Evaluation of the Accuracy of Polymer Gels for Determining Electron Dose Distributions in the Presence of Small Heterogeneities.

The aim of this study is to evaluate the application and accuracy of polymer gels for determining electron dose distributions in the presence of small heterogeneities made of bone and air. Different cylindrical phantoms containing MAGIC (Methacrylic and Ascorbic acid in Gelatin Initiated by Copper) normoxic polymer gel were used under the slab phantoms during irradiation. MR images of the irradiated gel phantoms were obtained to determine their R2 (spin-spin) relaxation maps for conversion to absorbed dose. One- and 2-dimensional lateral dose profiles were acquired at depths of 1 and 4 cm for 8 and 15 MeV electron beams. The results were compared with the doses measured by a diode detector at the same positions. In addition, the dose distribution in the axial orientation was measured by the gel dosimeter. The slope and intercept for the R2 versus dose curve were 0.509 ± 0.002 Gy s and 4.581 ± 0.005 s, respectively. No significant variation in dose-R2 response was seen for the two electron energies within the applied dose ranges. The mean dose difference between the measured gel dose profiles was smaller than 3% compared to those measured by the diode detector. These results provide further demonstration that electron dose distributions are significantly altered in the presence of tissue inhomogeneities such as bone and air cavity and that MAGIC gel is a useful tool for 3-dimensional dose visualization and qualitative assessment of tissue inhomogeneity effects in electron beam dosimetry.

Monte Carlo N Particle code - Dose distribution of clinical electron beams in inhomogeneous phantoms.

Electron dose distributions calculated using the currently available analytical methods can be associated with large uncertainties. The Monte Carlo method is the most accurate method for dose calculation in electron beams. Most of the clinical electron beam simulation studies have been performed using non- MCNP [Monte Carlo N Particle] codes. Given the differences between Monte Carlo codes, this work aims to evaluate the accuracy of MCNP4C-simulated electron dose distributions in a homogenous phantom and around inhomogeneities. Different types of phantoms ranging in complexity were used; namely, a homogeneous water phantom and phantoms made of polymethyl methacrylate slabs containing different-sized, low- and high-density inserts of heterogeneous materials. Electron beams with 8 and 15 MeV nominal energy generated by an Elekta Synergy linear accelerator were investigated. Measurements were performed for a 10 cm × 10 cm applicator at a source-to-surface distance of 100 cm. Individual parts of the beam-defining system were introduced into the simulation one at a time in order to show their effect on depth doses. In contrast to the first scattering foil, the secondary scattering foil, X and Y jaws and applicator provide up to 5% of the dose. A 2%/2 mm agreement between MCNP and measurements was found in the homogenous phantom, and in the presence of heterogeneities in the range of 1-3%, being generally within 2% of the measurements for both energies in a "complex" phantom. A full-component simulation is necessary in order to obtain a realistic model of the beam. The MCNP4C results agree well with the measured electron dose distributions.

Independent evaluation of an in-house brachytherapy treatment planning system using simulation, measurement and calculation methods.

Accuracy of treatment planning systems may significantly influence the efficacy of brachytherapy. The purpose of this work is a detailed, varied and independent evaluation of an in-house brachytherapy treatment planning software called STPS. Operational accuracy of STPS was investigated. Geometric tests were performed to validate entry and reconstruction of positional information from scanned orthogonal films. MCNP4C Monte Carlo code and TLDs were used for simulation and experimental measurement, respectively. STPS data were also compared with those from a commercial planning system (Nucletron PLATO). Discrepancy values between MCNP and STPS data and also those of PLATO and STPS at Manchester system dose prescription points (AL and AR) of tandem and ovoid configurations were 2.5% ± 0.5% and 5.4% ± 0.4%, respectively. Similar results were achieved for other investigated configurations. Observed discrepancies between MCNP and STPS at the dose prescription point and at 1 cm from the tip of the vaginal applicator were 4.5% and 25.6% respectively, while the discrepancy between the STPS and PLATO data at those points was 2.3%. The software showed submillimeter accuracy in its geometrical reconstructions. In terms of calculation accuracy, similar to PLATO, as attenuation of the sources and applicator body is not considered, dose was overestimated at the tip of the applicator, but based on the available criteria, dose accuracy at most points were acceptable. Our results confirm STPS's geometrical and operational reliability, and show that its dose computation accuracy is comparable to an established commercial TPS using the same algorithm.

Poster - Thur Eve - 69: Electron beam dosimetry in heterogeneous phantoms using the MAGIC normoxic polymer gel.

Recently, radiation sensitive polymer gels are being used as a reliable dosimetry method for three-dimensional (3D) verification of radiation doses in clinical use. Some properties of gel dosimeters have made them useful in verifying complex situations in electron therapy. The aim of this study was to experimentally evaluate the influence of tissue inhomogeneities on electron beam dose distributions by use of polymer gel dosimetry. Another purpose was to evaluate the appropriateness of polymer gels for electron beam dosimetry applications. A cylindrical phantom filled with MAGIC polymer gel with a polyacrilic wall (ρ = 1.18 g.cm-3 ) was placed in a Perspex water-filled tank exactly underneath the bone inhomogeneity region .Then, the slab phantom was irradiated with a dose of 5Gy of 8MeV electrons to measure the dose distribution beyond the heterogeneity region. Afterwards, another cylindrical gel phantom similar to the above was used and irradiated with the same dose of 15 MeV electrons to measure the dose distribution beyond the same heterogeneity region. The same mentioned setup was repeated for measurement of the dose distribution beneath the air heterogeneity and homogenous phantom. The results of gel dosimetry under bone inhomogeneity have shown a reduction in dose. This is related to the high mass stopping and mass scattering powers of bone tissue. In addition, dose enhancement is seen laterally near the bone-tissue interface, due to increased side scattering of electrons. Hot and cold scatter lobes under heterogeneity regions are other effects that can be seen. The results of gel dosimetry under the air inhomogeneity have shown an increase in dose. This is related to the low mass stopping and mass scattering powers of the air cavity. When a high energy beam passes through a low-density medium or an air cavity, electronic equilibrium is lost along the central axis of the beam .The dose rebuild up is a consequence of this electronic disequilibrium. An overall good agreement was found between measurements with gel and with a diode detector for the single beam experiment. Electron dose distributions are significantly altered in the presence of tissue inhomogeneities such as bone and air cavities which are related to mass stopping and mass scattering powers of heterogeneous materials.