The main objective of this study was to compare dosimetric characterisation of high-dose-rate brachytherapy (HDR-BT) with external beam intensity-modulated radiation therapy (EX-IMRT) as a means of delivering boost dose.

Five HDR patients were selected for IMRT planning. Patients underwent ultrasound-guided catheter placement for HDR. Computed tomography (CT) images were obtained and imported into the Nucletron PLATO Brachytherapy system. The prostate, urethra, bladder and rectum were contoured on axial slices. The dose was calculated and optimised by graphical optimisation. The CT images of these structures were exported from the PLATO to Eclipse workstation for IMRT planning. For each patient, the dose–volume histogram (DVH) of HDR and IMRT plans were generated, drawn on the same scale and compared.

The dose distribution in HDR plans was non-uniform and conformed peripherally inside the planned target volume (PTV). A small volume of the prostate received a very high dose from HDR.In IMRT plans, a uniform dose distribution was observed. The DVH curves for PTV dropped sharply and reached to a zero volume of the prostate at about 6·4 Gy. In HDR plans, the DVH curves for PTV showed a long tail up to a very high dose. About 10% of the prostate received about 13·3 Gy, which is 222% of the prescribed dose (6 Gy) in HDR plans. In contrast, the same volume in IMRT plans received <6 Gy (100%). The average dose for V90 was about 6·3 Gy for HDR and 5·8 Gy for IMRT plans. At a prostate volume of V100 level, the average dose in all plans was 5·0 Gy from HDR and 5·4 Gy from IMRT plans. In HDR plan, the V100 dose for urethra varied from 0·6 to 3·0 Gy (average 1·8 Gy). The range in IMRT plans varied from 3·6 to 6 Gy with an average of 4·7 Gy. At V90 level, the dose range in HDR and IMRT plans varied from 2·5 to 4·7 Gy (average 3·8 Gy) and 4·8 to 5·4 Gy (average 5·3 Gy), respectively. In general, the dose to the bladder and rectum was comparatively lower in HDR than in IMRT plans.

HDR brachytherapy may reduce normal tissue toxicities in prostate boost treatments, even though the dose homogeneity inside the PTV is far worse than in IMRT treatments. Another advantage of HDR over IMRT is that the organ motion is not a significant concern as in IMRT.