Accurate two‐dimensional IMRT verification using a back‐projection EPID dosimetry method

Abstract

The use of electronic portal imaging devices (EPIDs) is a promising method for the dosimetric verification of external beam, megavoltage radiation therapy-both pretreatment and in vivo. In this study, a previously developed EPID back-projection algorithm was modified for IMRT techniques and applied to an amorphous silicon EPID. By using this back-projection algorithm, two-dimensional dose distributions inside a phantom or patient are reconstructed from portal images. The model requires the primary dose component at the position of the EPID. A parametrized description of the lateral scatter within the imager was obtained from measurements with an ionization chamber in a miniphantom. In addition to point dose measurements on the central axis of square fields of different size, we also used dose profiles of those fields as reference input data for our model. This yielded a better description of the lateral scatter within the EPID, which resulted in a higher accuracy in the back-projected, two-dimensional dose distributions. The accuracy of our approach was tested for pretreatment verification of a five-field IMRT plan for the treatment of prostate cancer. Each field had between six and eight segments and was evaluated by comparing the back-projected, two-dimensional EPID dose distribution with a film measurement inside a homogeneous slab phantom. For this purpose, the y-evaluation method was used with a dose-difference criterion of 2% of dose maximum and a distance-to-agreement criterion of 2 mm. Excellent agreement was found between EPID and film measurements for each field, both in the central part of the beam and in the penumbra and low-dose regions. It can be concluded that our modified algorithm is able to accurately predict the dose in the midplane of a homogeneous slab phantom. For pretreatment IMRT plan verification, EPID dosimetry is a reliable and potentially fast tool to check the absolute dose in two dimensions inside a phantom for individual IMRT fields. Film measurements inside a phantom can therefore be replaced by EPID measurements.

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

“…The variability of the F factor is due to multiple effects. Most importantly, moving from the central axis (CAX) to the field edge, the dose decreases more rapidly than the signal due to a major loss of lateral electronic equilibrium in the phantom, thus causing F to increase 12 , 33 . A lesser effect is due to the flattening filter which causes beam hardening closer to the CAX.…”

“…The detector, being nonwater‐equivalent, has a response that is oversensitive to photons of lower energy, 34 , 35 producing a higher response for the same dose when farther from the CAX. This effect is actually overcompensated by the flood field correction (acquired with an empty beam), which does not account for the variation in beam attenuation caused by in‐patient hardening (12) . Because these two effects are most likely accelerator‐ and EPID‐dependent, we chose to account for them by an empirical, rather than model‐based, approach.…”

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

“…The complexity of the systems involved requires new and appropriate methods and potentially new tools in order to do this. Several devices dedicated to daily QA have been developed that can be used for IMRT and VMAT verification 6 , 7 , 8 , 9 , 10 , 11 , 12 …”

Impact of backscattered radiation from the bunker structure on EPID dosimetry

“…The response of amorphous silicon (aSi) EPIDs is reproducible over short and long periods of time, 8 , 10 and is linearly related to dose 8 , 11 , 12 . They have been used for verification of IMRT treatment plans 13 , 14 and for in vivo dosimetry measurements 15 , 16 . However, like any other dosimetry system, EPIDs have their own drawbacks.…”