Parasellar T2 dark sign on magnetic resonance imaging to differentiate lymphocytic hypophysitis from pituitary adenoma

BACKGROUND: Pituitary adenomas are the most common sellar masses in adults with magnetic resonance imaging (MRI) being the imaging modality of choice. Inflammatory pituitary lesions such as lymphocytic hypophysitis (LH) can mimic pituitary macroadenoma on imaging and are often misdiagnosed as such. Although the imaging appearance on most of the sequences on MRI has similar findings, LH has a characteristic dark signal on T2 images (called dark T2 sign) which can be very helpful to reliably differentiate the two conditions. CASE DESCRIPTION: A 68-year-old woman diagnosed with a “pituitary mass” on the MR study done at an outside facility was referred to our neurosurgery department. The case was discussed at our multidisciplinary tumor board, where the possibility of an inflammatory condition mimicking tumor was considered, given the very dark signal on T2-weighted sequences. Transsphenoidal endoscopic biopsy revealed a firm rubbery mass, which histopathology demonstrated fibrous connective tissue with inflammatory cells consistent with LH. CONCLUSION: Dark T2 signal on MR imaging can be very helpful in demarcating inflammatory pituitary conditions like LH from pituitary macroadenomas.