Pineal Cysts are usually small, benign (not malignant) cysts located in the Pineal Gland in the brain. Most are small (>5mm) and these small cysts are common, and often an incidental finding on MRI. Small cysts do not cause symptoms.
Larger Cysts (<10mm) are more rare, and may be symptomatic. The importance of Pineal Cysts is that they cannot be distinguished from Pineal Tumours on MRI alone. Larger cysts, or cysts with irregular features may actually be a Cystic Pineocytoma, a slow growing malignant pineal tumour. But a tumour cannot be definitively diagnosed without tissue samples, which involves invasive surgery. If it is a Cystic Pineocytoma, the prognosis is excellent, when the tumour is completely removed surgically. Treatment also sometimes requires radiation and chemotherapy in severe cases. The five year survival rate is 86%, so if you have to have a brain tumour, this is a good one to have.