Background and purpose: The two main modalities to deliver high dose to the prostate and prevent high doses to neighboring organs are intensity modulated radiotherapy (IMRT) or external beam radiotherapy combined with brachytherapy. Because of the different biological effectiveness the physical dose distributions were converted to 3-dimensional linear quadratic dose at 2 Gy per fraction (EQD(2)). From the latter, cumulative EQD(2)-volume histograms were determined for comparison of the modalities.
Material and methods: An IMRT plan was made on the contoured planning target volume (PTV1) and organs at risk (OAR) of 20 patients (IMRT-only). A dose of 70 Gy was prescribed on the PTV1 with a concomitant boost to a total of 76 Gy on a subvolume (PTV2). Also a 46 Gy IMRT plan was made combined with either a pulsed dose-rate (PDR) or a high dose-rate (HDR) brachytherapy boost. The EQD(2) on the PTV1 of the combined IMRT-PDR and IMRT-HDR plans were made equivalent to the EQD(2) of the 70 Gy IMRT-only plan. The alpha/beta-ratio for prostate was set to 1.5 Gy and 10 Gy. For normal tissues an alpha/beta-ratio of 3.0 Gy was taken. Several EQD(2)-volume histogram parameters were calculated for comparison and analyzed by two-way ANOVA.
Results: The mean EQD(2) to 95% of the prostate volume was slightly higher for the IMRT-only plan than for the brachytherapy modalities (P<0.001), in contrast to the mean EQD(2) to 50% of the prostate volume in which the opposite was the case (P<0.001). Rectum and bladder doses for IMRT-only are significantly higher (P<0.001). The urethra dose for IMRT-HDR was much higher than the other modalities only when the alpha/beta-ratio for prostate was 10 Gy.
Conclusion: Because of the high doses within an implant, the dose in 50% of the prostate volume is much higher with the brachytherapy modalities than IMRT-only which may have clinical consequences. With brachytherapy the doses to the OAR are lower or similar to IMRT-only. Dose escalation for prostate tumors is more easily achieved with brachytherapy than with IMRT alone. Therefore, brachytherapy might be the preferred modality to achieve further dose escalation.