Local Failure after Prostate SBRT Predominantly Occurs in the PI-RADS 4 or 5 Dominant Intraprostatic Lesion.

BACKGROUND:A positive post-treatment prostate biopsy following definitive radiotherapy carries significant prognostic implications. OBJECTIVE:To determine whether local recurrences after prostate stereotactic body radiation therapy (SBRT) are associated with the presence of and occur more commonly within the region of a PI-RADS 4 or 5 dominant intra-prostatic lesion (DIL) identified on pre-treatment multi-parametric magnetic resonance imaging (MRI). DESIGN, SETTING, AND PARTICIPANTS:247 patients with localized prostate cancer treated with SBRT at our institution from 2009-2018 underwent post-treatment biopsies (median time to biopsy: 2.2 years) to evaluate local control. INTERVENTIONS:Prostate SBRT (median 40 Gy in 5 fractions). OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS:MRIs were read by a single diagnostic radiologist blinded to other patient characteristics and treatment outcomes. The DIL presence, size, location, and extent were then analyzed to determine associations with the post-treatment biopsy outcomes. RESULTS AND LIMITATIONS:Among patients who underwent post-treatment biopsies, 39/247 (15.8%) were positive for Gleason-gradable prostate adenocarcinoma, of which 35/39 (90%) had a DIL initially present and 29/39 (74.4%) had a positive biopsy within the DIL. Factors independently associated with post-treatment biopsy outcomes included the presence of a DIL (OR 6.95; p = 0.001), radiographic T3 disease (OR 5.23, p < 0.001), SBRT dose ≥40 Gy (OR 0.26, p = 0.003), and use of androgen deprivation therapy (ADT; OR 0.28, p = 0.027). Among patients with a DIL (N = 149), the only factors associated with post-treatment biopsy outcomes included ≥50% percent cores positive (OR 2.4, p = 0.037), radiographic T3 disease (OR 4.04, p = 0.001), SBRT dose ≥40 Gy (OR 0.22, p < 0.001), and use of ADT (OR 0.21, p = 0.014). CONCLUSIONS:Our results suggest that men with PI-RADS 4 or 5 DILs have a higher risk of local recurrence after prostate SBRT and that most recurrences are located within the DIL. PATIENT SUMMARY:We found the presence of a dominant tumor on pre-treatment MRI was strongly associated with residual cancer within the prostate after SBRT and that most recurrences were within the dominant tumor.