The prostatic venous plexus drains into the internal iliac veins and communicates with the vertebral plexus. Therefore most hematogenous spread is towards the lungs first, filtering most metastases out. The drainage towards the vertebral plexus is thought to be the cause of spread towards the vertebrae. Also this is the reason why, when prostate cancer metastases are found in the brain, they are primarily seen in the posterior fossa.

In addition, there may be something in blood-brain barrier that prevents prostate adenocarcinoma metastatizing to the brain and this may be associated with immune response as well. Sutton et al speculated this in their case report, observing that patients with brain metastases usually had disseminated disease at time of diagnosis of brain implant.

Sutton M a, Watkins HL, Green LK, Kadmon D. Intracranial metastases as the first manifestation of prostate cancer. Urology [Internet]. 1996 Nov;48(5):789–93.

Brain metastases from prostate cancer are rare, occurring in 2.4 % of patients seen at Memorial Sloan-Kettering Cancer Center over a 7 year period (1993-2000) neurologic consultation. They often present with focal neurologic symptoms and occur in the setting of systemic metastatic disease. Whole brain radiation therapy is effective in minimizing neurologic morbidity and may extend survival. ( ASCO 2001)

Recently, a single brainstem metastasis from a prostate adenocarcinoma, who had previously been diagnosed with Muir-Torre syndrome has been reported by University of Navarra ( Spain).

All explanations, until now, are in the plane of speculation