Dosimetry of 131Cs Liquid-filled intracavitary balloon brachytherapy for enhanced treatment planning: utilizing TL dosimetry (TLD) and Monte Carlo calculation.

Abstract

An intracavity balloon filled with ¹³¹Cs liquid was investigated to be used in brachytherapy, particularly for the treatment of glioblastoma multiforme (GBM), with a reference of 40-60 Gy at a depth of 1 cm. The dosimetric characteristics of the liquid ¹³¹Cs, as a low-energy photon source for brachytherapy, were assessed. For this, thermoluminescence dosimetry was used for experimental validation, while Monte Carlo calculations were performed to evaluate dose rate distributions for treatment planning. To prepare the ¹³¹Cs solution, natural BaCO₃ was used as a target irradiated in a thermal neutron flux of 3.5 × 10¹³ n/cm²/s at the Tehran Research Reactor (TRR), and the separation of ¹³¹Cs with a radionuclide purity of 99% was confirmed using a radiochemical method. A well counter was employed to measure the activity of the ¹³¹Cs solution. The dosimetric measurements were conducted using a newly designed phantom in the form of a hollow sphere, as well as a water tank and thermoluminescence dosimeters (TLDs). The results were utilized to validate the accuracy of the dose distribution simulated with the MCNP5 Monte Carlo radiation transport code and a volumetric dose algorithm for predicting dose to water. The ± 8% agreement obtained between TLD measurements and MCNP5 simulations (with an uncertainty of ± 12.3%) confirms the validity of the model. The calculated dose rates in water and brain tissue were 0.0127 Gy/h and 0.0131 Gy/h at a reference depth of 0.5 cm per 37MBq, respectively. To calculate the activity relative to the balloon volume, two equations were deduced for volumes below and above 15 cm³. The results indicate that the validated dose rate can be used to develop a treatment planning method including a liquid ¹³¹Cs source for brachytherapy applications.