Can vegans eat oyster mushrooms?:
"The oyster mushroom is one of the few known carnivorous mushrooms. Its mycelia can kill and digest nematodes, which is believed to be a way in which the mushroom obtains nitrogen." 
Only one PCa study (but it has been widely studied in other cancers).
 (2006 - U.S.)
"Twenty species of edible mushrooms and three purified mushroom polysaccharides were screened for their antitumor potential on human androgen-independent cancer PC-3 cells. A water-soluble extract (POE) prepared from the fresh oyster mushroom Pleurotus ostreatus produced the most significant cytotoxicity on PC-3 cells among the mushroom species tested. At the same time, POE induced a rapid apoptosis on PC-3 cells ... when the cells were exposed to POE (150 microg/mL) for 2 hours."