Pineocytoma, also known as a pinealocytoma, is a benign, slowly-growing tumor

Tumor

A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

 of the pineal gland

Pineal gland

The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...

. Unlike the similar condition pineal gland cyst

Pineal gland cyst

A pineal gland cyst is a benign cyst in the pineal gland, a small endocrine gland in the brain. Historically, these fluid-filled bodies appeared on 1-4% of magnetic resonance imaging brain scans, but were more frequent at death, seen in 21-41% of autopsies. But a 2007 study by Pua et al...

, it is uncommon.

Diagnosis

Pineocytomas are diagnosed from tissue, i.e. a brain biopsy

Brain biopsy

Brain biopsy is the removal of a small piece of brain tissue for the diagnosis of abnormalities of the brain. It is used to diagnose Alzheimer's disease, tumors, infection, inflammation, and other brain disorders....

.

They consist of:

• cytologically
  Cytopathology
  Cytopathology is a branch of pathology that studies and diagnoses diseases on the cellular level. The discipline was founded by Rudolf Virchow in 1858. A common application of cytopathology is the Pap smear, used as a screening tool, to detect precancerous cervical lesions and prevent cervical...
  
   benign cells (with nuclei of uniform size, regular nuclear membranes, and light chromatin
  Chromatin
  Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
  
  ) and,
• have the characteristic pineocytomatous/neurocytic
  Neurocytoma
  Neurocytoma is a type of nervous system tumor which is primarily derived from nervous tissue.This is in contrast to the gliomas , which are derived from glial cells, which are not derived from nervous tissue.-External links:* http://moon.ouhsc.edu/kfung/JTY1/Com/Com307-1-Diss.htm*...
  
   rosettes, which is an irregular circular/flower-like arrangement of cells with a large meshwork of fibers (neuropil
  Neuropil
  In neuroanatomy, a neuropil, which is sometimes referred to as a neuropile, is a region between neuronal cell bodies in the gray matter of the brain and blood-brain barrier . It consists of a dense tangle of axon terminals, dendrites and glial cell processes...
  
  ) at the centre.

Pineocytomatous/neurocytic rosettes are superficially similar to Homer-Wright rosettes; however, they differ from Homer-Wright rosettes as they have (1) more neuropil at centre of the rosette and, (2) the edge of neuropil meshwork irregular/undulating.