Pituitary adenoma

Terminology

• Microadenoma: < 10 mm
• Macroadenoma: ≥ 10 mm

Epidemiology

• Among the most common of all CNS neoplasms (10-15% of primary intracranial neoplasms).
• Microadenoma is much more common than macroadenoma.
• Peak age of presentation is 4th-7th decades.
  • In a child with a diffusely enlarged, homogeneously enhancing gland, think pituitary hyperplasia (e.g. primary hypothyroidism, puberty, end-organ failure) before adenoma.

Clinical

General

• Diabetes insipidus at presentation is uncommon with adenoma and should make you think of other pathology (craniopharyngioma, metastasis, LCH, hypophysitis, etc.)
• Malignant degeneration into pituitary carcinoma is exceptionally rare.
  • Invasive-looking macroadenoma ≠ malignant → need craniospinal spread or distant metastasis to confirm malignancy.

Microadenoma

Most microadenomas are functioning; among these:

• Prolactinomas are the commonest
• Then GH (acromegaly/gigantism)
• Then ACTH (Cushing disease)
• TSH is rare.

Macroadenoma

• Macroadenomas generally present with mass effect (headache, bitemporal hemianopia).
• Slowly growing over a period of years.
• Cavernous sinus invasion is common in larger macroadenomas and is graded with Knosp.
  • Knosp 3–4 = definite invasion → key surgical planning point.

Imaging features

Macroadenoma

Large sellar ± suprasellar mass; normal gland often only a thin peripheral rim or not well-visualized.

CT

• NECT: variable attenuation (usually isodense with grey matter)
• Bone window: enlarged, remodeled sella turcica
  • Giant adenomas can erode and extensively invade the skull base
• Cysts (15-20%)
• Hemorrhage (10%)
• Calcification is rare (2%)
• CECT: Moderate but heterogeneous enhancement

MR

• Macroadenomas usually isointense with cortex on T1-and T2WIs
  • Can demonstrate heterogenous signal intensity on T2WI
• Small cysts and hemorrhagic foci are common
  • "Blooming" on T2* sequences
• Fluid-fluid levels can be present (ddx pituitary apoplexy)
• Strongly but heterogeneously enhance
• Posterior pituitary bright spot usually preserved
• Subtle dural thickening (a dural tail) can be seen (5-10%)
  Meningioma mimic, but adenoma tends to have more irregular invasion of cavernous sinus, no calcified/hyperostotic bone.
• Buzzwords
  • “Snowman / figure-of-eight” suprasellar extension (waist at diaphragma sellae).
  • Suprasellar mass elevating the optic chiasm from below.

DDx of macroadenoma

• Pituitary hyperplasia
• Meningioma of the diaphragmatic sellae
• Metastasis (uncommon)
  • Often loss of posterior pituitary bright spot + early DI.
  • Common primary includes:
    • breast
    • lung
    • systemic Lymphoma
      • infiltrative low T2 signal
• Craniopharyngioma
• Hypophysitis:
  • Symmetrical gland enlargement, homogeneous enhancement, thick, non-tapering stalk
  • Often in post-partum female or on immune checkpoint inhibitors.
• Aneurysm >> look for flow void and eccentricity in origin

Microadenoma

MR

• Pre-contrast: often mildly hypointense or inapparent.
• Standard delayed post-contrast: may be iso-enhancing → occult.
• Dynamic contrast:
  • Normal pituitary enhances early and avidly.
  • Microadenoma enhances more slowly, so on early dynamic phase it appears as a focal hypoenhancing nodule.
• Optional nuance: some microadenomas can be T1 hyperintense if haemorrhagic/proteinaceous.

DDx of microadenoma

• Intrapituitary cysts (esp. cystic microadenoma)
  • Rathke’s cleft cyst:
    Usually non-enhancing, may have intracystic nodule, often midline.
  • Pars intermedia cyst