Dosimetric Calculation of a Thermo Brachytherapy Seed: A Monte Carlo Study

by Khan, Nadeem

Research has proven that Hyperthermia and Radiation work in synergy with one another in killing cancer cells, with heat killing hypoxic cells and radiation killing oxygenated cells. The objective of this research project was twofold: a) to model the Best Medical, Inc. seed Model 2301 125I seed and run in Monte Carlo N-Particle Transport Code version 5 (MCNP5). To compute the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG43) factors using this model. Then compare the results to Book values as published by Sowards et al and the TG43 report. b) To model the new thermobrachytherapy seed and run the TG-43 factors, comparing the results to both the Book values and the calculated Best Model seed. In Liquid Water, the dose rate constant calculated for the Best model Seed is 1.056 ±0.0055 Gy*h-1*U-1 (Book value of 1.01: with a percentage difference of 4.6%). The value measured for thermobrachytherapy seed#1 is 1.057 ±0.031 cGy*h-1U-1 and thermobrachytherapy seed#2 is 1.058 ±0.031 cGy*h-1U-1 The value obtained for thermobrachytherapy Seed#1 varies by 0.091% from the measured Best Model value and by 4.69% from Best Model book value. Continuing the comparison, the value obtained for thermobrachytherapy Seed#2 varies by 0.15% from the measured Best Model value and by 4.75% from Best Model book value In Solid Water, the dose rate constant calculated for the Best model seed is 1.03 ±0.031 cGy*h-1*U-1 (Book value of 0.98: with a percentage difference of 5.1%). The value measured for thermobrachytherapy seed#1 is 1.03 ±0.031 cGy*h-1U-1 and thermobrachytherapy seeds#2 is1.029 ±0.031 cGy*h-1*U-1. The value obtained for thermobrachytherapy Seed#1 varies by 0.01% from the measured Best Model seed and by 5.1% from Best Model book value. The value obtained for thermobrachytherapy Seed#2 varies by 0.09% from the measured Best Model value and by 4.97% from Best Model book value. The Correction/Multiplicative Factor (conversion of dose rate constant) between the Solid Water and liquid water for the calculated Best Model 125I seed #2301 is 1.026 (Book Value of 1.05). Thermobrachytherapy Seed#1 has a factor of 1.026 and Thermobrachytherapy Seed#2 is 1.028. It once again shows close agreement for both Solid and liquid Water. Radial dose function was taken at 0.1cm to 10cm at 0.1cm intervals to 1cm and then at 0.5cm interval to 10cm.The figures for Radial dose functions can be compared and it shows superposition of the data points confirming good agreement between the measured and the book value. The Radial dose function from the two brachytherapy seeds is further compared to the measured values and book values of the Best Model seed and again a good agreement is seen in the results. Anisotropy Function was calculated for all the three seeds in both Liquid and Solid Water. The function was calculated from 1cm to 7cm in 1cm increments at 0°-90° at 10° increments. Similar to comparisons in the radial function, anisotropy function data points were superimposed confirming good agreement between the measured value for Best Model and the book value. The anisotropy function from the two brachytherapy seeds is further compared to the measured values and book values of the Best Model seed and again a good agreement is seen in the results. In liquid water, the Source Anisotropy Constant for Best Model 2301 Seed is 0.935, thermobrachytherapy seed#1 is 0.923 and thermobrachytherapy seed#2 is 0.927. For the Best Model 2301, the value deviates by 4.5% from the Book value. The values for thermobrachytherapy seed#1 deviates from the Monte Carlo calculated Best Model 2301 125I Seed by 1.28%, and from the Book value by 4.6%. Thermobrachytherapy seed#2 deviates from the Monte Carlo calculated Best Model 2301 125I Seed by 0.8% and the Book value by 5.4%. In Solid Water, the Source Anisotropy Constant for Monte Carlo calculated Best Model 2301 Seed is 0.926. Thermobrachytherapy seed#1 has a value of 0.918 and thermobrachytherapy seed#2 has a value of 0.927. In terms of deviation, the Monte Carlo calculated Best Model seed deviates 4.5% from the book value for the same seed. Thermobrachytherapy seed#1 deviates from the Monte Carlo calculated Best Model 2301 125I Seed by 0.08% and the Book value by 5.4%. Thermobrachytherapy seed#2 deviates from the Monte Carlo calculated Best Model 2301 125I Seed by 0.3% and the Book value by 4.8% Analyzing the data, there is very little difference between thermobrachytherapy seed#1 and thermobrachytherapy seed#2 in comparison to one another. The data between the two seeds is very comparable. Also, the results are quite similar, in terms of error percentage, between the Book values and measured value for the Best Model 2301 125I Seed. It is also important to note that the seed’s TG-43 factors have not deviated too much from the established data on the Best Model Seed. This ensures that the radio-activity from the seed is still established and there isn’t a loss of activity around the seed.