Is multiparametric MRI able to characterize margin for focal brachytherapy in low-grade prostate cancer?

Abstract

e555 Background: Focal brachytherapy is proposed in our institute as an alternative treatment to active surveillance for low-grade prostate cancer (PCa). This study aims at characterizing the tumor focus and its margin with multiparametric Magnetic Resonance Imaging (mpMRI). Methods: Patients pre-qualified for this study were positive for PCa (Gleason 3+3) on a previous standard biopsy series. New series of mp-MRI-guided and ultrasound-targeted biopsies were performed and in total, 17 patients with confirmed tumor and diameter < 20mm were included in this phase II clinical trial (NCT01902680). mpMRI were acquired on a 1.5T Magnetom Aera Siemens scan. Anatomic imaging consists in Fast Spin Echo T2-weighted MRI (T2-MRI) and functional Diffusion Weighted MRI (DWI-MRI) and Dynamic Contrast Enhanced MRI (DCE-MRI) were also performed. After mpMRI registration, tumor volumes of interest (VOI) were drawn on anatomic T2-MRI. VOI and VOI+2mm were reported on functional DWI-MRI and DCE-MRI. Extracted parameters were Apparent Diffusion Coefficient (ADC) and KTrans. All parameters distributions were analyzed with Olea Sphere v3.0 and compared to contralateral normal appearing tissue. Focal brachytherapy was then delivered to all patients with linked 125I seeds with a dose prescription of 152 Gy on the Planning Target Volume (PTV = VOI+2mm). Results: ADC parameters (mean, meadian, 25th and 75th percentiles) are found to be significantly lower in tumor volume (VOI) compared to contralateral normal tissue (p < 0.012 for all ADC parameters), confirming diffusion tumor mass restriction. Different distributions of ADC and KTrans were observed among patients: majority (66.66%) of low ADC and abnormal KTrans values were included in the VOI but interestingly, the 2mm margin allows us to treat additional abnormal ADC and KTrans volumes on 1/3 of the patients. Conclusions: This study confirms that mpMRI is a non-invasive technique able to characterize tumor margin for focal brachytherapy in low-grade PCa. Target volume margin definition is a hot topic when focal treatments (e.g. cryotherapy or HIFU) are considered and mpMRI can bring quantitative answers. Clinical trial information: NCT01902680.