MEASUREMENT OF ABSORBED DOSE TO ORGANS AT RISK IN HIGH DOSE RATE BRACHYTHERAPY OF THE CERVIX

ABSTRACT Radiation dose to bladder and rectum in the treatment of cervical cancer using high dose rate brachytherapy is a limiting factor to the delivery of the prescribed dose to the target. The objective of this study is to determine absorbed dose to the bladder and rectum (organs at risk) in the treatment of cervical cancer using multisource high dose rate brachytherapy treatment system in order to establish an in-vivo dosimetric and quality assurance protocol for patient treatment. A phantom was constructed using perspex for the measurement of dose distribution to the bladder and rectum. Gafchromic EBT3 films were used as dosimeters for measuring doses received by the bladder and rectum. The variation between doses calculated by the treatment planning system (TPS) and the doses measured by the Gafchromic EBT3 film at the bladder point was 15.38% (mean) ranging from -29.03% to 43.75%. The variation between doses calculated by the TPS and the doses measured by the film at rectum point was 14.73% ranging from -47.22% to 51.06%. Two mathematical algorithms were developed from the study to enable dose estimation to the bladder and rectum for error detection, variation analysis and verification of the treatment system. The variation between doses calculated by the TPS and the doses calculated by the model at the bladder point was 9.53% ±8.04. The variation between doses calculated by the TPS and the doses calculated by the model at the rectal point was 13.57% ±8.46. The results of the proposed models are within the ± 15% uncertainty proposed for dose delivery in radiation therapy (Hanson et al., 1994). The model is therefore recommended for clinical applications