ESTRO 36 Abstract Book

S395 ESTRO 36 2017 _______________________________________________________________________________________________

collected includes: linac model, year of install, measurement equipment and recording method. The dose response parameter, gamma, is the percentage change in treatment response caused by a percentage change in dose. Gamma values of 2.3 and 5.2 (representative for Head and Neck cancers) were used to estimate the effect on TCP and NTCP respectively [1]. Results Based on the collated data, the UK linac outputs had a mean of -0.01% with one standard deviation of 0.88%. There was a wide variety of recording methods, with 8 centres having no form of electronic record for daily checks. Measurement data for both constancy devices and ionisation chambers was provided for 29 linacs. Of these, 8 (28%) had a discrepancy between measurement devices of greater than 0.5%, with 3 linacs (10%) having greater than 1%. The greatest variation in the mean output of an individual linac was -2.1%, with 90% of linacs having a mean output within 1%of the national mean. No significant variations were observed based on the age of the linac. The maximum range within a single centre for the mean output for each linac was 2.3% (min:-1.1%, max:+1.2%) – see figure. Assuming patients are treated on a single linac for their treatment duration, this indicates a variation in TCP of 5.3% and a variation in NTCP of 12% dependent on which linac they are treated on at that centre. Conclusion The data collection process indicates that many departments still rely heavily on paper QC records. The variation in treatment outcomes caused by dose variation alone indicates the importance of accurate QC. Output adjustment is one of the simplest ways of maintaining treatment consistency between individual patients, and its significance should not be forgotten with the introduction of more advanced techniques. This variation in dose should be considered when participating in clinical trials. This applies both to small scale local trials in which the technique used may determine the treatment linac, and therefore the dose delivered, as well as large multi- centre trials where the dose variation should be considered for the trial power calculations. [1] Bentzen, S.M et al. (2000), Eur J Cancer, Mar: 36(5): pp.615-620. PO-0758 Development of patient-specific phantoms for verification of SBRT planning using 3D printer C.S. Hong 1 , D. Oh 1 , S.G. Ju 1 , M. Kim 1 , B.Y. Koo 1 , H.C. Park 1 , D.H. Choi 1 , H. Pyo 1 1 Samsung Medical Center- Samsung Biomedical Research Institute- Sungkyunkwan University School of Medicine, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective A new technique for manufacturing a patient-specific dosimetric phantom (PSDP) using three-dimensional printing (PSDP_3DP) was developed, and its geometrical and dosimetric accuracy was analyzed. Material and Methods External body contours and structures of the spine and metallic fixation screws (MFS) were delineated from CT images of a patient with MFS who underwent stereotactic body radiation therapy for recurrent thoracic spine metastasis. Contours were converted into a STereoLithography file format using in-house program. A hollow, four-section PSDP was designed and manufactured using three types of 3DP to allow filling with a muscle- equivalent liquid and insertion of dosimetric film and grass dosimeters in the axial and coronal planes, respectively. To evaluate the geometrical and dosimetric accuracy of PSDP_3DP, CT images were obtained and compared with patient CT data for volume, mean density, and Dice similarity coefficient (DSC) for contours. The dose distribution in the PSDP_3DP was calculated by applying the same beam parameters as for the patient, and the

Conclusion It was determined that the four testing materials were MR/CT compatible, showed expected PDD curves, and preliminary data show expected beam profiles with and without 1.5T on the MR-Linac. PO-0757 Variation of mean dose output from 204 UK linacs (Jan-June 2015) and its potential clinical impact. M. Bolt 1 , A. Nisbet 1 , C. Clark 2 , T. Chen 3 , R. Jena 4 1 St. Luke's Cancer Centre Royal Surrey County Hosp, Radiotherapy Physics, Guildford, United Kingdom 2 National Physical Laboratory, Radiation Dosimetry, Teddington, United Kingdom 3 University of Surrey, Chemical and Process Engineering, Guildford, United Kingdom 4 Addenbrooke's Hospital, Department of Oncology, Cambridge, United Kingdom Purpose or Objective Variation in dose delivered to patients directly impacts the effectiveness of radiotherapy treatments. The drift and daily fluctuations in linac beam calibration (output) is a contributing factor to the cumulative dose received by the patient. Knowledge of the variation in measured outputs on a national scale provides an insight into the uncertainties in dose delivery and its clinical impact. Material and Methods A request for 6MV output measurement data was sent to all UK radiotherapy centres. In total, data was provided for 204 linacs situated at 52 cancer centres across the UK. The data spans 6 months from January to June 2015, totalling almost 25,000 data points. Additional data