Immune Biomarkers in Prostate Cancer Patients Treated With Brachytherapy

This study is collecting blood samples and health information from prostate cancer patients who are being treated with brachytherapy (a type of internal radiation therapy using small radioactive seeds or high-dose implants). The goal is to look at immune markers in the blood to understand how the immune system responds to this treatment. **You may be eligible if...** - You have prostate cancer and are receiving brachytherapy (either I-125 seeds or HDR/PDR brachytherapy) - You are 18 or older - For the primary treatment group: you have not had any previous prostate cancer treatment - For the salvage (re-treatment) group: you have documentation of your prior treatment and have not had radiation within the last 3 months, and no history of prostate removal surgery **You may NOT be eligible if...** - You have a pre-existing autoimmune disease - You are on blood-thinning medication - You have a history of substance abuse Talk to your doctor to see if this trial is right for you.