40. Validation of a transit EPID device for a clinical use: application to iViewDose (Elekta)

Abstract

Introduction IMRT/VMAT and stereotactic treatments require patient specific quality assurance before and during treatment. In that context, Electronic Portal Imaging Device (EPID) can be used for pre-treatment and in vivo verification. The aim of this study was to evaluate a new in vivo portal dosimetry tool iViewDose (Elekta) for a clinical use. Methods The study was performed on a VersaHD LINAC equipped with Agility collimator and EPID iView v.3.4.1 (Elekta). iViewDose v.1.0.1 (Elekta) software allows users to reconstruct delivered dose by the LINAC in 3D on the planning CT scan using EPID images. Treatment plans were calculated with Pinnacle v.9.10 (Philips) for VMAT technique. Agreement between calculation and measurement was evaluated with gamma index analysis in 3D ( γ 3D, criteria: 3% global/3 mm, threshold 50%). Detector validation was realized in two steps: (i) detector intrinsic characteristics evaluation (repeatability, reproducibility, linearity, dose rate dependence) and (ii) evaluation of error detectability for two treatment plan complexities (prostate and Head & Neck) delivered on an anthropomorphic phantom with: leaves shift from 1 to 5 mm, collimator rotation from 1 to 5° and MU increase from 1% to 5%. The in vivo agreement between calculation and measurement was investigated for nine patients (H&N, liver, esophagus, iliac bone) corresponding in a total of 86 fractions. According to the obtained results, a parallel analysis with CBCT images was carried out. Results Dose response of the detector was linear (R2 = 1). A x ¯ / σ of 0.06% was obtained for measurements performed with a dose rate varying from 70 to 500 MU/min. For a same irradiation, reconstructed dose was repeatable ( x ¯ / σ = 0.07%) and reproducible ( x ¯ / σ = 0.2%). For H&N treatment plan, iViewDose could detect a 1 mm bank shift ( γ 3D percentage decreased from 91.7% to 67.6%), a 2° collimator rotation ( γ 3D percentage decreased to 76.8%) and an increase of 1% MU ( γ 3D percentage decreased to 84.6%). A two bank shift in the same direction was not detected. Similar results were obtained for prostate treatment plans. In vivo calculation/measurement coherence for five patients (liver, esophagus, iliac bone) showed on average a γ 3D percentage of 90.9% (30 fractions). Those measurements showed punctually a wrong matching for a liver treatment case near diaphragm ( Fig. 1 b). For H&N treatment plans, in vivo calculation/measurement coherence is deteriorated during treatment: 84.4% of points have a gamma Fig. 1 a). Conclusions This study shows the iViewDose capability to detect LINAC and patient potential errors. With that kind of detector, it becomes necessary to correlate those results to a clinical impact and to define action thresholds justifying treatment re-planning. It could be very useful in an adaptive radiotherapy and would be more accurate with dose calculation on CBCT images of the day.