ABSTRACT
Background: One major challenge in brachytherapy is to verify the accuracy of dose distributions calculated by the treatment planning system. In this project, a new phantom design has been introduced for quality assurance of dose distributions in gynocological [GYN] brachytherapy implants using EBT GafChromic film. Materials and Methods: This phantom has been designed and fabricated from 90 slabs of 18×16×0.2 cm[3] Perspex to accommodate a tandem and ovoids assembly, which is normally used for GYN brachytherapy treatment. In addition, this phantom design is allowing the use EBT GafChromic films for dosimetric verification of GYN implants with Cs-137 Selectron LDR system. With this assembly, GafChromic films were exposed using a plan designed to deliver 2.5 Gy dose to point "A" in Manchester system for tandem and ovoids configurations and to deliver 1.5 Gy of dose to 0.5 cm distance from the lateral surface of ovoids for using ovoid-pair. The measured dose distributions with GafChromic films were compared with the TPS isodose lines both numerically and spatially. For a quantitative analysis of the results, the measured doses values at several points of interest were evaluated with the treatment planning data and values obtained following the TG-43 dose calculation formalism. Results: The results of these investigations have indicated that the new phantom design enables us to measure differences of greater than +/-6% for LDR brachytherapy GYN treatments. Conclusion: The new phantom design could be utilized for the QA procedure of the brachytherapy remote after loading systems to confirm the accuracy of dose distribution in GYN implants
ABSTRACT
The dose rate distribution delivered by a low dose rate [137]Cs pellet source, a spherical source used within the source trains of the Selectron gynecological brachytherapy system, was investigated using the MCNP4C Monte Carlo code. The calculations were performed in both water and Plexiglas and the absolute dose rate distribution for a single pellet source and the AAPM TG-43 parameters were computed. A spherical phantom with dimensions large enough [60 cm] was used to provide full scattering conditions. In order to score dose at different distances from the source centre, this sphere was divided into a set of 600 concentric spherical shells of 0.05 cm thickness. The calculations were performed up to a distance of 10 cm from the source centre. To calculate the effect of the applicator and dummy pellets on dose rate constant and radial dose function, a single pellet source was simulated inside the vaginal applicator, and spherical tally cells with radius of 0.05 cm were used in the simulations. The F6 tally was used to score the absolute dose rate at a given point in the phantom. The dose rate constant for a single active pellet was found to be 1.102 +/- 0.007 cGyh[-1]U[-1], and the dose rate constant for an active pellet inside the applicator was 1.095 +/- 0.009 cGyh[-1]U[-1]. The tabulated data and 5th order polynomial fit coefficients for the radial dose function along with the dose rate constant are provided for both cases. The effect of applicator and dummy pellets on anisotropy function of the source was also investigated. The error resulting from ignoring the applicator was reduced using the data of a single pellet. The results indicate that F[r, theta] decreases towards the applicator
ABSTRACT
Grid radiation therapy, using the megavoltage X-ray beam, has been proven to be an effective method for management of large and bulky malignant tumors. This treatment modality is also known as Specially Fractionated Radiation Therapy [SFRT]. In this treatment technique a grid block converted the open radiation field into a series of pencil beams. Dosimetric characteristics of an external beam grid radiation field have been investigated using experimental and Monte Carlo simulation technique. Dose distributions [%DD as well as the beam profiles] of a grid radiation field have been determined using experimental and Monte Carlo simulation technique, for 6- and 18 MV X-ray beams from a Varian Clinics 2100C/D. The measurements were performed using LiF TLD and film in Solid Water phantom Material. Moreover, the MCNP Monte Carlo code was utilized to calculate the dose distribution in the grid radiation field in the same phantom material. The results of the experimental data were compared to the theoretical values, to validate this technique. Upon the agreement between the two techniques, dose distributions can be calculated for the grid field with different patterns and sizes of holes, in order to find an optimal design of the grid block. The results of dose profiles for 6 MV X-ray beams obtained with the Monte Carlo simulation technique was in good agreement with the measured data. In addition, the 3D dose distribution of the grid field generated by the Monte Carlo simulation gave more detailed information about the dose pattern of the grid. The grid block can be used as a boost for treatment of bulky tumors. The Monte Carlo simulation technique can be utilized to optimize the pattern, size and spacing between the holes, for optimal clinical results
ABSTRACT
MD-55-2 is one of the Radiochromic film models with the sensitivity suitable for dose measurements ranging from 5 to 100 Gy. However, this lower limit makes the film impractical for its applications in many areas such as brachytherapy source dosimetry. n this project, the useful range of the film has been extended by using a multilayer film technique. In this technique, single-, double-, and triple- layers of films were exposed to the doses ranging from 0.5 to 10 Gy using a Co-60 photon beam. Calibration curves for corresponding layers of films were obtained with a spectrophotometer using a 680nm wavelength. The results indicated that the sensitivities of double and triple layers were approximately 200% and 300%, respectively, higher than a single-layer film. The impact of multilayer film arrangement on the energy dependence of the MD-55-2 Radiochromic film has also been examined using 100KVp, 80 KVp, and 6 MV X-ray beams. The results indicated an insignificant [within 5%] change in film responses with the beam energy. Therefore, the multilayer technique enhances the Radiochromic film sensitivity and expands its application to the low dose range in field of brachytherapy source dosimetry