Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pineocytoma ?
    ### Response:
    A pineocytoma is a tumor of the pineal gland, a small organ in the brain that makes melatonin (a sleep-regulating hormone). Pineocytomas most often occur in adults as a solid mass, although they may appear to have fluid-filled (cystic) spaces on images of the brain.
    