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Objective To assess the relationship between the volume of the index lesion (IL) measured at multiparametric magnetic resonance imaging (mpMRI; MRIvol) and at radical prostatectomy (RPvol), stratifying it according to Prostate Imaging‐Reporting and Data System (PI‐RADS) score. Patients and Methods We identified 332 men with a positive mpMRI (single lesion with PI‐RADS ≥3) who underwent systematic plus targeted biopsy and subsequent RP at two tertiary referral centres between 2013 and 2018. All mpMRIs were reviewed by experienced radiologists using PI‐RADS scores. The study outcome was to assess the relationship between MRIvol (based on planimetry from MRI sequence best showing tumour) and RPvol (based on tumour involved area of each RP pathology slice). To achieve this endpoint, we performed a multivariable linear regression analysis (LRA) to predict RPvol using PI‐RADS, prostate‐specific antigen level, prostate volume, age, digital rectal examination, Gleason score at MRI‐targeted biopsy, biopsy history and time from mpMRI to RP as covariates. Non‐parametric locally estimated scatterplot smoothing (LOESS) function was used to graphically explore the relationship between MRIvol and RPvol, stratifying for PI‐RADS score. Results Overall, 24%, 49% and 27% of men had visible PI‐RADS 3, 4 and 5 lesions at mpMRI. The median (interquartile range [IQR]) MRIvol and RPvol were 0.67 (0.29–1.76) mL and 1.39 (0.58–4.23) mL. At LRA, MRIvol was significantly correlated with a RPvol underestimation (slope: 2.4, 95% confidence interval [CI] 0.1–46.3). The non‐parametric LOESS analysis showed a non‐linear relationship between MRIvol and RPvol. Significant underestimation was reported across all volumes with the highest differences between MRIvol and RPvol in the low volume range (<2 mL), where RPvol almost doubled MRIvol. A similar effect was observed across all PI‐RADS scores subgroups. Conclusions In the present study, mpMRI significantly underestimated the exact volume of the IL, especially for small visible lesions, regardless of PI‐RADS score. This should be considered when planning tailored focal therapy approaches often delivered to men with smaller prostatic lesions.

AbstractBackgroundThe clinical course in metastatic castrate‐resistant prostate cancer (mCRPC) can be complicated when patients have disease progression after prior treatment with second generation hormone therapy (second HT), such as enzalutamide or abiraterone. Currently, limited data exist regarding the optimal choice of chemotherapy for mCRPC after failing second generation hormone therapy. We sought to evaluate three common chemotherapy regimens in this setting.MethodsWe retrospectively identified 150 mCRPC patients with disease progression on enzalutamide or abiraterone. Of these 150 patients, 92 patients were chemo‐naïve while 58 patients had previously received docetaxel chemotherapy before being started on second HT. After failing second HT, 90 patients were assigned for docetaxel‐alone (group A), 33 patients received carboplatin plus docetaxel (group B), while 27 patients received cabazitaxel‐alone (Group C). A favorable response was defined by more than or equal to 50% reduction in prostate‐specific antigen from the baseline level after a complete course of chemotherapy. Survival outcomes were assessed for 30‐month overall survival.ResultsPatients in group (B) were 2.6 times as likely to have a favorable response compared to patients in group (A) (OR = 2.625, 95%CI: 1.15‐5.99) and almost three times compared to patients in group (C) (OR = 2.975, 95%CI: 1.04–8.54) (P = .0442). 30‐month overall survival was 70.7%, 38.9% and 30.3% for group (B), (A), and (C), respectively (P = .008). We report a Hazard Ratio of 3.1 (95% CI, 1.31‐7.35; P = .0037) between patients in group (A) versus those in group (B) and a Hazard Ratio of 4.18 (95% CI, 1.58‐11.06; P = .0037) between patients in group (C) compared to those in group (B)ConclusionThis data demonstrates improved response and overall survival in treatment‐refractory mCRPC with a chemotherapy regimen of docetaxel plus carboplatin when compared to docetaxel alone or cabazitaxel alone. Further investigations are required.

e17540 Background: The clinical course in metastatic castrate-resistant prostate cancer (mCRPC) can be complicated when patients have disease progression after treatment with 2nd generation hormone therapy (2nd-HT), such as enzalutamide or abiraterone. Currently, limited data exist regarding the optimal choice of chemotherapy for mCRPC after failing 2nd-HT. We sought to evaluate three common chemotherapy regimens in this setting. Methods: We retrospectively identified 150 patients with mCRPC with disease progression on enzalutamide or abiraterone. 92 patients were chemo-naïve, while 58 patients had previously received docetaxel chemotherapy prior to 2nd-HT. After failing 2nd-HT, 90 patients received docetaxel-alone (group A), 33 patients received carboplatin plus docetaxel (group B), while 27 patients received cabazitaxel-alone (Group C). Favorable response was defined by ≥50% reduction in PSA level from baseline after a complete course of chemotherapy. Survival outcome was assessed for 30-month overall survival. Results: Mean (SD) age was 71.2 (8.28), 69.5(8.38) and 67.2 (8.36) for group (A), (B) and (C), respectively. Mean (SD) pre-chemotherapy PSA was 63.8 (138.18), 58.5 (118.15) and 53.7 (88.15) for group (A), (B) and (C), respectively. Mean (SD) Gleason score was 7.9 (1.1), 8.4 (0.88) and 8.1 (1.06) for group (A), (B) and (C), respectively. Patients in group (B) were 2.6 times more likely to have a favorable response compared to group (A) (OR = 2.625, 95%CI: 1.15 - 5.99) and almost 3 times compared to patients in group (C) (OR = 2.975, 95%CI: 1.04 – 8.54) (p-value = 0.0442). We report a Hazard Ratio (HR) of 3.1 (95% CI 1.31-7.35; p = 0.0037) between patients in group (A) versus group (B), and a HR of 4.18 (95% CI 1.58-11.06; p = 0.0037) between patients in group (C) versus group (B). Thirty-month overall survival was 70.7%, 38.9%, and 30.3% for group (B), (A), and (C) respectively (p-value = 0.008). Conclusions: Our data demonstrate improved response and cancer-specific survival in patients with treatment-refractory mCRPC on docetaxel plus carboplatin compared to docetaxel or cabazitaxel alone. Selection bias is inherent in any retrospective study; however, our finding suggests that clinicians may consider docetaxel plus carboplatin in mCRPC patients who fail 2nd-HT. Further prospective studies are warranted.

5577 Background: Prostate specific antigen (PSA) has well-recognized limitations as a marker for treatment response and disease progression. A post hoc analysis of the PREVAIL trial reported 24.5% of chemotherapy naïve mCRPC patients on enzalutamide had radiographic progression on conventional imaging with non-rising PSA. In this study, we sought to retrospectively compare PSA levels with C-11 choline positron emission tomography/ computed tomography (PET/CT) images in patients with m-CRPC on 2nd-HT with prior use of chemotherapy. Methods: We identified 123 patients with mCRPC on 2nd-HT following prior use of docetaxel chemotherapy (Abiraterone, n = 106; Enzalutamide, n = 17). Patients underwent serial PSA testing and C-11 choline PET/ CTs every 3–6 months. Disease progression was defined by the increase in blood pool corrected maximum standardized uptake value (SUVmax) of the index lesion on C-11 choline PET/CT scan. Suspicious lesions were confirmed by biopsy and/or conventional imaging. Results: Approximately 43% (n = 53) of patients had radiographic disease progression while on 2nd-HT. At time of radiographic progression, 60.4% of patients showed a parallel rise in PSA (Group-A), while 39.6% showed a paradoxical response; defined as radiographic progression with stable or down-trending PSA (Group-B). Median PSA at time of progression was 3.1 ng/ml for Group-A, and 1.3 ng/ml for Group-B (p-value = 0.0176). Median SUVmax was the same (4.9 Group-A, 4.6 Group-B; p-value = 0.6072). Bone-predominance progression was more significant in Group-B (90%) versus Group-A (65%) (p-value = 0.0309). The median time for radiographic progression was 9.5 months versus 3.9 months for Group-A and Group-B, respectively (Log-Rank = 0.0063). Conclusions: Metabolic imaging is a useful tool that should complement PSA in the evaluation of treatment response and disease progression in mCRPC patients on 2nd-HT, especially considering the paradoxical response observed in our data.

144 Background: Despite having well-recognized limitations, urologists often rely on serial PSA testing as a marker for treatment response or disease progression. To determine if PSA was indeed a reliable marker for treatment response or disease progression, we compared PSA levels against C-11 choline PET/CT in the evaluation of patients with advanced prostate cancer treated with second generation hormonal therapy (2nd-HT). Methods: We retrospectively identified 239 patients who were undergoing treatment with 2nd-HT (enzalutamide or abiraterone) for advanced prostate cancer. While on treatment, patients underwent serial PSA testing and C-11 choline PET/ CTs every 3 – 6 months. Paradoxical response was defined as increasing blood pool-corrected SUVmax of known choline-avid lesions and/or identification of new choline-avid lesions, despite stable or down-trending PSA. Results: Median (IQR) age was 70.4(64.3 – 75.7) years and median (IQR) primary Gleason Score was 8 (7 – 9). In our study, 19% of patients (n = 46/239) who were receiving 2nd-HT exhibited paradoxical response. Median (IQR) PSA and corrected SUVmax at baseline evaluation were 1.3 ng/mL (0.3 – 12.8 ng/mL) and 3.5 (1.8 – 5.8), respectively. Median (IQR) PSA and corrected SUVmax at the time of paradoxical response were 0.4 ng/mL (0.1 – 5.4 ng/mL) and 4.5 (2.8 – 6.8), respectively. The median duration of 2nd-HT treatment prior to detection of paradoxical response was 4.8 months (2.9 – 10.1 months). No significant difference was noted between patients receiving enzalutamide versus abiraterone (p = 0.35). Independent predictors of paradoxical response were prior primary systemic treatment (i.e. hormonal/chemo-hormonal therapy versus local therapy) and patient’s age at time of 2nd-HT initiation on univariate and multivariate analysis. Conclusions: Our retrospective review demonstrated prostate cancer disease progression discordant with PSA down-trending in 19% of patients receiving 2nd-HT. We conclude that in this subset of patients with advanced prostate cancer, PSA may not be a reliable marker of treatment response of disease progression, and routine radiographic evaluation in these patients is warranted.

143 Background: Systemic treatment in the management of metastatic prostate cancer is inevitable. However, there is a growing interest in metastases-directed therapy (MDT). We sought to investigate the efficacy of MDT in treating patients with solitary metastatic prostate cancer and hence, delaying initiation of systemic treatment (i.e., Androgen deprivation therapy or chemotherapy). Methods: We retrospectively identified 61 patients treated with targeted therapy in the form of surgery (n = 30), stereotactic body radiation therapy (SBRT) (n = 25), or cryotherapy (n = 7) for their solitary metastases prostate cancer. Complete response was defined by achieving a PSA value of ≤0.2 ng/ml plus resolution of the solitary metastatic lesion on C-11 choline PET choline scan, while incomplete response was defined by a residual PSA of > 0.2 ng/ml and/or radiographic evidence of disease following metastases-targeted therapy. Results: Mean (±SD) age was 68.4 (±7.8) yrs., median (IQR) primary Gleason Score was 7 (7-9) and median (IQR) pre-MDT PSA was 2 (1.3-3.8) ng/ml. Median (IQR) time from primary treatment of the prostate to MDT was 5.1 (2.7-10.1) years. None of the patients were on hormone therapy at the time of presentation with solitary metastases prostate cancer. 30 patients had bone metastases, 29 patients had lymph node metastases, 1 patient had soft tissue metastasis (pelvic metastatic mass), and another patient had visceral metastasis (to the lung). 42% of the patients (n = 26) achieved complete response to targeted therapy. Median time to initiation of 2nd line systemic treatment following MDT was 17.8 months for the complete responders versus 9.3 months for incomplete responders. 11% of the patients (n = 7) did not require 2nd line therapy after their MDT for a mean (±SD) time of 56.9 (±22.5) months. Conclusions: The use of targeted therapy in the management of patients with solitary metastatic disease or low-volume metastatic disease can provide comparable outcomes to those of systemic treatment. Further studies are warranted.