ESTRO 2021 Abstract Book

S1345

ESTRO 2021

Conclusion Exhibiting minimal directional dose-response dependence, in agreement with the corresponding literature, the nanoDots can provide precise dosimetry measurements allowing for the experimental verification of HT treatment plans. PO-1624 Dose Response characteristics and ghosting effect analysis for 3D EPID in-vivo dosimetry system F. Biltekin 1 , G. Ozyigit 1 1 Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey Purpose or Objective We aimed to investigate the dose response characteristics and impact of ghosting effects, namely image lag and change in sensitivity or gain, for EPID-based three dimesional (3D) EPID in-vivo dosimetry under clinical considiration. Materials and Methods All measurements were performed on Elekta Versa HD linear accelerator using EPID iViewGT v.3.4.1 MV panel and iViewDose software v.1.0.1 working in conjunction with the existing EPID panel. After image acquisition process, iViewDose software converts detected signal to the absolute dose value in the reconstruction plane estimated from CT data of the phantom or patients. Then, reconstructed dose distribution can be compared with calculated dose distribution using 3D gamma analysis method. In the present study, we performed two sets of measurement to evaluate dose response characteristics and impact of ghosting effects for 3D in-vivo dosimetry system. In the first part, treatment field size was set as 10 cm x 10 cm and measurements were carried out for different beam-on time including 1, 2, 5, 10, 25, 50, 100, 200, 500, 1000 monitor unit (MU). Linearity of the EPID response with changing beam-on time was analyzed using 3D gamma analysis method and point dose difference at 10 cm depth for defined field size. In the second part, measurements were performed to evaluate the impact of ghosting effect. The workflow for the measurement and evaluation can be divided into four steps, i) expose a small field 5 cm × 5 cm, ii) immediately expose a large field of 10 cm × 10 cm and this image was called as Image #1, iii) then, expose the same large field (10 cm × 10 cm) without any pre- irradiations and this image was called as Image #2, iv) Compare Image #1 and Image #2 to evaluate the increase in the signal in the center 5 cm × 5 cm of Image #1. During the analysis, gamma passing threshold (%γ ≤ 1) was set as 90% for 3D per fraction analysis. Additionally, percentage dose differences at dose reference point (ΔDRP), γ mean and γ 1% parameters were evaluated. Results As given in Table 1, dose response of 3D EPID-based in-vivo dosimetry system was almost linear with changing beam-on time except for 1 MU. However, maximum dose differences between calculated and measured at dmax was found as 1.98% for 1 MU irradiation. According to 3D gamma analysis method, %γ ≤ 1 was calculated greater than 90% for beam-on time greater than 2 MU. In terms of ghosting effect, the maximum increase in relative response between image #1 and image #2 was found less than 0.1% (Table 2).