Pituitary carcinomas or metastatic PitNET are rare tumors that are histologically indistinguishable from pituitary adenomas (PitNET) on imaging, defined only by the presence of central nervous system or systemic metastases.

Terminology

In the 5th edition of the WHO classification of CNS tumors and WHO classification of endocrine tumors, the term "pituitary carcinoma" no longer exists as a distinct diagnosis. Rather it favors the use of the term “metastatic PitNET” recognizing that no established histologically these tumors are no different to more indolent tumor ³.

Epidemiology

The incidence is estimated at less than 0.5% of the pituitary symptomatic tumors ^1,5. 

Clinical presentation

They are usually diagnosed as invasive macroadenomas and, as such, have a similar clinical presentation. In many instances, metastatic disease only becomes evident years after a routine pituitary adenoma is treated ⁵. Many tumors are functional, and therefore, clinical features may include Cushing disease, acromegaly, hyperprolactinemia, etc. ^4,5.

Pathology

Pituitary carcinomas are composed of adenohypophysial cells and do not show obvious microscopic features of malignancy. The majority are hormonally active ^1-3. They sometimes demonstrate elevate mitotic index (e.g. Ki67 > 10%) ⁵.

Metastases may occur through the adjacent lymphatic and vascular spaces, hematogeneously, or from invasion into the subarachnoid space. 

Radiographic features

They are indistinguishable from pituitary macroadenomas on imaging, and local invasion to the cavernous sinus and adjacent structures are features of both tumors. The diagnosis may be inferred when metastases are identified. 

Nuclear medicine

PET

Positron emission tomography (PET) scan using F-18 fluorodeoxyglucose (FDG) have been used for staging and follow-up of the metastatic lesions. Imaging using gallium-68 dotatate is useful when the tumor is positive for somatostatin receptors ⁴.

Treatment and prognosis

Treatment depends on location and and extent of disease and the specifics of the tumor. External beam radiotherapy, chemotherapy (e.g. temozolomide, everolimus (mTOR inhibitor), sunitinib, bevacizumab) have been used. If the tumor is positive for somatostatin receptors then somatostatin analogs and peptide receptor radionuclide therapy (PRRT) can be used ⁵.

Due to the rarity of these tumors, no robust prognostic information is available. Patients generally tend to have a poor prognosis with a reported mean survival time between 1 to 2 years ², although, protracted survival has also been reported ⁵.

Differential diagnosis

• metastasis to the pituitary gland

  • as pituitary carcinomas are a rare entity, it is more reasonable to consider other primary sites in patients presenting with a pituitary lesion and concomitant peripheral metastases