A recent study observed a survival benefit in men diagnosed with metastatic prostate cancer (mPCa) and managed with local treatment of the primary tumor (LT; either radical prostatectomy plus pelvic lymph node dissection or radiation therapy). We tested the hypothesis that only specific mPCa patients would benefit from LT and that the potential benefit would vary based on primary tumor characteristics. A total of 8197 mPCa patients at diagnosis (M1a, M1b, and M1c) were identified using the Surveillance Epidemiology and End Results database (2004-2011) and were divided according to treatment type: LT versus nonlocal treatment of the primary tumor (NLT; either androgen deprivation therapy or observation). Multivariable Cox regression analysis was used to predict cancer-specific mortality (CSM) in patients that received NLT. To assess whether the benefit of LT was different by baseline risk, we tested an interaction with CSM risk and LT. At multivariable analysis, all predictors were significantly associated with CSM, and the interaction test was statistically significant (p < 0.0001). Local treatment of the primary tumor, compared with NLT, conferred a higher CSM-free survival rate in patients with a predicted CSM risk <40%. The number needed to treat according to the predicted CSM risk at 3 yr after diagnosis remained substantially constant from 10% to 30%, whereas it exponentially increased for predicted CSM risk >40%. These results should serve as a foundation for future prospective trials. Patient summary Among metastatic prostate cancer patients, the potential benefit of local treatment to the primary tumor depends greatly on tumor characteristics, and patient selection is essential to avoid either over- or undertreatment.
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