Pineocytoma

Pineocytomas are relatively benign (WHO grade 1) pineal parenchymal tumors that have a relatively good prognosis.

On imaging, they generally appear as a well-demarcated tumor less than 3 cm in its greatest dimension, iso- to hyperattenuating on CT, hypo- to isointense on T1, and hyperintense on T2. An avid and homogeneous enhancement is characteristic. Cystic changes may be present, which in some cases, can make it difficult to differentiate from a pineal cyst. 

Epidemiology

Pineocytomas can be encountered at any age but mostly occur in adults (20-60 years of age) ⁹. Unlike pineal germinomas, which have a strong predilection for males, pineocytomas are somewhat more frequently encountered in females (M:F 0.6:1) ⁹.

Clinical presentation

As with all other pineal region masses, clinical presentation is mainly from obstructive hydrocephalus secondary to compression of the tectum of the midbrain and obstruction of the aqueduct. Compression of the superior colliculi can also lead to a characteristic gaze palsy, known as Parinaud syndrome.

Pathology

Pineocytomas are well-differentiated tumors and considered WHO grade 1 tumors in the 5th edition (2021) WHO classification of CNS tumors, believed to arise from pinealocytes ⁹.

Macroscopic appearance

These tumors are well circumscribed, with cystic or hemorrhagic change sometimes evident ⁹. They have a grey or tan cut surface.

Microscopic appearance

Under light microscopy, pineocytomas are composed of small cells similar in appearance to normal pinealocytes, arranged in sheets; pineocytomatous pseudorosettes are characteristic and not seen in normal pineal gland tissue ⁹.

As is the case with the rest of the pineal gland, pineocytomas do not have a well-formed blood brain barrier and as such enhance vividly with contrast ⁶.

In keeping with their low grade, mitotic figures are rare ⁹. 

Immunophenotype

• synaptophysin: positive
• neurofilament: positive
• neuron-specific enolase: positive
• other neuronal markers (e.g. MAPT, chromogranin-A, 5-HT etc..): variable
• NeuN: negative

Radiographic features

Typically, pineocytomas are slow-growing and well-circumscribed tumors (compared to pineoblastomas that tend to be larger, and less well-circumscribed). They tend to be solid, although focal areas of cystic change or hemorrhage do occur. When the cystic component is large, distinguishing them from pineal cysts can be difficult, although it has been postulated that this is not so much the case if complete and accurate imaging is obtained (i.e. good quality thin-section MRI with contrast) ⁶. 

CT

CT demonstrates the mass to be of intermediate density, similar to the adjacent brain. Pineal calcifications tend to be dispersed peripherally. This is the same pattern seen in other pineal parenchymal tumors, which is helpful in distinguishing these tumors from pineal germinomas that tend to engulf pineal calcification. 

MRI

MRI is the modality of choice for examining tumors of the pineal region. 

• T1: hypo- to isointense to brain parenchyma
• T2
  • solid components are isointense to brain parenchyma
  • areas of cystic change are common
  • sometimes the majority of the tumor is cystic
• T1 C+ (Gd): solid components vividly enhance

Treatment and prognosis

Pineocytomas are treated surgically and have an excellent prognosis when a complete resection is achieved (which is most of the time as they are well-circumscribed lesions). A 5-year survival rate of 86% has been reported ⁷. 

Local recurrence and even CSF metastases are reported, but are rare ⁵. 

Differential diagnosis

On CT or MRI consider:

• pineal cyst
  • at most, thin smooth (<2 mm) peripheral enhancement
• other pineal parenchymal tumors
  • pineal parenchymal tumor with intermediate differentiation
  • pineoblastoma: larger, poorly defined
  • papillary tumor of the pineal region
• germ cell tumors
  • germinoma
    • marked male predominance
    • engulfed calcification
  • embryonal carcinoma
  • choriocarcinoma
  • teratoma: may contain fat
• astrocytoma of the pineal gland
• metastasis