Pourandokht Naseri

Initial investigation of a novel light-scattering gel phantom for evaluation of optical CT scanners for radiotherapy gel dosimetry

There is a need for stable gel materials for phantoms used to validate optical computerized tomography (CT) scanners used in conjunction with radiation-induced polymerizing gel dosimeters. Phantoms based on addition of light-absorbing dyes to gelatine to simulate gel dosimeters have been employed. However, to more accurately simulate polymerizing gels one requires phantoms that employ light-scattering colloidal suspensions added to the gel. In this paper, we present the initial results of using an optical CT scanner to evaluate a novel phantom in which radiation-exposed polymer gels are simulated by the addition of colloidal suspensions of varying turbidity. The phantom may be useful as a calibration transfer standard for polymer gel dosimeters. The tests reveal some phenomena peculiar to light-scattering gels that need to be taken into account when calibrating polymer gel dosimeters.

Mechanisms for Covalent Immobilization of Horseradish Peroxidase on Ion-Beam-Treated Polyethylene

The surface of polyethylene was modified by plasma immersion ion implantation. Structure changes including carbonization and oxidation were observed. High surface energy of the modified polyethylene was attributed to the presence of free radicals on the surface. The surface energy decay with storage time after treatment was explained by a decay of the free radical concentration while the concentration of oxygen-containing groups increased with storage time. Horseradish peroxidase was covalently attached onto the modified surface by the reaction with free radicals. Appropriate blocking agents can block this reaction. All aminoacid residues can take part in the covalent attachment process, providing a universal mechanism of attachment for all proteins. The native conformation of attached protein is retained due to hydrophilic interactions in the interface region. The enzymatic activity of covalently attached protein remained high. The long-term activity of the modified layer to attach protein is explained by stabilisation of unpaired electrons in sp2 carbon structures. A high concentration of free radicals can give multiple covalent bonds to the protein molecule and destroy the native conformation and with it the catalytic activity. The universal mechanism of protein attachment to free radicals could be extended to various methods of radiation damage of polymers.

Optimisation of exposure conditions for in vitro radiobiology experiments.

Despite the long history of using cell cultures in vitro for radiobiological studies, there is to date no study specifically addressing the dosimetric implications of flask selection and exposure environment in clonogenic assays. The consequent variability in dosimetry between laboratories impedes the comparison of results. In this study we compare the dose to cells adherent to the base of three types of commonly used culture flasks or plates. The cells are exposed to a 6MV clinical photon beam using either an open or a half blocked field. The depth of medium in each flask is varied with the medium surrounding the flask either water or air. The results show that the dose to the cells is more affected by the scattering conditions surrounding the flasks than by the level of filling within the flask. It is recommended that water or a water equivalent phantom material is used to surround the flasks or plates to approximate full scatter conditions at the cell layer. However for modulated fields, surrounding the 24 well plates with water-equivalent material is inadequate because of the large volume of air surrounding individual wells. Our results stress the importance of measuring the dose for new experimental configurations.

Scintillation dosimeter arrays using air core light guides: simulation and experiment

The performance of a scintillation dosimeter that uses a silvered air core light guide is examined by Monte Carlo (MC) simulations and by experiment to determine its suitability for array dosimetry in external beam radiotherapy. The air core light guide avoids the generation of the Cerenkov background that is produced in a conventional optical fibre. MC simulations using a 6 MV photon beam showed that silver thicknesses of less than 1 microm compensated for the effects of the other material components, to give the dosimeter water equivalence within 0.5%. A second dosimeter located adjacent to the primary dosimeter in any direction affected the dose measurement by less than 1.5%, when the centre-to-centre spacing was 1.3 mm or greater. When the dosimeter array is located perpendicular to the beam central axis, with a spacing of 2.5 mm, the calculated deviation from the dose deposited in water was less than 2%. When the dosimeter array is located parallel to the beam central axis with a spacing of 10 mm, the calculated dose readings deviated from water by less than 2.5%. The simulation results were confirmed with experiment for two neighbouring dosimeters and a small densely packed array. No proximity effects were measured within the experimental error of +/-1.5%. These results confirm the dosimetric accuracy of the air core dosimeter design without the need for correction factors. The dosimeter has excellent potential for use in arrays.

Initial investigation of a novel phantom for simulating optical scattering of dosimetric gels

To evaluate the performance of an optical CT scanner for scanning dosimetric gels, it is useful to have a gel phantom in which the 3-dimensional shapes and extinction (attenuation) coefficients of the internal features can be manufactured at will. We describe and evaluate an improved version of the Oldham phantom, employing scattering gels.

An Air Core Cerenkov Free Scintillation Dosimeter: Monte Carlo Simulation for External Beam Radiation Therapy

We examine the water equivalent properties of two versions of a new type of scintillation optical fibre dosimeter designed for use in external beam applications. The dosimeter has an air core light guide to avoid the generation of a Cerenkov background. The dosimeter has a small sensitive volume and has been designed for use in high dose gradients. One version has a silver coating on the scintillator, while the other has no silver coating. A Monte Carlo (MC) method is used to determine the impact of various structures of the dosimeter on the dose deposited in the sensitive volume. MC is also used to determine the effect of a nearby dosimeter of the same type on the dose deposited, in order to determine the suitability of the dosimeter for array dosimetry. The theoretical predictions are compared to experimental measurements in a 6MV photon beam. We show that the dosimeter is effectively water equivalent when no silver is used on the scintillator, with absolute dose readings in agreement with the control values. When normalised at their maximum, the shape of all depth dose curves is the same as for the water equivalent control. We also show that dosimeter proximity effects are negligible, confirming the suitability of this dosimeter for use in close packed arrays.

Array of square waveguides for scintillation dosimetry in external radiotherapy

An array of air core scintillation dosimeters (of round or square cross section) is an efficient solution for managing the problem of Cerenkov background light in megavoltage radiation. This array generates a high-resolution dose map in a way that satisfies ICRU dosimetric accuracy recommendations without the need for correction factors. Efficient scintillation signal transportation is vital to sensitivity of the dosimeter. The attenuation of the light irradiance as a function of waveguide length in PMMA and silver hollow square and round waveguides is studied experimentally and theoretically. In practice, the silvered square waveguide has the least attenuation while the PMMA square waveguide performs almost as well as commercially sourced silvered tubes. The attenuation of the commercially sourced tubes is increased by the rough internal silver surfaces.

Light propagation in multimoded square hollow waveguides

The irradiance–distance relation for light propagation in multimoded hollow waveguides is calculated from theory and compared with measurement for visible light. These are the first measurements of this relation for square waveguides and the first for round waveguides for visible light. The square waveguides had either silvered glass surfaces or uncoated PMMA surfaces. A geometric optics method based on image formation in a kaleidoscope is used for calculations. The measurements agreed with the predictions of theory in all cases. The loss is not described by a fixed value of an attenuation coefficient as the angular divergence of the light changes with distance. At larger distances the logarithm of the irradiance is linearly dependent on the logarithm of distance, regardless of material type, surface properties and waveguide cross section. The silvered square waveguide shows the lowest measured attenuation while PMMA square waveguides constructed without any special surface preparation performed almost as well as silvered round tubes sourced commercially. The surface roughness present in the silver coating of commercially sourced round tubes compromised their performance. An array of square PMMA waveguides was used to demonstrate an application for signal transport in megavoltage radiation fields for profiling a medical radiation beam and measuring the variation of dose rate with depth in water.

Air core metallic light guides for scintillation dosimetery in radiotherapy

Cerenkov radiation introduces an unwanted background in scintillation dosimeters coupled to an optical fibre used for megavoltage radiotherapy, causing inaccuracies. We analyse propagation in air core light guides which do not suffer from this problem.

SU‐GG‐T‐282: Investigation of Optical Artifacts in a Complex Light‐Scattering Gel Phantom for Optical CT Polymer Gel Dosimetry

Introduction: A test phantom of complex shape was developed to simulate artefacts in dosimetric polymer gels (e.g. PAGAT [1]) when read using cone beam optical CT. Here we report the results of modelling of experimental data using this phantom. Method and Materials: Using a method reported earlier [2], opacity change in an exposed dosimetric gel was simulated by adding known concentrations of Dettol™ to gelatin. Different doses were simulated with gelatin of different opacity (optical density, OD). The phantom was contained is a cylindrical PETE vessel (9.5 cm diameter, 13 cm high). It consisted of an outer mantle of pure gelatin (“zero dose”) surrounding a central, vertical hollow finger. The hollow finger was constructed of 10 transverse layers 15 mm thick of increasing OD, the uppermost layer having the lowest OD. The cavity through the finger centre was filled with a pith of pure “zero dose” gelatin. Inclusion of this central pith simulates a treatment where a high dose region surrounds a low dose region. The phantom was scanned with a Vista Optical CT Scanner (Modus Medical Devices Inc.). Horizontal profiles of OD versus position through each of the 10 layers were obtained from the reconstruction (resolution; 0.25 mm per voxel). Results and Discussion: Consistent with earlier work [1], higher OD slices exhibited dishing artefacts — inner regions within the slice exhibiting an apparent reduction in OD, similar to dishing artefacts in X‐ray CT. More interestingly, the central “zero dose” pith exhibited spurious increases in OD. These effects are modelled by taking into account the back‐reflection of scattered light.