Re-distribution of brachytherapy dose using a differential dose prescription adapted to risk of local failure in low-risk prostate cancer patients.

ABSTRACT

BACKGROUND AND PURPOSE: We investigated the application of a differential target- and dose prescription concept for low-dose-rate prostate brachytherapy (LDR-BT), involving a re-distribution of dose according to risk of local failure and treatment-related morbidity. MATERIAL AND METHODS: Our study included 15 patients. Multi-parametric MRI was acquired prior to LDR-BT for gross tumor volume (GTV) delineation. Trans-rectal ultrasound (US) images were acquired during LDR-BT for prostate gland- (CTV(Prostate)) and organs at risk delineation. The GTV contour was transferred to US images after US/MRI registration. An intermediate-risk target volume (CTV(Prostate)) and a high-risk target volume (CTV(HR)=GTV+5 mm margin) were defined. Two virtual dose plans were made Plan(risk-adapt) consisted of a de-escalated dose of minimum 125 Gy to the CTV(Prostate) and an escalated dose to 145-250 Gy to the CTV(HR); Plan(ref) included the standard clinical dose of minimum 145 Gy to the CTV(Prostate). Dose-volume-histogram (DVH) parameters were expressed in equivalent 2 Gy fractionation doses. RESULTS: The median D(90%) to the GTV and CTV(HR) significantly increased by 44 Gy and 17 Gy, respectively when comparing Plan(risk-adapt) to Plan(ref). The median D(10%) and D(30%) to the urethra significantly decreased by 9 Gy and 11 Gy, respectively and for bladder neck by 18 Gy and 15 Gy, respectively. The median rectal D(2.0cm(3)) had a significant decrease of 4 Gy, while the median rectal D(0.1cm(3)) showed an increase of 1 Gy. CONCLUSIONS: Our risk adaptive target- and dose prescription concept of prescribing a lower dose to the whole gland and an escalated dose to the GTV using LDR-BT seed planning was technically feasible and resulted in a significant dose-reduction to urethra and bladder neck.