Abstract
Brachytherapy is a technique by which sealed radioactive sources are introduced into the body and positioned near the tumour using appropriate applicator devices. This technique can deliver the desired dose to the tumour while avoiding damage to healthy tissues.
To achieve the best possible treatment planning, it is very important to know as precisely as possible the dose distribution around not only the source but also the applicator.
In the thesis, an Ir-192 source, Microselectron type from the firm Nucletron has been used in a gynaecological applicator that consists of a tube intrauterine in stainless steel and two vaginal cylinders in polymer. A dose distribution evaluation has been performed by both calculations and measurements. Measurements have been made using an ionization chamber type Wellhöfer CC04 as well as radiochromic films type Gafchromic © EBT. Radiochomics films have been used to obtain isodoses around the applicator and the ionization chamber to obtain depth dose. To ensure proper positioning of both source and measurement devices, a phantom has been developed.
Dose distribution calculations have been performed using the treatment planning system (TPS) PLATO v14.3 and also the MCNP5 code based on the Monte Carlo method. A model of the source, applicator and phantom has been developed. As commercial TPS does not take into account the attenuation in applicator materials, it is important to quantify the influence of this effect on the dose evaluation.
After comparing calculations and measurements, it is possible to say that a sensible shielding effect of the intra-uterine tube is limited to the very near field of the tube, for distances lower than 1 cm.
Gafchromic©EBT films and the Wellhöfer CC04 ionization chamber, used during the thesis, have allowed the measurement of dose distribution as well as absolute dose and have lead, in some simple geometrical situation, to the validation of the model developed for the MCNP5 code. The MCNP5 code is a powerful tool to simulate sources and devices in order to help physicists in treatment planning and to evaluate doses where physical measurement can not be performed, for example in contact with the intrauterine tube or very close to it.
For distances greater than 1 cm from the source, doses estimated by the PLATO system are consistent with measurements and MCNP5 calculations.
Finally, a complete analysis of the uncertainties relative to the utilization of the MCNP5 code has been performed. The effect of the different elements related to the modelling, but also with the code has been studied. It is necessary to indicate that an evaluation of the global uncertainty is essential in case of comparison with other systems of calculation or with the effective measures.