Risk of skeletal related events among elderly prostate cancer patients by site of metastasis at diagnosis

The purpose of this study was to estimate the risk of developing skeletal-related events (SREs) based on site of metastasis at diagnosis and identify other predictors of developing SREs among metastatic prostate cancer patients. We conducted a retrospective cohort study using linked SEER (Surveillance, Epidemiology, and End Results) and Medicare data and identified men over the age of 65 with incident metastatic prostate cancer diagnosed during 2005-2009. SREs included radiation (RAD), pathological fractures (PF), bone surgery (BS), and spinal cord compression (SCC). The association between site of metastasis at diagnosis and SRE was examined using a Cox proportional hazards model that accounts for death as a competing risk. Among 4404 men (median age: 79 years) with incident metastatic prostate cancer, 44% experienced SREs at a median of 9.6 months post diagnosis. Compared to bone metastasis only, our model showed that patients were significantly less likely to develop SREs if they had LN-only metastasis at diagnosis (Sub-Hazard Ratio [SHR] 0.56; 95% Confidence Interval [CI]: 0.43-0.72) or unknown site of metastasis (SHR: 0.79; CI: 0.64-0.97). Other predictors of reduced SRE risk were age 80+ years (SHR: 0.83; CI: 0.75-0.91), non-Hispanic Black (SHR: 0.77; CI: 0.65-0.90), or being diagnosed in year 2009 (SHR: 0.85; CI: 0.72-0.99). Patients were significantly more likely to develop SREs if they received androgen deprivation therapy (SHR: 1.73; CI: 1.48-2.02) or had Gleason score 8-10 disease (SHR: 0.79; CI: 0.64-0.97). Compared to patients who present with bone metastasis only at diagnosis, patients presenting with other metastatic sites have similar risk of developing SREs, with the exception of those presenting with lymph node only metastasis who have a significantly reduced risk of SREs.