Masses in the pineal region can be essentially cystic or neoplastic,
with a significantly lower rate of vascular malformations of adjacent structures that can mimic,
especially if thrombosed,
a mass of the pineal gland (like an aneurysm of the vein of Galen).
Other imaging features to adress,
pineal gland calcifications are detected at CT scan in about 70% of the general adult population and are associated with aging brain (they are extremely rare in children up to 14yo,
and their detection in younger age should suggest an underlying neoplastic lesion).
The extent of calcifications inside the gland may have an inverse correlation with melatonine secretion; moreover,
their appearance inside a tumor mass can be a key factor in the differential diagnosis: germ cell tumors tend to engulf the existing calcifications while pineal parechimal tumors fragment and dislocate them towards the periphery in their growth.
Cystic lesions
Cysts of the pineal gland are usually discovered as incidentalomas in about 25% of MRI studies; they are asymptomatic unless very large; their prevalence increases with age and they show a slight predominance in females.
Their dimensions range up to 15 mm and usually remain stable over time; follow-up is advised only if they show atypical features such as septa,
nodular enhancement or wall thickening.
The pineal region can also harbor arachnoid cysts,
filled with CSF.
MRI findings: round and well-circumscribed,
they appear hypointense on T1-weighted images and hyperintense on T2; on fluid-attenuated inversion-recovery (FLAIR) images the signal may not be completely suppressed due to the proteinaceous contents.
Contrast administration is essential to differentiate them from pineocitomas: cyst enhancement is usually thin (under 2 mm) and confined to the rim (Figure 1).
Arachnoid cysts follow the signal characteristics of cerebrospinal fluid (CSF).
Neoplastic lesions
During embryogenesis,
extragonadal totipotential germ cells migrate intracranially around the third ventricle and give rise to essentially two subtypes of tumors: germinomas and non-germinomatous germ cell tumours (teratomas,
choriocarcinoma,
yolk cell tumours,
and embryonal carcinoma).
All of them tend to affect males under 20 years of age.
The possibility of CSF seeding necessitates imaging evaluation of the entire neuroaxis,
both for diagnosis and follow-up.
In addition,
they produce some oncoproteins (α-fetoprotein,
β-hCG,
placental alkaline phosphatase) whose elevated serum levels can support the diagnosis.
MRI findings: Germinomas are the most frequent tumors of the pineal region.
Their high cellular density determinates their MRI characteristics: they are solid and circumscribed lesions that may have cystic components,
iso- to hyperintense to gray matter on T1- and T2-weighted images and demonstrate avid,
homogeneous enhancement on postcontrast images; reduced diffusion may be seen (Figure2).
They promote calcification of the pineal gland which is subsequently engulfed by the tumor.
Teratomas are the second most common tumors of the pineal region: they are heterogeneous multilobulated masses with cystic components and foci of spontaneous high-intensity on T1- and T2-weighted images depending on fat components and proteins depots.
The differential diagnosis for germ cell tumors includes the primary pineal neoplasms,
high-grade gliomas and lymphomas,
but they can be differentiated from them for the presence of oncoproteins in the CSF and the “engulfed” pattern of calcifications.
Pineocytomas are rare,
slow-growing (WHO grade I),
well-circumscribed lesions occurring more frequently in adults after the age of 40.
On MRI they present as homogeneous iso-signal T1 and high-signal T2 lesions,
strongly enhanced by a contrast agent.
They may have a cystic appearance,
making differentiation from a pineal cyst difficult; however,
at post-contrast imaging,
pineocytomas demonstrate internal or nodular wall enhancement.
Pineoblastomas are highly malignant tumors with a poor prognosis,
primarily affecting young subjects with no sex preference.
At the time of diagnosis they already show large dimensions,
infiltration of adjacent brain structures and often CSF seeding; obstructive hydrocephalus is frequently observed.
The mass is solid,
lobulated,
homogeneous; pineal calcifications,
if seen,
appear fragmented,
“exploded” and pushed at the periphery by the rapid growth of the lesion.
At MR imaging,
pineoblastomas are heterogeneous in appearance,
with the solid portion appearing hypo- to isointense on T1-weighted images and iso- to mildly hyperintense to the cortex on T2-weighted images,
with heterogeneous enhancement on post-contrast sequences.
Hemorragic and necrotic regions may be present,
whereas cystic changes rarely occur.
Diffusion can be restricted in relation to high cellularity (Figure 3).
Papillary tumor of the pineal region (PTPR) is a rare neuro-epithelial neoplasm both in children and adults,
recently included in the WHO classification as a grade II-III tumor.
On MRI,
PTPR presents as a relatively large solid mass,
often indistinguishable from a pineocytoma: however,
the detection of foci of spontaneous hyperintense T1 signal (due to protein secretory inclusions) or CSF seeding is characteristic of PTPRs (Figure 4).
Gliomas of all histologies have been described in the pineal region,
mainly astrocytomas and ependymomas,
but also high grade glioblastomas (Figure 5).
They don't generally possess patognomonic features on MR imaging; low-grade lesions usually don't show contrast enhacement,
whereas high-grade ones tend to have dishomogeneous enhancement and necrotic areas.
Depending on the grade of the lesion,
CSF seeding and invasion of adjacent structures can be observed.
Slow-growing,
low grade astrocytomas,
like pilocytic astrocytomas,
however,
are characterized by prominent and relatively homogeneous contrast enhancement; they usually occur in the region of the tectum,
presenting with symptoms of obstructive hydrocephalus.
Meningiomas,
although infrequent in this region,
are easy to identify: they are hypo- to isointense on T1-weighted images and iso- to hyperintense on T2-weighted images and,
after contrast administration,
enhance avidly and show the typical “dural tail” image.
Metastases are also infrequent but,
in consideration of the particular location of the pineal gland outside of the blood-brain barrier,
they can be observed even in the absence of other parenchymal focalizations.
The most common solid tumors to spread to the pineal region are those of the lung,
breast,
kidney and gastro-intestinal tract.
Also focalizations from a lymphoma have been observed (Figure 6).