ESTRO 2021 Abstract Book

S1355

ESTRO 2021

F. Biltekin 1 , G. Yazici 1 , G. Ozyigit 1 1 Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey

Purpose or Objective To evaluate the feasibility of electronic portal imaging device (EPID)-based 3D in-vivo dosimetry system for monitoring relevant interfractional variations in head and neck radiotherapy. Materials and Methods In the present study, the results of the 3D in vivo dose measurements for the first five fractions of eight head and neck patients treated with double arc VMAT technique were analyzed retrospectively. All treatments were performed in Elekta Versa HD linear accelerator and iViewDose™ software v. 1.0.1 working in conjunction with the existing EPID iViewGT v.3.4.1 MV panel was used for 3D in-vivo dose measurements. During the analysis, 5 mm distance to agreement (DTA) and 5% dose differences (DD) was used as a gamma evaluation criteria. Gamma passing threshold (%γ ≤ 1) was set as 90% for 3D per fraction analysis. Additionally, percentage dose difference at dose reference point (ΔDRP) (threshold ≤ 3% for protocol and ≤ 4% for minor variation), γ mean (threshold ≤ 0.5 for protocol and ≤ 0.7 for minor variation) and γ 1% (threshold ≤ 2 for protocol and ≤ 2.5 for minor variation) parameters were evaluated for all fractions. Results As presented in Table 1, mean value of the ΔDRP between calculated and measured dose was within 3.66 % at isocenter. When we analyzed the mean of the total gamma passing rate for the first five fractions, all values were inside of the clinical tolerance level (> 90%) except for one patient (88.9% for patient #8). The reason of the dose differences were found to be associated with shoulder position in the 2 nd fraction of treatment. Gamma analysis and dose line profile for the 1 st and 2 nd fractions of the patient #8 were illustrated in Figure 1.

Conclusion The presented approach provides fast and accurate in-vivo EPID dosimetry and it was found to be sensitive to detect potential errors due to the shoulder positions in head and neck radiotherapy.