Treatment of prostate cancer patients who have aggressive cancer (Gleason 9-10) with external beam radiotherapy and hormones or surgery, results in poor outcomes. Therapies using prostate brachytherapy with external radiotherapy and hormones have shown more promising outcomes. In a recent study, this combined treatment was compared to surgery or external beam radiotherapy with hormones and the results demonstrated a reduced number of metastasis and prostate cancer death in the group using brachytherapy. Currently, it is not clear how a local treatment, such as prostate brachytherapy, results in less metastasis. One possible explanation for this clinical finding is that an extremely high dose of radiation could result in significant tumour death, and aid the immune system in recognizing tumour cells. This proof of principle research project aims to study and compare the underlying immune response between patients treated with external radiotherapy and hormones or brachytherapy plus external radiotherapy and hormones.