Background: The association of peripheral monocyte count and prostate cancer progression is not well characterized. Objective: Our aim was to investigate the prognostic value of absolute monocyte count (AMC), which is thought to modulate immune response in the tumor microenvironment, in castration-resistant prostate cancer (CRPC) patients treated with docetaxel chemotherapy. Methods: We retrospectively reviewed the medical records of 214 CRPC patients who received docetaxel therapy and were used as the training and validation set. Docetaxel at a dose of 75 mg/m2 was administered every 3 or 4 weeks. Clinicopathological factors and laboratory data were collected to assess the prognostic factors for overall survival (OS) and progression-free survival (PFS). Results: In the training set, the median age was 73.0 years, and the median prostate-specific antigen (PSA) value was 31.7 ng/ml at initial treatment. The median OS and PFS were 23.0 months (range 1.20–84.0) and 11.2 months (range 3.6–78.0), respectively. According to multivariable Cox regression analysis, AMC ≥400/uL, PSA level ≥20 ng/ml, and Hb <10 mg/dL were associated with increased risk of PSA progression [hazard ratio (HR) 2.06, p = 0.005; HR 2.39, p = 0.002; and HR 2.38, p = 0.024, respectively]. Moreover, multivariate analysis for OS indicated that AMC ≥400/uL, pretreatment PSA level ≥20 ng/ml, presence of visceral metastasis, and alkaline phosphatase ≥284 U/L were independent prognostic factors for shortened OS (HR 2.07, p = 0.004; HR 2.18, p = 0.007; HR 2.11, p = 0.011; and HR 1.67, p = 0.048, respectively). According to the validation set, high AMC remained an independent prognostic factor for PFS and OS (HR 2.26, p = 0.001; and HR 3.10, p < 0.001, respectively). Conclusions: Elevated monocyte counts were associated with aggressive tumor features and poor survival outcomes of patients with CRPC treated with docetaxel chemotherapy.
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