Can pineal tumors be a cause of psychosis?

Rarely: There are case reports in the medical literature of pineal tumors causing psychosis. That means it happens so rarely that when it does, someone writes a paper about it!

Yes: It is possible. Pineal tumors can be associated with alterations in hormone levels that could lead to psychosis. But it isn't very common.

I'm no surgeon, but: I don't think so. Before the VP shunt is placed, the pt would have sxs of increased pressure: vision changes, balance problems, headache, etc. After the shunt, those sxs should abate. Psychosis is hearing voices, seeing things that aren't there, held beliefs despite evidence to the contrary; and should not be related to a pineal tumor.