ESTRO 2020 Abstract book

S1041 ESTRO 2020

intensity modulated techniques used at the North Coast Cancer Institute (NCCI) over the last 3.5 years treating 3000 cases, 85% are volumetric modulated arc therapy (VMAT) and with 3.7% SBRT. This trend is similar to many other radiotherapy clinics globally. There is an increasing trend of the use of SBRT which justifies the implementation and use of FFF beams. The other aspect of this is the financial and human resource burden on the procurement and maintenance of these beams along with the conventional flat beams. This is important, considering the estimates that staff costs represent the dominant cost in radiotherapy (4). This study investigates the optimum number and energy of the photon beams required for a modern radiotherapy department. Material and Methods 6MV, 15MV, 6MVFFF and 10MVFFF beams were commissioned on Elekta® linac and were modelled in Monaco® TPS for Monte Carlo (MC) dose calculation algorithm. The basic validation of the beams is performed in a stack of water equivalent plastic. Further testing is performed in low density materials like lung tissue equivalent slabs and phantoms for simple and complex cases. Peripheral dose to the tumour was assessed using normoxic polymer gel and EBT3 films in lung equivalent phantoms. Treatment time and plan pass rates were compared for 6MV, 6MVFFF and 10MVFFF beams for 5 clinical sites for 5 different doses, totalling 75 VMAT plans. Results In water, calculated dose (Dc) agreed to delivered dose (Dd) for fields down to 2x2cm 2 for all beams. With decrease in the electron density of the medium and increase in beam energy, the dose agreement becomes poor, about 15% for 15MV and 10MVFFF for 2x2cm 2 . On the periphery, the films and polymer gel shows that Dc overestimates the Dd, more so for 10MVFFF than 6MVFFF and worst for 15MV (Fig 1). This is due to the effect of inaccurate modelling of the secondary dose build-up from lung tissue to tumour. For the 75 VMAT plans tested, FFF plans showed significant treatment time advantage over 6MV, specifically above 4Gy but no significant difference between the 6MVFFF and 10MVFFF (Fig 2). There is no significant difference in the gamma pass rates between energy and doses with a mean gamma of 99.2% for 3%,3mm and 96.3% for 2%, 2mm. Conclusion This study shows that, 6MVFFF has the best dosimetric accuracy in all media. 10MVFFF doesn’t have significant time advantage over 6MVFFF for VMAT. 15MV and 10MVFFF have dosimetric concerns in low density targets for small fields. Hence, these factors are important to consider while planning an RT department as it can influence the cost of bunker design, machine selection and QA requirements. PO-1777 Validation of the Pylinac software platform for MLC log file analysis J.F. Calvo Ortega 1 , M. Hermida-López 2 , S. Moragues- Femenía 3 , C. Laosa-Bello 3 , J. Casals-Farran 3 1 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.Servicio de Oncología Radioterápica- Hospital Universitari Dexeus- Barcelona- Spain, 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.S, ; 2 Servei de Física i Protecció Radiològica. Hospital Vall d'Hebron. Barcelona Spain, Servei de Física i Protecció Radiològica. Hospital Vall d'Hebron. Barcelona Spain, Barcelona, Spain ; 3 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.Servicio de Oncología Radioterápica- Hospital Universitari Dexeus- Barcelona- Spain, 1.Servicio de Oncología Radioterápica- Hospital Quirónsalud- Barcelona- Spain & 2.Ser,

software (LifeLine Software, Inc., Austin, TX, USA) was used to reconstruct the 3D dose distribution based on the MVCT detector sinogram acquired during plan delivery with the couch out of the bore. The assessment of plans deliverability was performed through global gamma analysis. The adopted criteria were 3%/3mm and 2%/2mm with 10% threshold (TH). The correlation between the computed texture features and deliverability was investigated using the Spearman’s rank correlation coefficients (r s ), with a significance level of 5%.

Results For the 20 H&N plans, sinograms presenting a higher contrast and a lower homogeneity tended to be associated with a poorer delivery performance. After a comprehensive correlation analysis of the results, contrast presented a significant correlation with the gamma passing rates for a criteria of 2%/2mm 10%TH, with values of r s =- 0.532, p=0.016 (for d=3) and r s =-0.588, p=0.006 (for d=5). Homogeneity generally showed a moderate correlation with gamma passing rates, when considering both evaluation criteria (3%/3mm, and 2%/2mm 10% TH). Concerning the other two texture features as correlation and energy, deliverability results did not show a significant dependency (r s <0.4, p>0.05). Conclusion The presented study indicated that the texture features may have potential to be used as indicators of the degree of modulation of the planned sinograms in HT. In addition, contrast and homogeneity showed likelihood to predict the plan deliverability. PO-1776 Optimum photon beam procurement in modern radiotherapy departments M. Mundayadan Chandroth 1 , A. Venning 1 , B. Chick 1 , B. Waller 1 1 Mid North Coast Cancer Institute, Radiation Oncology, Port Macquarie, Australia Purpose or Objective Modern radiotherapy (RT) utilizes intensity modulated treatment delivery techniques for better clinical outcomes (1). The use of photon beams above 10MV are questionable for such treatments (2). The introduction of flattening filter free (FFF) beams have made extracranial stereotactic radiotherapy (SBRT) significantly time efficient and improved patient stability (3). Of all the

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