Evaluation of Bremsstrahlung radiation dose in stereotactically radiocolloid therapy of cystic craniopharyngioma tumors with 32P radio-colloid.

Over 90% of craniopharyngeal brain tumors are cystic, which enables the injection of beta emitters such as phosphorus-32 (32P) radio-colloid into cysts for their treatment. The aim of this study was to evaluate the clinical and theoretical modelling of Bremsstrahlung radiation dose resulting from stereotactic radio-colloid therapy of cystic craniopharyngioma tumors with 32P. 32P radio-colloid with appropriate activity concentration was injected to a head phantom, and then the Bremsstrahlung radiation spectrum and planar images were obtained using a gamma camera. Both phantom and gamma camera were simulated using MCNPX code, and the results were compared with practical results. Bremsstrahlung radiation spectrum was measured using a handheld gamma spectrometer for two patients treated with stereotactic radio-colloid therapy with 32P in different positions and compared to Monte Carlo simulation. Results of counting and determining sensitivity coefficients in the air and the attenuating environment were obtained. Also, comparing the counting sensitivity from practical and simulation methods indicated the agreement of the data between the two methods. Comparison of the spectra from different positions around patient's head indicated the ability to use this detector to quantify the activity in the operating room. Selection of the spectrum is important in Bremsstrahlung radiation imaging. We can take advantage of spectrometry measurement using gamma camera, handheld gamma spectrometer for patient, and theoretical modeling with Monte Carlo code to evaluate radiopharmaceutical distribution, leakage, as well as estimate activity and predict therapeutic effects in other adjacent structures and ultimately optimize radio-colloid therapy in cystic craniopharyngeal patients.

Monte Carlo calculations and experimental measurements of the TG-43U1-recommended dosimetric parameters of 125I (Model IR-Seed2) brachytherapy source.

A new design of 125I (Model IR-Seed2) brachytherapy source has been manufactured recently at the Applied Radiation Research School, Nuclear Science and Technology Research Institute in Iran. The source consists of six resin beads (0.5 mm diameter) that are sealed in a cylindrical titanium capsule of 0.7 mm internal and 0.8 mm external diameters. This work aims to evaluate the dosimetric parameters of the newly designed 125I source using experimental measurements and Monte Carlo (MC) simulations. Dosimetric characteristics (dose rate constant, radial dose function, and 2D and 1D anisotropy functions) of the IR-Seed2 were determined using experimental measurements and MC simulations following the recommendations by the Task Group 43 (TG-43U1) report of the American Association of Physicists in Medicine (AAPM). MC simulations were performed using the MCNP5 code in water and Plexiglas, and experimental measurements were carried out using thermoluminescent dosimeters (TLD-GR207A) in Plexiglas phantoms. The measured dose to water in Plexiglas data were used for verification of the accuracy of the source and phantom geometry in the Monte Carlo simulations. The final MC simulated data to water in water were recommended for clinical applications. The MC calculated dose rate constant (Λ) of the IR-Seed2 125I seed in water was found to be 0.992 ± 0.025 cGy h-1U-1. Additionally, its radial dose function by line and point source approximations, gL(r) and gp(r), calculated for distances from 0.1 cm to 7 cm. The values of gL(r) at radial distances from 0.5 cm to 5 cm were measured in a Plexiglas phantom to be between 1.212 and 0.413. The calculated and measured of values for 2D anisotropy function, F(r, θ), were obtained for the radial distances ranging from 1.5 cm to 5 cm and angular range of 0°-90° in a Plexiglas phantom. Also, the 2D anisotropy function was calculated in water for the clinical application. The results of these investigations show that the uncertainty of the experimental data is within ± 7% between the measured and simulated data in Plexiglas. Based on these results, the MC-simulated dosimetric parameters of the new 125I source model in water are presented for its clinical applications in brachytherapy treatments.

Dosimetry of (32)P radiocolloid for treatment of cystic craniopharyngioma.

In Tajrish Shohada Hospital, patients with either cystic craniopharyngiomas or cystic astrocytomas have been treated with (32)P radiocolloid by stereotactic procedure. The total activity was prescribed for delivery dose between 200 and 300 Gy to the cyst wall thicknesses of 1-3mm. In this project, MD-55-2 radiochromic film was utilized to determine the dose distribution around the source. The film dosimetry data were compared with Monte Carlo simulated values calculated with MCNP4C code. In addition, the clinical and dosimetric factors such as the cyst volume, and method of radiocolloid injection were evaluated. The required activity and distribution of radial dose in and out of cyst wall have been investigated.