of films causes delay between QA delivery and the availability of results. Recently, the SRS MapCHECK™ detector (Sun Nuclear Corporation, Melbourne, FL, USA) was reviewed for the pre-treatment verification of SABR treatments. The detector is a diode array, offering high spatial resolution and results processing in real time. This work verifies the SRS MapCHECK detector for SABR PSQA to replace film measurement. Material and Methods 12 SABR treatment plans on a range of sites were retrospectively analysed in this study. The plans were produced on the Eclipse™ (v15.6) treatment planning system (Varian Medical Systems, Palo Alto), using the Anisotropic Analytical Algorithm (AAA). The plans were delivered using a Varian Edge® radiosurgery system, and dose was measured by EBT3 GAFchromic film and SRS MapCHECK for each plan. Gamma analysis was used to compare the delivered dose distribution to the planning system’s calculation, and the two detectors’ results compared. Local tolerance for MapCHECK was determined based on gamma analysis results. Bland Altman (B-A) statistical analysis was used to analyse interchangeability between the two detector systems, with film as the current standard for SABR PSQA. A SABR plan was scaled with a 2.1% drop in plan dose, for sensitivity testing of local tolerances. Results The B-A plots (figures 1, 2) show good agreement between the two detector systems, where all results lie within the 95% confidence level.

Figure 1. B-A plot for 2%/2mm gamma criteria

Figure 2. B-A plot for 3%/2mm gamma criteria

The mean SRS MapCHECK gamma pass rate is 99.6% and 99.9% on 2%/2mm and 3%/2mm respectively, with all 12 plans scoring above 99.0% and 99.7%, on the two gamma criteria. Local tolerance for MapCHECK was set at 98% on 2%/2mm, based on two standard deviations away from the mean gamma pass rate. For 11 out of 12 patients, MapCHECK results were higher than the film results, which is evident in the negative bias of the B-A plots in figures 1 and 2 (-0.94 for 2%/2mm and -0.69 for 3%/2mm). Only one outlier sits outside of two standard deviations of the bias, on 3%/2mm. Sensitivity testing using the erroneous plan produced results of 96.0 % and 92.7%, on 2%/2mm, for SRS MapCHECK and film, respectively. These results are below the local set tolerance of 98% and 96%, on 2%/2mm, for the two devices, respectively. Conclusion SRS MapCHECK and GAFchromic film are interchangeable detector systems for SABR PSQA, and a local tolerance based on gamma analysis was determined for SRS MapCHECK. PO‐1400 Comparing dose calculation algorithms in low dose regions for patients with brain metastases M. AlahmarI 1 , C. Wolfs 1 , Y. Temel 2 , D. Eekers 1 , F. Verhaegen 1 1 GROW – School for Oncology and Developmental Biology- Maastricht University Medical Centre, Department of Radiation Oncology Maastro, Maastricht, The Netherlands ; 2 School for Mental Health and Neuroscience- Faculty of Health- Medicine and Life Sciences- Maastricht University Medical Centre, Department of Neurosurgery, Maastricht, The Netherlands Purpose or Objective Dose calculation algorithms are used to precisely calculate the dose distribution to the target volumes and surrounding tissues. However, uncertainties still exist in dose distributions and usually occur beyond the high dose area of the planning target volume (PTV). This may