47 Background: Magnetic resonance imaging (MRI) allows for delineation of the dominant intraprostatic lesions (DIL) thus minimizing the risk of a geographic miss. At our center, using MRI planning following the brachytherapy implant, the DIL is defined as the gross tumor volume (GTV) and receives at least the prescribed dose. This retrospective analysis studies the biochemical response and urinary symptoms of a MRI-guided escalated dose to the DIL during prostate brachytherapy. Methods: Intermediate- or high-risk patients with a visible DIL on the diagnostic MRI, treated between 2013 and 2016, with combined external-beam radiation therapy (46 Gray) and a high-dose rate whole prostate brachytherapy (15 Gray), were analyzed. Baseline characteristics and dosimetric values were obtained as well as PSA and International prostate symptom score (IPSS) at baseline, 3, 6, 12, 18, 24, 36 and 48 months (mos). Results: A total of 117 patients were studied, of which 58% were intermediate-risk and 42% high-risk. Median follow-up was 31 months (10-69 months). The biochemical progession-free survival (bPFS) was 97.4%. The median GTV D100 (dose to 100% of the GTV) and CTV V100 (percentage of the clinical target volume, defined as the whole prostate, receiving 100% of the prescribed dose) were 119% and 97.97%. The median PSA at 12, 24, 36 and 48 months for all patients and for patients treated without androgen deprivation therapy (ADT), was 0.18, 0.11, 0.06, 0.07 μg/L, and 0.73, 0.35, 0.1, 0.08 μg/L, respectively. The median IPSS score at baseline was 6 and remained stable over time. Conclusions: MRI-guided dose escalation to the GTV during brachytherapy boost yields an excellent bPFS and a rapid PSA drop without an increase in urinary symptoms. [Table: see text]
