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Pituitary macroadenoma

Pituitary macrocadenomas are the most common suprasellar mass in adults, and responsible for the majority of transsphenoidal hypophysectomies. They are defined a as pituitary adenomas greater than 10 mm in size and are approximately twice as common as pituitary microadenomas 10

For a general discussion, including epidemiology, please refer to the article on pituitary adenomas

Clinical presentation

Patients typically present with symptoms of local mass effect on adjacent structures (especially optic chiasm). Some may present hormonal imbalance, with symptoms of hypopituitarism (from compression) or secretion. Hormonal imbalance due to overproduction tends to present earlier and tumours are thus usually small at presentation. This mode of presentation is discussed in the article on pituitary microadenomas

Rarely pituitary apolplexy may present acutely and often catastrophically. 

Optic chiasm compression

The optic chiasm is located directly over the pituitary gland in 80% of individuals. The rest are divided between pre and post fixed chiasms. A prefixed optic chiasm is located anterior to its normal position over the tuberculum sellae, whereas a postfixed chiasm is located over the dorsum sellae 10

A macroadenoma growing superiorly out of the pituitary fossa (of for that matter other pituitary region masses) will contact, elevate and compress the central part of the chiasm in most individuals. This central part carries fibers from the nasal retina, and thus results in the classical bitemporal hemianopia 10. Patients typically complain of bumping into things or having car accidents, but as the macular fibres are often spared, they may not be aware of actual visual deficits. 

In cases of prefixed or postfixed chiasms, or when the macroadenoma grows asymmetrically then the optic nerves or optic tracts can be compressed, resulting in a variety of visual deficits. 

Cavernous sinus invasion

Some macroadenomas demonstrate invasive growth, and extension into the cavernous sinuses is characteristic. Prolactin secreting tumours are most frequently responsible for cavernous sinus extension, and typically prolactin level increase significantly when the tumour gains access to the sinus 10. Once in the sinus, these tumours are difficult to completely resect. 

They may compress cranial nerves resulting in deficits, although this is uncommon, seen in only 1-14% of cases 10. Occulomotor nerve (CN III) is most commonly involved, followed by the abducens nerve (CN VI) 10

Radiological and surgical invasion grading system characterised by areas distinguished from each other by a medial, median and lateral intercarotid lines: medial, lateral and central tangent on the intra- and supracavernous internal carotid arteries on coronal view 11.

Pathology

Most macroadenomas are non-secretory (endocrinologically inactive). They are a type of benign epithelial tumour composed of adenohypophysial cells

Radiographic features

Pituitary macroadenomas are by definition >10 mm mass arising from the pituitary gland, and usually extending superiorly. Indentation at the diaphragma sellae can give a snowman or figure eight configuration 10

CT

No contrast attenuation can vary depending on haemorrhagic, cystic and necrotic components. Adenomas which are solid, without haemorrhage, typically have attenuation similar to brain (30-40HU) and demonstrates moderate contrast enhancement; less marked than one typically sees in meningiomas. 

Calcification is rare.

MRI

MRI is the preferred imaging modality, not only able to exquisitely delineate the mass, but also clearly visualise the optic chiasm, anterior cerebral vessels and cavernous sinuses. 

Overall signal characteristics can significantly vary depending on tumour components such as haemorrhage, cystic transformation or necrosis.

  • T1
    • typically isointense to grey matter 10
    • larger lesions are often heterogeneous and vary in signal due to areas of cystic change/necrosis/haemorrhage
  • T1 C+ (Gd)
    • solid components demonstrates moderate to bright enhancement
  • T2
    • typically isointense to grey matter 10
    • larger lesions are often heterogeneous and vary in signal due to areas of cystic change / necrosis / haemorrhage
  • T2* gradient echo
    • most sensitive for detecting any haemorrhagic components, which appear as areas of signal loss
    • calcification is rare, but should be excluded by reviewing CT scans

Assessment of cavernous sinus invasion can be difficult. The most convenient method is to assess the degree of encasement of the cavernous portion of the internal carotid artery. Less than 90 degrees makes involvement of the sinus very unlikely, where as greater than 270 degrees makes involvement almost certain 10

Differential diagnosis

The differential of a pituitary macroadenoma is essentially the list of conditions leading to a pituitary region mass. The most common considerations include:

  • craniopharyngioma (papillary type)
    • more likely cystic
    • more likely to have areas of calcification (although still a minority of cases)
    • more likely to have areas of T1 intrinsic hyperintensity (although blood can result in similar appearances)
  • pituitary metastasis
    • often in the setting on known disseminated malignancy
    • often less well defined
    • bone destruction rather than remodelling may be seen
  • pituitary carcinoma
    • indistinguishable on imaging
    • CSF seeding may be evident
    • rare
  • meningioma
    • dural tail usually visible
    • enhancement more vivid
    • hyperdense on non-contrast CT

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