ESTRO 37 Abstract book

S1209

ESTRO 37

created under the local reference condition (95 cm SSD) to test the Eclipse Algorithm utilizing various field sizes. Measurements at 5 cm were used to evaluate the output factor modelling. Measurements at 10 and 20 cm were used to evaluate the energy modelling. Additional verifications included MLC DLG measurements, IMRT, and RapidArc test plans. QA measurements of the plans were performed using the Sun Nuclear Arc Check with a farmer chamber at the centre to evaluate absolute dose. Results Measured photon PDD were within 0.5% overall agreement with the pre-configured beam data; the largest discrepancies were found for the smaller and for largest field size. Profiles showed good agreement within and outside the field region. Some discrepancy was observed on the penumbra region. The output factors were in agreement within 0.6% for field sizes ≥3x3 cm 2 . However, up to a 1% discrepancy was observed for field sizes <3x3 cm 2 . The AAA point dose tests agreed with measured data within 1% with the exception of the 2x2 cm 2 where the variation was 1.7%. MLC DLG data agreed within 0.1 mm. The test plan Arc Check QA measurements resulted in gamma analysis of >95% (3%/3mm, 10% threshold). The mean ion chamber measurements at the Arc Check centre showed an agreement within 2%. Conclusion The beam model preconfigured data was found to be in good agreement with the measured data down to 3x3 cm 2 . Most of the reference data set was measured using a larger chamber compared to the measurements carried out in this work. The effect of the volume variations is more evident for smaller field sizes. The preconfigured AAA model resulted in good agreement between measured and predicted dose data for clinical IMRT and RapidArc plans. EP-2184 Clinical Implementation of a Nomogram as a HDR Prostate Pre-Treatment Second Check S. Cavanaugh 1 , J. Wei 2 , T. Law 2 , J. Dick 2 , P. Schantz 2 , T. Tsui 2 , S. Crawford 2 , J. Swanson 2 1 Cancer Treatment Centers of America, Radiation Oncology, Newnan GA, USA 2 Landauer Medical Physics, Radiation Oncology, Newnan GA, USA Purpose or Objective Nomograms are well established in Low Dose Rate (LDR) Brachytherapy to estimate the number of seeds for a prostate volume. This concept can be applied to High Dose Rate (HDR) Brachytherapy to provide a second check of the dose calculation and assess plan and implant quality. In 2011, Pujades et al. published findings on the use of a nomogram in LDR and HDR prostate brachytherapy at 2 separate institutions in Valencia, Spain. This work seeks to provide updated patient data to validate the use of nomograms in HDR brachytherapy and provide a linear fit correlation from the perspective of another institution. Material and Methods The HDR prostate brachytherapy treatment plan data were collected from February 2014 through September 2017, including several monotherapy and boost prescriptions from a single institution, Cancer Treatment Centers of America Southeastern (CTCA SERMC). The Ir- 192 HDR source and the Varian VariSource™ iX Afterloader were used in conjunction with the Varian BrachyVision (versions 10.0, 11.0, and 13.6) for treatment planning and dose calculation. All treatment plans were evaluated based on American Brachytherapy Society consensus guidelines for HDR prostate brachytherapy. The air kerma strength, S k , (µGy-m 2 /h) and total treatment time, T, (sec) were recorded and normalized by the prescription dose, Rx, (Gy). The normalized total air kerma strength

EP-2183 Dosimetric evaluation of Halcyon data – experience of the early implementers G. Ghebremariam 1 , G. Beyer 2 , L. Crees 1 , R. Hulley 1 1 Barking- Havering and Redbridge University Hospitals, Radiotherapy Physics, London, United Kingdom 2 Medical Physics Services Intl. Ltd., MPSI, Cork, Ireland Purpose or Objective This work describes the data analysis from the commissioning process carried out for the new Halcyon linear accelerator (Varian) for the first clinically operational unit in the UK. Material and Methods Halcyon, Varian’s newest linear accelerator, offers fully image-guided IMRT and RapidArc treatments with 6MV FFF and was specifically designed to improve clinical workflow. It includes double layered high-speed MLC leaves and a rapidly rotating gantry. The Eclipse AAA calculation algorithm for Halcyon (V. 15.1.1) is preloaded using the Varian reference data set. The commissioning process involved the evaluation of how well the dosimetric characteristics of the Halcyon conformed to the Varian’s pre-configured reference beam data and model. The IBA CC04 chamber was used in the beam data validation process to minimise the volume effect for small fields and the ion recombination effect for FFF energy. Scanned data was measured using the IBA Blue Phantom 2 setup at 90 cm SSD. The PDDs and profiles were evaluated at d max , 5, 10, and 20 cm. Output factors (square and rectangular fields) were measured at 95 cm SSD and 5 cm depth. Measurements were performed from 2x2 cm 2 to the maximum field size (28x28 cm 2 ). Plans for a water-equivalent phantom in Eclipse were