ESTRO 2020 Abstract book

S771 ESTRO 2020

probability of DQA failure is increased. Therefore, these parameters should consider when making a plan. PO‐1364 Beam modelling and matching of eight Varian TrueBeam linear accelerators M. Ghazal 1 , L. Södergren 1 , J. Söderström 1 , M. Westermark 1 , T. Pommer 1,2 1 Karolinska University Hospital, Department of Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden ; 2 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden Purpose or Objective Eight Varian TrueBeam linear accelerators (linacs) were accepted for photon energies: 6X, 15X, 6XFFF and 10XFFF and modelled in Analytical Anisotropic Algorithm ( AAA ). The beam-model was optimized in terms of Dosimetric Leaf Gap ( DLG ) and Spot Size . All linacs were matched to achieve equivalency. The scope of this report is to present the level of agreement achievable: (i) between the beam- model and the beam-data, (ii) among the linacs. Material and Methods Beam-data measured at the first linac were set as reference for matching and beam modelling. The photon energies were matched among the linacs by measuring 40 x 40 cm 2 diagonal profiles at D max in water and comparing the region within the penumbra with the reference profile. The energy matching was verified by Tissue Phantom Ratio ( TPR 20,10 ) measurements according to IAEA TRS-398 (6X and 15X) and TRS-483 (6XFFF and 10XFFF). An inhouse imager- based method was developed for the calibration of the jaw-collimators using jaw-position encoders and mathematical prediction model. The DLG was matched among the linacs. The Spot Size in AAA was optimized by calculating plans with varying Spot Size and comparing them to measured lateral dose profiles in water. DLG was optimized in AAA by calculating several dynamic representative plans with varying DLG in AAA and compare the dose difference to measured plans using Delta4 and Quasar phantoms. Verification of both the matching among the linacs and the beam-model were performed by profile measurements in different geometries in water. Results The regions within the penumbra of the 40 x 40 cm 2 diagonal profiles for all linacs and energies were matched to < 0.5 %. The TPR 20,10 measurements were within 0.5 % for all energies. The global average gamma evaluation index (2 %, 2 mm) of all matching verification measurements was 100 %. The DLG for 6X was matched to 1.40±0.03 mm resulting in a DLG of 1.56±0.04 mm (15X), 1.25±0.03 mm (6XFFF) and 1.45±0.04 mm (10XFFF). The jaws were calibrated in each linac with a precision of 0.3 mm, and the jaw junction at isocenter was calibrated with a precision of 0.2 mm, significantly improving the calibration method provided by the vendor (Table 1). Varying the Spot Size in AAA resulted in a negligible effect on output. The Spot Size was therefore optimized based on penumbra widths of small MLC-defined lateral dose profiles and resulted in 0.5 mm and 0 mm in X- and Y-axis respectively for all energies (vendor recommendation is X=1 mm and Y=0 mm). The global average gamma evaluation index (2 %, 2 mm) of all AAA verification measurements was 100 % (Figure 1). DLG in AAA was optimized for 6X (6XFFF and 10XFFF) to 1.8 mm (1.6 mm, 1.8 mm) to match a 1.4 mm measured DLG (1.25 mm and 1.45 mm). No DLG optimization was done for 15X because no dynamic treatments are used clinically for this energy.

Conclusion Use of Mobius3D as an independent calculation system can identify plans that would fail an ion chamber measurement. The higher sensitivity of Mobius3D could be used to identify a subset of plans that require ion chamber measurements, freeing up significant clinical resources by identifying potentially failing plans without physical measurements. Additionally, Mobius3D can be helpful in identifying deficiencies in linac beam models that may otherwise be difficult to diagnose. PO‐1363 Dosimetric impact of leaf open time and other planning parameters on DQA in helical tomotherapy K.H. Chang 1 , D.M. Jung 1 , C. Hong 1 , M. Han 1 , J. Kim 1 , S.C. Han 1 , H. Lee 1 , K. Park 1 , D.W. Kim 1 , J. Cho 1 , Y.B. Kim 1 , J.S. Kim 1 1 Yonsei Cancer Center, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective Tomotherapy recommended that leaf open time (LOT) below 100 ms should be less than 30 % because the risk of multileaf collimator errors was increased and higher DQA failures. This study aims to investigate leaf open time (LOT) values by analyzing the impact of LOT on delivery quality assurance (DQA) results for HT cases. Moreover, we evaluate the DQA failure pattern according to the LOT, modulation factor (MF), pitch, and gantry period (GP). Material and Methods A total of 71 patients who passed and failed for DQAs results in the first measurement randomly were retrospectively selected to evaluate the LOT pattern for the patient's plan. There were 49 patients and 22 patients who passed and failed in DQAs, respectively. These included brain tumor (n=11), head and neck (H & N) (n=13), spine (n=7), prostate (n=15), rectal (n=9), pelvis (n=5) and CSI and large field (, n=11) patients. LOT and DQA results (dose difference and gamma passing rate) in patients passed, and failed cases were analyzed. The criteria of point dose difference and gamma passing rate with 3%/3 mm below ±5 % and 90 %, respectively. Seven new DQA plans for each patient have generated its plan qualities and LOT values were analyzed. Moreover, then, DQA measurements performed with ArcCheck. To evaluate the correlation among the DQA results (absolute point dose difference and gamma passing rate) of each studied case and the plan parameters of the four DQA measurements. Pearson correlation coefficients (r) with the corresponding p values were calculated. Results The average percentage of LOTs below 100 ms in passed and failed cases for DQA results was 18.14 and 35.60%, respectively. The average value of pitch in passed and failed cases for DQA results was 0.43 and 0.32, respectively. The average value of GP in passed and failed cases for DQA results was 19.07 and 16.20, respectively. In case of the proportion of LOT below 100 ms is more than 25%, DQA fail was accounted for about 95.5 % of the total failures. In case of the GP below 13 sec/rotation, all DQAs were failed and DQA fail was accounted for about 36.4% of the total failures. In case of the pitch below 0.25, all DQAs were failed. A weak correlation was observed between the point dose difference and modulation factor (r = -0.383, p = 0.001) and No correlations were observed in the correlation between the gamma passing rate and field width and LOT for all the measurements. However, p- values were less than 0.05. For failing DQA case, plan quality after re-planning with changed pitch, and modulation factor does not change the uniformity of target and organ at risks dose constraints. Conclusion We confirmed that plans having a proportion of LOT below 100 ms is more than 25% could affect DQA failures. In case of pitch < 0.251, actual MF > 1.9 and GP < 13, the