ESTRO 2021 Abstract Book

S1343

ESTRO 2021

result of coplanar VMAT. We investigated CTV coverage with respect to PD and motion at lateral and superior/inferior (S/I) aspects of tumours. The distance to 50Gy equivalent isodose depends on PD. Laterally at centre tumour slice average distance (AD) from disease specific CTV edge to 50Gy EQD 2 is: 11mm for 54/3, 9.8mm for 60/5 and 7.5mm for 60/8. Laterally 6mm and 8mm CTV is amply covered at each PD. Dose fall off laterally was motion independent. 12 of the 20 cases had vertical motion of < 5mm and are considered static. Average slope of fall of in S/I direction for static tumours was 11.5%/mm. The 50Gy EQD 2 isodose covered the CTV in the S/I direction by a range of 2mm for 54/3 to 0.8mm for 60/8. For tumours with sup/inf motion >1cm (n=12) the AD is 5.8 mm from CTV to 50Gy EQD 2 at S/I (for 54/3). For targets moving >5mm to <1cm (n=1) the AD is 5.5 mm to 50Gy EQD 2 . This compares to 2mm for static tumours. This shows that significant motion improves S/I CTV coverage. Conclusion Incidental CTV coverage is related to dose, fractionation and motion. Dose fall off in S/I direction is more than three times steeper compared to laterally. Our data show that a S/I SABR PTV margin needs to be 5mm at a minimum if CTV is to be covered, especially for static tumours. The currently used 5mm isotropic PTV margin is adequate to cover microscopic disease and may contribute to excellent local control and the low rate of marginal failures. As treatment delivery accuracy and image guidance improves, some centres may wish to reduce PTV margins to below 5mm. This may lead to inadequate coverage of microscopic disease for those treated with linac based lung SABR. PO-1621 Commissioning of the Eclipse TPS using a Monte Carlo software J. Calvo-Ortega 1 , M. Hermida-López 2 , J. Casals-Farran 1 1 hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain; 2 hospital Vall D'hebron, Servei De Física I Protecció Radiològica, Barcelona, Spain Purpose or Objective To assess the accuracy of the dose calculation performed by Eclipse TPS configured using the Varian Representative Beam Data using Monte Carlo simulation as reference. Materials and Methods Eclipse v 13.5 has been configured in our department for the AAA and Acuros XB (AXB) algorithms for a 6 MV beam with flattening filter (WFF) from a TrueBeam linac, using the Representative Beam Data beam data package provided by Varian. As part of the Eclipse commissioning process, basic dose distributions calculated in a water phantom were compared to the corresponding dose distributions estimated by the PRIMO Monte Carlo software v 0.3.1.1774 (www.primoproject.net). PRIMO used the Varian TrueBeam phase space file (PSF) version 2 for 6 MV WFF obtained from the website www.myvarian.com/montecarlo. The accuracy of PRIMO with this PSF was investigated by Rodríguez et al. (see PO-1393 from ESTRO 2020), obtaining gamma passing rates of 99.7% for criteria 1%/1 mm for 6 MV WFF beams of the TrueBeam linac. Doses distributions for 1×1, 2×2, 3×3, 10×10, 30×30 and 40×40 cm 2 field sizes calculated using AAA and AXB algorithms in a water phantom were compared to the PRIMO-based dose distributions. 3D Global (G) gamma passing rates (GPRs) were calculated for 1%(G)/1 mm and 2%(G)/2 mm, with a dose threshold of 1% of the maximum dose, to include in the analysis low-dose regions such as the profile tails.

Results

GPRs are shown in the table. Statistical uncertainty ( k = 2) of the simulated dose distributions was < 1% in all cases.

Conclusion Excellent Eclipse-PRIMO agreement was obtained. PRIMO software is a helpful tool to be used during the commissioning of the Eclipse TPS, to perform complementary tests to measured-based comparisons.

PO-1622 A comparison between Gafchromic EBT3 and EBT-XD films for pre-treatment QA of SBRT/SRS plans T. Santos 1 , T. Ventura 1 , M. Capela 1 , J. Mateus 1 , M.D.C. Lopes 1 1 Instituto Portugues de Oncologia Francisco Gentil, E.P.E., Medical Physics Department, Coimbra, Portugal Purpose or Objective Gafchromic TM EBT3 films have been used at our department for pre-treatment QA. The increasing number of stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS) treatments demands an evaluation of the tools used for dose verification. This work aims at comparing the performance of EBT3 and EBT-XD films by irradiating simultaneously both films.