The aim of this study was to investigate the over-response of an inorganic optical fibre sensor (OFS) when measuring percentage depth dose curves (PDDs) with respect to an ion chamber by means of physical measurements and Monte Carlo (MC) simulations. The sensor was constructed by filling a cavity (700 μm diameter and 7 mm deep), which was made in a PMMA (polymethyl methacrylate) plastic optical fibre, with an inorganic scintillating material: terbium doped gadolinium oxysulphide (Gd2O2S:Tb). The MC software packages BEAMnrc and DOSXYZnrc were used to develop a MC model of an Elekta Versa HD linear accelerator (linac), which was then used to simulate the Gd2O2S:Tb scintillator. The results of the PDD measurements showed a depth dependence of the OFS, however the percentage differences between the ion chamber and the OFS measurements showed that as the radiation field size decreases, the difference between the two measurements decreases from 16.5% to 5.1% for 10x10 cm2 and 2x2 cm2, respectively. The MC simulation of the sensor showed a good agreement compared to physical measurements at shallow depth in the phantom; however, discrepancies were observed at depth, which was less pronounced for 4x4 cm2 than for 10x10 cm2. The results of this study indicate that including Cerenkov radiation measurements is essential to accurately quantify the overresponse and the higher discrepancy between the measured and simulated PDD profiles of the OFS.
A revised version of this paper, published originally on 2016, was published on 6 January, 2017, replacing the original paper. To correct for a set-up error, the simulated treatment plan was re-run using CT images of a solid water phantom with/without the Autoscan US probe in contact with its surface. The revised paper is available at http://dx.doi.org/10.1117/12.2216653.
Changes to original text:
1. A solid water phantom is employed as opposed to a Rando phantom.
2. Figure 1 is changed to illustrate new phantom.
1. New treatment isocentre.
2. Figure 3 is changed to illustrate dose distribution in new phantom.
1. Text of first paragraph has been revised to reflect new results.
2. Table 2 has been revised to reflect new results.
3. Figure 6 has been revised to reflect new results.
1. Text of second paragraph has been revised to reflect new results.
The aim of this study was to quantify the dosimetric effect of the AutoscanTM ultrasound probe, which is a 3D transperineal probe used for real-time tissue tracking during the delivery of radiotherapy. CT images of a solid water phantom, with and without the probe placed in contact with its surface, were obtained (0.75 mm slice width, 140 kVp). CT datasets were used for relative dose calculation in Monte Carlo simulations of a 7-field plan delivered to the phantom. The Monte Carlo software packages BEAMnrc and DOSXYZnrc were used for this purpose. A number of simulations, which varied the distance of the radiation field edge from the probe face (0 mm to 5 mm) were performed. Perineal surface doses as a function of distance from the radiation field edge, with and without the probe in place, were compared. The presence of the probe was found to result in negligible dose differences when the radiation field is not delivered through the probe. A maximum surface dose increase of ≈1% was found when the probe face to field edge distance was 0 mm. Surface doses with and without the probe in place agreed within Monte Carlo simulation uncertainty at distances ≥ 3 mm. Using data from three patient volunteers, a typical probe face to field edge distance was calculated to be ≈20 mm. Our results therefore indicate that the presence of the probe does not adversely affect a typical patient treatment, due to the relatively large probe face to field edge distance.
This work outlines the development of a multi-pinhole SPECT system designed to produce a synthetic-collimator image of a small field of view. The focused multi-pinhole collimator was constructed using rapid-prototyping and casting techniques. The collimator projects the field of view through forty-six pinholes when the detector is adjacent to the collimator. The detector is then moved further from the collimator to increase the magnification of the system. The amount of pinhole-projection overlap increases with the system magnification. There is no rotation in the system; a single tomographic angle is used in each system configuration. The maximum-likelihood expectation-maximization (MLEM) algorithm is implemented on graphics processing units to reconstruct the object in the field of view. Iterative reconstruction algorithms, such as MLEM, require an accurate model of the system response. For each system magnification, a sparsely-sampled system response is measured by translating a point source through a grid encompassing the field of view. The pinhole projections are individually identified and associated with their respective apertures. A 2D elliptical Gaussian model is applied to the pinhole projections on the detector. These coefficients are associated with the object-space location of the point source, and a finely-sampled system matrix is interpolated. Simulations with a hot-rod phantom demonstrate the efficacy of combining low-resolution non-multiplexed data with high-resolution multiplexed data to produce high-resolution reconstructions.
We have recently developed a digital x-ray image receptor for use in mammographic procedures. The detector is based upon a photoconductor, amorphous selenium (a-Se), coupled to a polymer dispersed liquid crystal (PDLC) layer. A potential is applied across the structure to create a bias electric field in the photoconductor. When x-rays are absorbed in the photoconductor, electron-hole pairs are released. The created charges are swept to the a-Se /PDLC interface via the applied electric field, which causes potential variations across the PDLC. These variations lead to liquid crystal (LC) molecule re-orientation, which affects the propagation of readout light from an external source through the display. The readout light can be bright in this arrangement so that no secondary quantum sinks are present. Since this system is independent of light creation in contrast with a phosphor screen system, the image brightness can be adjusted independently of the number of x-rays used to make the image. The image can be digitized with a CCD camera and a frame grabber. Results will be presented on the PDLC characteristics, the system model and initial images from the detector.