The dose distribution close to an 192Ir wire source

: EGS4 Monte Carlo calculations

Abstract

Monte Carlo simulation using the EGS4 (PRESTA version) code has been employed as an investigative tool to calculate the dose close to Ir wire implants. Most commercial treatment planning computers employ the Sievert integral (Sievert 1921) and Meisberger polynomial (Meisberger et al 1968) for dose calculations and neglect the contribution from the energetic secondary electrons due to the high atomic number of platinum (Pt) used in the encapsulation. It has been shown that the GE Target II Sun Sparc treatment planning system was still able to reproduce the EGS4 results at radial distances of 1mm and farther away. An experiment using thin TLD chips was performed in air to test the user code of the EGS4 indirectly. The user code after modification was able to predict correctly the percentage difference, at radial distances from 2 to 9cm, before and after the insertion of the perspex (PMMA) cylinder around the Ir wire source. Furthermore, another experiment using GafChromicTM film was performed to prove the validity of the user code of the EGS4 which calculates the dose close to the wire source at a low energy range. The user code after another modification could predict the trend of the net optical density against the radial distance close to the Ir wire source. Although there is no clinical need to calculate the dose rate close to the source in water, the Monte Carlo results serve as a reference for medical physicists, radiographers and radiation therapists in hospital because the experiments are difficult to perform very accurately near the surface of the Ir wire source.

Date of Award	3 Nov 1995
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chien Ming Enoch YOUNG (Supervisor) & Kwan Ngok Peter YU (Co-supervisor)