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Item does not contain fulltext PURPOSE: At the Netherlands Cancer Institute--Antoni van Leeuwenhoek Hospital in vivo dosimetry using an electronic portal imaging device (EPID) has been implemented for almost all high-energy photon treatments of cancer with curative intent. Lung cancer treatments were initially excluded, because the original back-projection dose-reconstruction algorithm uses water-based scatter-correction kernels and therefore does not account for tissue inhomogeneities accurately. The aim of this study was to test a new method, in aqua vivo EPID dosimetry, for fast dose verification of lung cancer irradiations during actual patient treatment. METHODS: The key feature of our method is the dose reconstruction in the patient from EPID images, obtained during the actual treatment, whereby the images have been converted to a situation as if the patient consisted entirely of water; hence, the method is termed in aqua vivo. This is done by multiplying the measured in vivo EPID image with the ratio of two digitally reconstructed transmission images for the unit-density and inhomogeneous tissue situation. For dose verification, a comparison is made with the calculated dose distribution with the inhomogeneity correction switched off. IMRT treatment verification is performed for each beam in 2D using a 2D gamma evaluation, while for the verification of volumetric-modulated arc therapy (VMAT) treatments in 3D a 3D gamma evaluation is applied using the same parameters (3%, 3 mm). The method was tested using two inhomogeneous phantoms simulating a tumor in lung and measuring its sensitivity for patient positioning errors. Subsequently five IMRT and five VMAT clinical lung cancer treatments were investigated, using both the conventional back-projection algorithm and the in aqua vivo method. The verification results of the in aqua vivo method were statistically analyzed for 751 lung cancer patients treated with IMRT and 50 lung cancer patients treated with VMAT. RESULTS: The improvements by applying the in aqua vivo approach are considerable. The percentage of gamma values

Contains fulltext : 50013.pdf (Publisher’s version ) (Closed access) 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.

For each radiation treatment machine, the dose per monitor unit must be known at a reference point in a water-phantom under reference conditions. It varies with field size, due to: ( 1) changes in radiation scattered from the head of the treatment machine to the reference point and into the monitor chamber and ( 2) changes in the radiation scattered from the irradiated part of the phantom to the reference point. The field size dependence is usually described by the total scatter correction factor, Scp• If the field size defined by the collimator setting at the reference source-surface distance ( SSD) does not correspond with the field size at the phantom surface, Sec must be separated into two factors, the head or collimator scatter correction factor, Sc, and the phantom scatter correction factor, Sc. This is the case when a source-surface distance different from the SSD of the reference condition is used, when tissue is missing in the beam, or when shielding blocks are applied. The factor Sc describes the influence of the setting of the collimator on the total scatter correction factor; Sp describes the influence of the field size at the phantom surface, and thus of the irradiated volume on this correction f actor. In this report, recommendations are given for the measurement of Scp, Sc and Sp in a photon beam at a reference depth of 10 cm. The reference field is defined as the open field, with a collimator setting yielding a 10 cm x 10 cm field when the SSD is set equal to the source-axis distance ( SAD) . For the determination of Sc, the use of a narrow cylindrical beam-coaxial phantom (the mini-phantom) is recommended. In this way, measurements can be performed in small fields and the disturbance of contaminating electrons, reaching the point of interest from the head of the treatment machine, becomes negligible. Construction details of the mini-phantom are described. Furthermore, a consistent set of relations is presented for the use of these factors in dose calculations for symmetric ally collimated square, rectangular and arbitrarily shaped fields, at an arbitrary SSD. These include blocked and wedged fields. A procedure to use Sc and Sp data in the calculation of monitor units is presented. Relations are given to calculate the collimator and phantom scatter data for the reference situation from already available data, measured at a non­ reference depth and at a non-reference SSD. The influence of asymmetric set-up of the collimating jaws or multi-leaf collimators on the scatter correction factors is, however, not yet considered. Scp, Sc and Sp data sets are presented in this report for different types of treatment machines and for a wide range of photon beams, with beam qualities ranging from 6°Co to 25 MV. Sp is shown to be a smooth f unction of the beam quality if the same reference depth of 10 cm and an SSD of 100 cm is chosen for all beam qualities. However, Sc is shown to depend on the design of the head of the linear accelerator.

For each radiation treatment machine, the dose per monitor unit must be known at a reference point in a water-phantom under reference conditions. It varies with field size, due to: ( 1) changes in radiation scattered from the head of the treatment machine to the reference point and into the monitor chamber and ( 2) changes in the radiation scattered from the irradiated part of the phantom to the reference point. The field size dependence is usually described by the total scatter correction factor, Scp• If the field size defined by the collimator setting at the reference source-surface distance ( SSD) does not correspond with the field size at the phantom surface, Sec must be separated into two factors, the head or collimator scatter correction factor, Sc, and the phantom scatter correction factor, Sc. This is the case when a source-surface distance different from the SSD of the reference condition is used, when tissue is missing in the beam, or when shielding blocks are applied. The factor Sc describes the influence of the setting of the collimator on the total scatter correction factor; Sp describes the influence of the field size at the phantom surface, and thus of the irradiated volume on this correction f actor.In this report, recommendations are given for the measurement of Scp, Sc and Sp in a photon beam at a reference depth of 10 cm. The reference field is defined as the open field, with a collimator setting yielding a 10 cm x 10 cm field when the SSD is set equal to the source-axis distance ( SAD) . For the determination of Sc, the use of a narrow cylindrical beam-coaxial phantom (the mini-phantom) is recommended. In this way, measurements can be performed in small fields and the disturbance of contaminating electrons, reaching the point of interest from the head of the treatment machine, becomes negligible. Construction details of the mini-phantom are described.Furthermore, a consistent set of relations is presented for the use of these factors in dose calculations for symmetric ally collimated square, rectangular and arbitrarily shaped fields, at an arbitrary SSD. These include blocked and wedged fields. A procedure to use Sc and Sp data in the calculation of monitor units is presented. Relations are given to calculate the collimator and phantom scatter data for the reference situation from already available data, measured at a non­ reference depth and at a non-reference SSD. The influence of asymmetric set-up of the collimating jaws or multi-leaf collimators on the scatter correction factors is, however, not yet considered.Scp, Sc and Sp data sets are presented in this report for different types of treatment machines and for a wide range of photon beams, with beam qualities ranging from 6°Co to 25 MV. Sp is shown to be a smooth f unction of the beam quality if the same reference depth of 10 cm and an SSD of 100 cm is chosen for all beam qualities. However, Sc is shown to depend on the design of the head of the linear accelerator.