ESTRO 2022 - Abstract Book

S1338

Abstract book

ESTRO 2022

1 The Beacon Hospital, Radiotherapy , Dublin, Ireland; 2 Hospital of Wilhelmshaven, Radiation Oncology, Wilhelmshaven, Germany; 3 Heinrich-Heine Universität Düsseldorf, Science, Düsseldorf, Germany; 4 Nation university of Ireland, Galway, Medical Physics, Galway, Ireland; 5 Sun Nuclear Corp, Medical Physics, Melbourne, USA Purpose or Objective The aim of this study was to compare the SRS MapCHECK and ArcCHECK detector arrays (both Sun Nuclear Corp., Melbourne, FL, USA), operated with and without high density merge, in their ability to recognize artificially introduced MLC errors in SRT treatment plan deliveries and to establish recommended gamma criteria for each device. Materials and Methods Various treatment plans with target volumes ranging between 1.5 cm and 3 cm diameter were created in Eclipse TPS (Varian Medical Systems, USA) at The Beacon Hospital, Dublin. These treatment plans were modified by introducing predetermined MLC and/or jaw errors according to Kim et al. (2014). The introduced MLC and jaw offsets ranged from 0.5 mm to 2 mm. To determine the clinical impact of these misalignments, dose volume histograms (DVHs) of the modified plans were calculated and compared to the DVHs of the respective unmodified plans. The modified treatment plans were then irradiated onto the SRS MapCHECK array contained within the StereoPHAN End- to-End phantom (also Sun Nuclear Corp.) as well as the ArcCHECK array. Additional, shifted measurements were taken to double the effective spatial resolution of the arrays (high density merge). The measured dose maps of the modified treatment plans were compared to the TPS calculated dose map of the unmodified treatment plan. The accompanying SNC Patient software was used to establish the gamma passing rates. The gamma analysis was carried out using various gamma criteria ranging from 1%/1mm to 3%/3mm, to see which were sensitive enough to pick up on the misalignments and which may be oversensitive. The passing criteria was set to be 95% for a plan to be determined viable for treatment. The results each array was compared to each other to see which array was successfully able to pick up on the smallest, clinically relevant misalignment. Results By comparing DVHs of modified plans to the original plans, it could be seen that misalignments as small as 1 mm induced in the system can have a detrimental effect on the PTV coverage, which could potentially lead to an under-dosage to the treatment volume, therefore any gamma criteria selected would be required to be able to pick up on shifts of 1 mm. The SRS MapCHECK was able to pick up all introduced MLC errors using Gamma criteria of 2%/1mm. Doubling the spatial resolution of the SRS MapCHECK did not affect the error detection capability or passing rates. The ArcCHECK could detect introduced errors in deliveries of treatment plans with larger target volumes of about 3 cm using gamma criteria of 3%/1mm. However, the ArcCHECK was insensitive to introduced errors in deliveries of treatment plans with small target volumes of about 1.5 cm. Conclusion The SRS MapCHECK successfully detects clinically significant errors in patient specific QA of SRT treatment plans. The ArcCHECK has shown to struggle in detecting errors in very small fields, but performs well in larger field sizes. Purpose or Objective TPS plays an important part in the dosimetric accuracy of small fields radiotherapy. Improper beam modelling of small fields will affect the dose delivery. As Eclipse TPS only considers beam profile data above 3x3 cm 2 , modelling of small field by the MLC (while keeping jaw at 3x3cm 2 ) might produce a better dose accuracy as compared to jaw delineated field size. In this work, both convolution-based anisotropic analytical algorithm (AAA) and the Linear Boltzmann Transport Equation solvers AcurosXB calculation algorithm (AXB) are evaluated on their dose accuracy under various jaw and MLC-delimited field sizes. Different beam parameters such as dosimetric leaf gap (DLG) of the MLC and effective spot size (ESS) of the beam are also varied to evaluate the dosimetric accuracy. Materials and Methods Four different energies are used in this study: - 6X, 6FFF, 10X and 10FFF. Jaws delineated field size of 3x3cm 2 , 2x2cm 2 , 1x1cm 2 , 1x0.5cm 2 are measured at 95cm SSD with a fixed 100MU using PTW 31006 PinPoint detector. For MLC delineated field size, in additional to the above field size, 1x0.4cm 2 , 1x0.3cm 2 , 1x0.2cm 2 and 1x0.1cm 2 were also added in to evaluate the dosimetric accuracy. The measured results are then compared again TPS calculated values with different parameters. DLG was first varied by changing from the measured values of 0.1350, 0.120, 0.149, 0.140 for 6X, 6FFF, 10X and 10FFF using sliding gap technique to 0.030 across all energy. 0.25cm calculation grid size and ESS was varied from 0mm in both X and Y direction (0-0) to 1mm in both directions (1-1) for AAA. The measurement was repeated using AXB algorithm except the use of 0.10cm for calculation grid size. The percentage discrepancy between measurement and TPS are reported for various TPS configurations. Results There are missing lines in the figures due to overlapping results. Across all plots for AAA, jaws delineated field are found to have similar percentage errors regardless of the values of ESS with the smallest field size of 1x0.5cm 2 having the biggest error. For MLC delineated fields, DLG value of 0.030 with ESS of 1-1 is seen to have the biggest percentage error at 1 x 0.1cm 2 while other MLC delineated fields have similar percentage errors with the biggest error at 1x0.4cm 2 and 1x0.3cm 2 field size. PO-1556 small field dosimetric accuracy in Eclipse TPS using jaw and MLC delineated field size K.S. Lew 1 , H.Q. Tan 1 , J.H. Phua 1 , Y.G. Mok 1 , G.A.C. Chua 1 , W.Y.C. Koh 1 , K.W. Ang 1 , S.Y. Park 1 , J.C.L. Lee 1 1 National Cancer Centre Singapore, Division of Radiation Oncology, Singapore, Singapore