ESTRO 2020 Abstract book

S795 ESTRO 2020

and distance to agreement (DTA) ranging from 1-3mm, with cut-off value of 50%. Results Table 1 shows the results of gamma evaluation for the lung IMRT plans. For single MLC positioning fault of up to 3mm, the panel fails to detect any dose deviation as the resulting gamma values evaluated using different gamma criterions were comparable to the original plan without any MLC positioning error. However when the entire leaf bank is shifted, the results is apparent even with an induced positioning error of only 1mm. The results of the SRS dynamic arc measurements are shown in Figure 1. When gamma criterion of 3%/3mm is applied with a passing threshold of 95%, all deviations went undetected except the shift of the entire leaf bank by 3mm . The Octavius SRS 1000 system could possibly detect an incidental MLC positioning errors of up to 1mm when the analysis is carried out using gamma criterion of 3%/1mm. When DTA of >1mm is applied, all incidental errors of up to 3mm would go unnoticed. The use of more stringent gamma criterion is crucial in detecting such small errors, although it might also increase the incident of false negative findings.

Material and Methods Using an anthropomorphic phantom, we made two radiation treatment plans covering the left breast and IMNs; the conventional wide two-field technique and the VMAT. The prescribed dose was 50 Gy in 25 fractionations. Two radiochromic film patches (EBT3, Ashland, USA) and three dosimetric elements (GD-302M, Asahi techno glass, Japan) were inserted near the breast parenchyma and mammary nodes to measure the actual doses absorbed in the lesions. To simulate the respiratory movement in 3- dimensions, we fabricated a dynamic platform by combining a motorized jack and a commercial moving platform (Model 008PL, CIRS, USA). For each plan, we repeated the irradiation three times, oscillating the phantom with amplitudes of 0 mm, 5 mm and 10 mm, respectively. The period was fixed at 3 seconds. Results In most cases, the gamma pass rate evaluated from the films with the oscillation against no oscillation significantly reduced, and the pass rate decreased as the amplitude of respiration increased. Exceptionally, one of cases irradiated by VMAT presented the opposite result that the pass rate increased as the amplitude increased. In two- field cases, the mean gamma pass rates of 5mm and 10mm amplitudes against the zero amplitude were 70.9% and 30.7%, respectively. Meanwhile, in VMAT cases, they were 68.4% and 65.3%, respectively. The error of point doses measured by the dosimetric elements was increased as the amplitude increased in all cases. The mean errors of 5mm and 10mm amplitudes were 2.9% and 4.7%, respectively (two-field case), and were 3.7% and 6.5%, respectively (VMAT case). Conclusion This study demonstrates that the interplay effect of respiratory motion to dose distribution in IMN areas is not negligible. Differences between static and moving deliveries were most prominent in tangential 2-field plan in large amplitude respiration. In the case of large respiration movement, the delivered dose of the tangential treatment could be compromised significantly compared to that of VMAT. PO-1407 Sensitivity of Octavius 4D system in detecting systematic or incidental MLC positioning errors C.L. Ong 1 , M. Alimohamad 1 , E. Franken 1 1 Hagaziekenhuis, Radiotherapy, Den Haag, The Netherlands Purpose or Objective Modern radiotherapy techniques often use the multileaf collimator (MLC) to create highly conformal dose distributions. Therefore, the MLC leaf positioning accuracy during treatment delivery is very important. Any missteps during preparation such as transfer error or machine miscalibration could lead to MLC leaf mispositioning, and in turn, dosimetric deviation. This study aims to examine the possibility to detect any dosimetric deviation caused by systematic or incidental MLC positioning errors using Octavius 4D system (PTW). Material and Methods A lung IMRT treatment plan and a SRS dynamic arc brain metastases plan, both optimized using Pinnacle TPS, were included in this study. For both plans, incidental and systematic faults were introduced by misplacing one MLC leaf or all MLC leaves of the entire leaf bank by 1-3mm, respectively. All plans were delivered on an Elekta Synergy linac with Agility collimator. Patient QA of treatment plans with large fields (long IMRT) were carried out using Octavius 4D 1500 system while SRS plans were measured using Octavius 4D 1000 SRS system. The detector size of both systems are 0.06 and 0.003cc, respectively. The 3D- measured doses were analyzed using Verisoft software with gamma criterion of dose difference ranging from 2-5%

Conclusion Incidental MLC positioning errors might go undetected when patient QA is carried out using the Octavius 4D 1500 system. However, for treatments with large fields that require larger margin, the clinical dosimetric impact of such incidental errors might be insignificant. In contrast, this impact could be profound in SRS treatment which usually apply margins of only 1-2mm. The Octavius SRS 1000 is capable of detecting such small positioning errors when a DTA of 1mm is use for analysis. The use of a larger DTA could potentially mask the present of small MLC positioning errors in SRS plan. PO-1408 End-to-end empirical validation of 3D dose distributions using an anthropomorphic pelvis phantom M. Marot 1 , A. Elter 1 , P. Mann 1 , A. Schwahofer 1 , C. Lang 1 , W. Johnen 1 , S.A. Körber 2 , B. Beuthien-Baumann 3 , C. Gillmann 1 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 University Hospital Heidelberg, Radiation Oncology and Radiotherapy, Heidelberg, Germany ; 3 German Cancer Research Center DKFZ, Radiology, Heidelberg, Germany Purpose or Objective To develop an end-to-end test for empirical validation of 3D dose distributions in prostate and rectum using an anthropomorphic pelvis phantom.