Pituitary microadenomas are a minority of all pituitary adenomas, but can pose imaging and management challenges on account of their size and protean clinical presentations.
By definition a microadenoma is less than 10 mm in size. If the same tumour is greater than 10 mm in size it is then considered a pituitary macroadenoma. Differences in presentation and imaging merely represent a difference in size rather than any fundamental difference in biology.
For a general discussion, including epidemiology, treatment and prognosis , please refer to the article on pituitary adenomas.
A pituitary microadenoma is confined to the sella, and as such has no scope to produce symptoms due to mass effect. As such they are most frequently diagnosed as the result of investigating hormonal imbalance (usually excess production of one or more hormones). Rarely they can be an incidental finding, however as by their very nature microadenomas are difficult to identify on anything other than dedicated pituitary imaging.
Skull radiographs and CT
Historically, before the advent of MRI, the pituitary was images with lateral skull x-rays (looking for remodelling of the pituitary fossa), and later with CT. Although CT was able to detect up to 80-90% of microadenomas between 5-10mm in size, it was highly technique and radiologist dependent, and had difficulty in identifying smaller nodules 2.
MRI is the mainstay of imaging for pituitary microadenomas, and required dedicated pituitary sequences (thin slice, small field of view, dynamic contrast acquisition). Contrast enhanced MRIs have a sensitivity of 90%.
Post contrast and especially thin section dynamic contrast enhanced imaging is an important part of a pituitary MRI and has significantly improved diagnostic accuracy 2-3. Some often subtle morphology changes can be identified on non-contrast images however. These include bulkiness of the gland on the side of the microadenoma, subtle remodelling of the floor of the sella, deviation of the pituitary infundibulum away from the adenoma 2.
- T1: usually isointense to normal pituitary
T1 C+ (Gd)
- dynamic sequences demonstrate a rounded region of delayed enhancement compared to the rest of the gland 1
- delayed images are variable, ranging from hypo-enhancement (most common) to isointense to the rest of the gland, to hyperintense (retained contrast)
- T2: variable, but often a little hyperintense
An important fact of life needs to be kept in mind when reporting pituitary MRIs: small pituitary incidentalomas are relatively common, with up to 2-30% of autopsies identifying small asymptomatic microadenomas 4.
Inferior petrosal sinus sampling
Inferior petrosal sinus sampling is now reserved for one of two situations where patients are suspected of having a pituitary microadenoma, despite normal MRI:
- confirm presence of a microadenoma, rather than a non-pituitary source 4; this is especially the case of Cushing's disease, as there are many sources of extra-pituitary ACTH (e.g. some lung cancers)
- lateralize the microadenoma, to aid in surgical exploration
The differential is broadly that of other pituitary regions masses, but is predominantly composed of:
normal pituitary gland
- especially the periphery of the pituitary can be challenging to image
Rathke's cleft cyst
- no enhancement rather than hypo-enhancement
- rarely purely intrasellar
- usually present larger
- calcification common
Pituitary region masses
- pituitary adenoma (commonest in the adult population)
- hypothalamic astrocytoma/glioma
- chiasmatic astrocytoma
- optic nerve glioma
- dermoid (CNS) / epidermoid / intracranial teratoma
- pituitary metastases
- granular cell tumour of the pituitary (pituitary choristoma)
- pilocytic astrocytoma of the neurohypophysis (infundibuloma)
- cellular infiltrates
- other lesions
- 1. Newton HB, Jolesz FA. Handbook of neuro-oncology neuroimaging. Academic Pr. (2008) ISBN:012370863X. Read it at Google Books - Find it at Amazon
- 2. Peck WW, Dillon WP, Norman D et-al. High-resolution MR imaging of pituitary microadenomas at 1.5 T: experience with Cushing disease. AJR Am J Roentgenol. 1989;152 (1): 145-51. AJR Am J Roentgenol (abstract) - Pubmed citation
- 3. Kucharczyk W, Bishop JE, Plewes DB et-al. Detection of pituitary microadenomas: comparison of dynamic keyhole fast spin-echo, unenhanced, and conventional contrast-enhanced MR imaging. AJR Am J Roentgenol. 1994;163 (3): 671-9. AJR Am J Roentgenol (abstract) - Pubmed citation
- 4. Sano T, Rayhan N, Yamada S. [Pathology of pituitary incidentaloma]. Nippon Rinsho. 2004;62 (5): 940-5. - Pubmed citation