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, st(w,L), by the EPID and the phantom midplane dose, Dm(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 st(w,L)/st'(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
Highlights
219 Cilla et al.: Elekta amorphous silicon (aSi)-electronic portal imaging devices (EPIDs) used as transit dosimeterplanning system (TPS)
The increasing complexity of techniques in radiotherapy requires an accurate verification of the dose delivered to the patient, and several studies have addressed reconstruction of dose delivered to the patient during the treatment by means of electronic portal imaging devices (EPIDs).(2) When compared to the traditional EPIDs such as fluoroscopic screen/camera-based and liquid-filled matrix ionization chambers, the new generation of EPIDs, equipped with amorphous silicon flat panels, supply more stable transit signals and are suitable as transit detectors
The authors have developed an in-vivo dosimetry method based on correlation functions defined by the ratios between the transit signal, st, measured by aSi EPIDs, and the solid water phantom midplane doses, Dm, measured by an ion chamber positioned along the central axis.(3)
Summary
219 Cilla et al.: Elekta aSi-EPIDs used as transit dosimeterplanning system (TPS). Discrepancies may be due to possible errors from previous steps in the radiotherapy process such as errors in the data transfer from the TPS to the radiotherapy unit, errors in the functioning of the treatment equipment, and errors in the accuracy of the dose calculation algorithms employed by the TPS, in addition to errors due to the patient setup or patient morphology changes.A standard in-vivo dosimetry technique is based on the entrance dose reconstruction using a solid-state detector on the patient surface.(1) These in-vivo dosimetry techniques are generally applied only for an initial check because they require workload for detector positioning and corrections for their X-ray fluence absorption.The increasing complexity of techniques in radiotherapy requires an accurate verification of the dose delivered to the patient, and several studies have addressed reconstruction of dose delivered to the patient during the treatment by means of electronic portal imaging devices (EPIDs).(2) When compared to the traditional EPIDs such as fluoroscopic screen/camera-based and liquid-filled matrix ionization chambers, the new generation of EPIDs, equipped with amorphous silicon (aSi) flat panels, supply more stable transit signals and are suitable as transit detectors. The authors have developed an in-vivo dosimetry method based on correlation functions defined by the ratios between the transit signal, st, measured by aSi EPIDs, and the solid water phantom midplane doses, Dm, measured by an ion chamber positioned along the central axis.(3). The aim of the present paper is the determination of the generalized correlation functions for the recent Elekta IviewGT aSi EPIDs needed to implement an in-vivo transit dosimetry method for the 3D conformed radiotherapy (3DCRT). Use of these correlation functions avoids the need for measurements in solid water phantom
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