Correlation functions for Elekta aSi EPIDs used as transit dosimeter for open fields

Abstract

In‐vivo dosimetry techniques are currently being applied only by a few Centers because they require time‐consuming implementation measurements, and workload for detector positioning and data analysis. The transit in‐vivo dosimetry performed by the electronic portal imaging device (EPID) avoids the problem of solid‐state detector positioning on the patient. Moreover, the dosimetric characterization of the recent Elekta aSi EPIDs in terms of signal stability and linearity make these detectors useful for the transit in‐vivo dosimetry with 6, 10 and 15 MV photon beams. However, the implementation of the EPID transit dosimetry requires several measurements. Recently, the present authors have developed an in‐vivo dosimetry method for 3D CRT based on correlation functions defined by the ratios between the transit signal, normalsnormalt(w,L), by the EPID and the phantom midplane dose, normalDnormalm(w,L), at the source to axis distance (SAD) as a function of the phantom thickness, w, and the square field dimensions, L. When the phantom midplane was positioned at distance, d, from the SAD, the ratios normalsnormalt(w,L)/snormalt'(d,w,L) were used to take into account the variation of the scattered photon contributions on the EPID as a function of d and L.The aim of this paper is the implementation of a procedure that uses generalized correlation functions obtained by nine Elekta Precise linac beams. The procedure can be used by other Elekta Precise linacs equipped with the same aSi EPIDs, assuming the stabilities of the beam output factors and the EPID signals. The procedure here reported avoids measurements in solid water equivalent phantoms needed to implement the in‐vivo dosimetry method in the radiotherapy department. A tolerance level ranging between ±5% and ±6% (depending on the type of tumor) was estimated for the comparison between the reconstructed isocenter dose, Diso, and the computed dose, Diso,TPS, by the treatment planning system (TPS).PACS number: 87.55.Qr; 87.56.Fc

Initial clinical experience with Epid-based in-vivo dosimetry for VMAT treatments of head-and-neck tumors

“…The linac is equipped with Elekta IviewGT EPID, based on aSi panels XRD 1640 AL5 (PerkinElmer Optoelectronics, Fremont, CA, USA) operating at the fixed source-EPID distance (SED) equal to 160 cm. The IviewGT characteristics are discussed in [15]. An integrated transit signal St(x,y) can be obtained by the following equation:…”

“…In recent years, the authors developed a generalized procedure for the isocenter dose reconstruction of 3D-CRT treatments (Diso), based on the correlation functions between EPID transit signals and the doses at phantom mid-plane [11][12][13][14][15]. This research led to the foundation of an Italian national project financed by the Istituto Nazionale di Fisica Nucleare (INFN) and the Diso software was extensively applied in clinical routine in several Italian centers.…”

Two‐dimensional in vivo dose verification using portal imaging and correlation ratios

“…Further work has enabled this group to implement their in vivo dose-at-isocenter verification in various centers in Italy (29) and to apply it in numerous treatment situations. (30)(31)(32) We expanded the work by Piermattei et al to calculate 2D dose maps in an anthropomorphic phantom during three-dimensional conformal RT (3D CRT). To this aim, we had to characterize the off-axis variability of the correlation ratios F, here defined as the ratios of the EPID signals and the TPS doses in equivalent water phantom midplanes.…”

Epid‐based in vivo dose verification for lung stereotactic treatments delivered with multiple breath‐hold segmented volumetric modulated arc therapy

“…At our institution, we developed a generalized procedure for the daily in‐vivo isocenter dose reconstruction of radiation treatments, leading to the foundation of an Italian national project financed by the Istituto Nazionale di Fisica Nucleare and to the development of the SOFTDISO software (Best Medical Italy, Italy). The analysed results for conformal radiotherapy reported clinically relevant differences between planned and delivered dose, detecting the presence of dose discrepancies in more than 10% of the tests with respect to our tolerance levels .…”