Radiological thickness measurement using a liquid ionization chamber electronic portal imaging device

The effect of respiratory motion on electronic portal imaging device dosimetry

There is an increasing need to develop methods for in vivo verification of the delivery of radiotherapy treatments. Electronic portal imaging devices (EPID's) have been demonstrated to be of use for this application. The basic principle is relatively straightforward, the EPID is used to measure a two-dimensional (2D) planar exit or portal dose map behind the patient during the treatment delivery that can provide information on any errors in linear accelerator output or changes in the patient anatomy. In this paper we focused on the effect of intra-fraction motion, particularly respiratory motion, on the measured 2D EPID dose-response. Measurements were made with a breast phantom undergoing one-dimensional (1D) sinusoidal motion with a range of amplitudes (0.5, 1.0, and 1.5 cm) and frequencies (12, 15, and 20  cycles/min). Further measurements were made with the phantom undergoing breathing sequences measured during patient planning computed tomography simulation. We made use of the quadratic calibration method that converts the EPID images to a surrogate for dose, equivalent thickness of Plastic Water^® . Comparisons were made of the 2D thickness maps derived for the different motions compared to the static phantom case and the resulting dose difference analyzed over the "breast" region of interest. A 2D gamma analysis within the same region of interest was performed of the motion images compared to static reference image. Comparisons were made of 1D thickness profiles for the moving and static phantom. The 1D and 2D analyses show the method to be sensitive to the smallest motion amplitude of 0.5 cm tested in the phantom measurements. The results using the phantom demonstrate the method to be a potentially useful tool for monitoring intra-fraction motion during the delivery of patient radiotherapy treatments as well as more generally providing information on the effects of motion on EPID based in vivo dosimetric verification.

A dual two dimensional electronic portal imaging device transit dosimetry model based on an empirical quadratic formalism

OBJECTIVE

This study describes a two dimensional electronic portal imaging device (EPID) transit dosimetry model that can predict either: (1) in-phantom exit dose, or (2) EPID transit dose, for treatment verification.

METHODS

The model was based on a quadratic equation that relates the reduction in intensity to the equivalent path length (EPL) of the attenuator. In this study, two sets of quadratic equation coefficients were derived from calibration dose planes measured with EPID and ionization chamber in water under reference conditions. With two sets of coefficients, EPL can be calculated from either EPID or treatment planning system (TPS) dose planes. Consequently, either the in-phantom exit dose or the EPID transit dose can be predicted from the EPL. The model was tested with two open, five wedge and seven sliding window prostate and head and neck intensity-modulated radiation therapy (IMRT) fields on phantoms. Results were analysed using absolute gamma analysis (3%/3 mm).

RESULTS

The open fields gamma pass rates were >96.8% for all comparisons. For wedge and IMRT fields, comparisons between predicted and TPS-computed in-phantom exit dose resulted in mean gamma pass rate of 97.4% (range, 92.3-100%). As for the comparisons between predicted and measured EPID transit dose, the mean gamma pass rate was 97.5% (range, 92.6-100%).

CONCLUSION

An EPID transit dosimetry model that can predict in-phantom exit dose and EPID transit dose was described and proven to be valid.

ADVANCES IN KNOWLEDGE

The described model is practical, generic and flexible to encourage widespread implementation of EPID dosimetry for the improvement of patients' safety in radiotherapy.

Correction for dose-response variations in a scanning liquid ion chamber EPID as a function of linac gantry angle

The response of electronic portal imaging devices (EPIDs) of the scanning liquid ionization chamber (SLIC) type is known to vary with linear accelerator gantry angle. This work considered several contributing factors, quantified the artefacts, monitored their reproducibility and investigated the effects of repeated gantry rotations. Unflatness of up to 5% was found. A correction technique was devised using nonlinear regression of a three-variable sinusoidal modulation. Comparison with two existing techniques found our method to be the most effective, providing a flatness well within 2%. This improved accuracy is expected to benefit more accurate dosimetric studies in particular. The post-acquisition correction process required no change in imaging protocols. Applicability of the new technique was demonstrated on images acquired on different days and with different beam sizes. Since the artefacts compromise both accurate dosimetry and image quality, their successful removal should benefit a broad range of SLIC EPID applications.

Three-dimensional dose reconstruction of breast cancer treatment using portal imaging

In this study, we present an algorithm for three-dimensional (3-D) dose reconstruction using portal images obtained with an electronic portal imaging device (EPID). For this purpose an algorithm for 2-D dose reconstruction, which was previously developed in our institution, was adapted. The external contour of the patient was used to correct for absorption of primary photons, but the presence of inhomogeneities was not taken into account. The accuracy of the algorithm was determined by irradiating two anthropomorphic breast phantoms with 6 MV photons. The dose values derived from portal images were compared with results from 3-D dose calculations, which, in turn, were verified with data obtained with an ionization chamber and film dosimetry. It was found that the application of contour information significantly improves the accuracy of 2-D dose reconstruction. If the total dose at the isocenter plane resulting from all treatment beams is reconstructed, the average deviation from the planned dose is 0.1%+/-1.7% (1 SD). If contour information is not available, the differences increase up to +/-20% for the individual beams. In that case, the dose can only be reconstructed with reasonable accuracy when (nearly) opposing beams are used. The average deviation of the 3-D reconstructed dose from the planned dose in the irradiated volume is 1.4%+/-5.4% (1 SD). If the irradiated volume is enclosed by planes less than 5 cm distant from the isocenter plane, then the average deviation is only 0.5%+/-3.4% (1 SD). It can be concluded that the proposed algorithm for a 3-D dose reconstruction allows a determination of the dose at the isocenter plane and the dose-volume histogram with an accuracy acceptable for an independent verification of the treatment.

Monte Carlo simulation of portal dosimetry on a rectilinear voxel geometry: a variable gantry angle solution

A software solution has been developed to carry out Monte Carlo simulations of portal dosimetry using the BEAMnrc/DOSXYZnrc code at oblique gantry angles. The solution is based on an integrated phantom, whereby the effect of incident beam obliquity was included using geometric transformations. Geometric transformations are accurate within +/- 1 mm and +/- 1 degrees with respect to exact values calculated using trigonometry. An application in portal image prediction of an inhomogeneous phantom demonstrated good agreement with measured data, where the root-mean-square of the difference was under 2% within the field. Thus, we achieved a dose model framework capable of handling arbitrary gantry angles, voxel-by-voxel phantom description and realistic particle transport throughout the geometry.

Full forward Monte Carlo calculation of portal dose from MLC collimated treatment beams

This work deals with a full Monte Carlo (MC) simulation of a radiotherapy treatment facility including a multi-leaf collimator (MLC) and electronic portal imaging device (EPID). A method for a planar calibration of the EPID response in terms of dose using the MC technique is presented. Calibration measurements and simulations with several blocks of attenuating material are carried out down to approximatively 5% of the open field transmitted dose. A linear relationship is shown between the squared EPID signal and the MC calculated dose. The calibrated EPID was used as a dosimetric system to validate a MC model for the MLC. Computations and measurements agreed within 2% of dose difference (or 2 mm in regions of high dose gradient). The technique described herein is not significantly limited by physics transport model constraints. Therefore it can potentially provide a more accurate verification of dose delivery to inhomogeneous anatomical regions in patients undergoing complex multi-field conformal or intensity-modulated radiation therapy.

Portal imaging

Portal imaging is the acquisition of images with a radiotherapy beam. Imaging theory suggests that the quality of portal images could be much higher if the efficiency of the imaging media in detecting radiation could be improved. Introduction of new media (films and electronic portal imaging devices) has confirmed this by markedly increasing the quality of portal images. Images from these devices can then be used to verify a patient's treatment. Geometric verification requires the portal image to be registered with a reference image. Dosimetric verification requires the portal imager to be calibrated for dose. This review gives a brief overview of the current areas of interest in portal imaging: imaging theory; imaging media, film and electronic portal imaging devices; image registration; and dosimetry using these devices.

Analytical calculation of the portal scatter to primary dose ratio: an EGS4 Monte Carlo and experimental validation at large air gaps

An analytical approximation for the scatter to primary dose ratio (SPR) on the central axis was validated against Monte Carlo results and experimental measurements for homogeneous and inhomogeneous phantoms. The analytical approximation only included first-order Compton scatter. The contribution to the total SPR from first-order Compton scatter, multiply scattered photons and electron scatter was investigated using Monte Carlo simulation for homogeneous phantoms (up to 30 cm thick for 6 and 18 MV beams; source to detector distances from 150 to 230 cm) as well as for a neck, thorax and pelvis phantom. SPRs were measured on the central axis with an ionization chamber for water phantoms (up to 20 cm thick at 4 MV, 30 cm for 6 MV and 10 MV and 40 cm for 18 MV; source to detector distances of 185 and 200 cm) and for phantoms representing the neck, thorax and pelvis (for air gaps of 50 cm and larger). The mean difference between the experimental and analytical SPRs on the central axis for source to detector distances of 170 cm or greater was within: -0.003 (neck); -0.012 (thorax); -0.028 (pelvis, 10 MV) and 0.008 (pelvis, 18 MV) respectively.

Reconstruction of megavoltage photon spectra from electronic portal imager derived transmission measurements

Three variants of the Schiff equation are investigated to model the spectra produced by megavoltage linear accelerators. These models are tested against well-validated Monte Carlo (MC) generated spectra on the central axis of large-area fields, and show excellent agreement. Numerical reconstructions of 6 and 10 MV spectra using the same models are then presented, using experimental attenuation data derived from an electronic portal imager. The process of deriving spectra from experimental attenuation data is shown to be inherently badly constrained mathematically, with the derived spectrum being highly sensitive to noise in the source data, and non-unique. By placing a priori constraints on the Schiff model from both physical knowledge of the construction of the accelerator and MC data, physically useful results are gained and presented for both the energy dependence and off-axis behaviour of photon spectra.