While there are maitake cancer studies on PubMed, there are none for PCa.
"In 2009, a phase I/II human trial, conducted by Memorial Sloan–Kettering Cancer Center, showed Maitake could stimulate the immune systems of breast cancer patients. Small experiments with human cancer patients have shown Maitake can stimulate immune system cells, like NK cells. In vitro research has also shown Maitake can stimulate immune system cells. An in vivo experiment showed that Maitake could stimulate both the innate immune system and adaptive immune system.
In vitro research has shown Maitake can induce apoptosis in various cancer cell lines as well as inhibit the growth of various types of cancer cells. Small studies with human cancer patients revealed that a portion of the Maitake mushroom, known as the Maitake D-fraction, possesses anti-cancer activity. In vitro research demonstrated the mushroom has potential anti-metastatic properties.
Research has shown Maitake has a hypoglycemic effect, and may be beneficial for the management of diabetes. The reason Maitake lowers blood sugar is because the mushroom naturally contains an alpha glucosidase inhibitor.
Maitake contains antioxidants and may partially inhibit the enzyme cyclooxygenase.[ An experiment showed that an extract of Maitake inhibited angiogenesis via inhibition of the vascular endothelial growth factor (VEGF)."
 (2009 - U.S. - Memorial Sloan-Kettering)
This is the study mentioned above. However, a note of caution:
"Increasing doses of Maitake increased some immunologic parameters and depressed others; the dose–response curves for many endpoints were non-monotonic with intermediate doses having either immune enhancing or immune suppressant effects compared with both high and low doses."
"Oral administration of a polysaccharide extract from Maitake mushroom is associated with both immunologically stimulatory and inhibitory measurable effects in peripheral blood. Cancer patients should be made aware of the fact that botanical agents produce more complex effects than assumed, and may depress as well as enhance immune function."