Thanks again for your thoughtful comments, you were all "right on the money" from the beginning. This is indeed a Psammomatous Melanotic Schwannoma. (PMS)
PMS was first described by Carney as a "distinctive, heritable tumor associated with cardiac myxoma and Cushing's syndrome". PMS commonly arise in spinal nerve roots, GI tract and bones. Grossly, they are described as "dumbbell shaped", unencapsulated, sometimes cystic tumors with a distinct black, blue-brown pigmentation. Microscopically, these neoplasms are composed of spindle and epithelioid cells arranged in fascicles or whorls. Melanin pigment is abundant in tumor cells, and melanophages, Psammoma bodies are present, and occasionally one might see focal microvesicular change with some resemblance to fat.
Fifty percent of PMS are associated with Carney's complex which is a multiple endocrine neoplasia (MEN) syndrome that affects the adrenal cortex, pituitary, thyroid glands, and gonads. The complex is also associated with skin and mucosal pigmentation abnormalities and neoplasms of myxoid, neuroectodermal and mesenchymal origin. (Carney's complex should not to be confused with Carney's triad: multiple paragangliomas, gastric stromal sarcoma, and pulmonary chondroma or the recently described syndrome of familial paraganglioma and gastric stromal sarcoma).
On MRI most peripheral nerve sheath tumors are hypointense on T1 and hyperintense on T2 weighed images, this is the opposite of PMS and other melanin containing tumors that are distinctly hyperintense on T1 and hypointense on T2 weighed images. The high T1 signal in melanotic tumors is attributed to the abundance of paramagnetic free radicals in melanin.
The histologic differential diagnosis includes: conventional schwannoma, pigmented neurofibroma, melanocytoma, metastatic melanoma and clear cell sarcoma of soft tissues (See AFIP fascicle of peripheral nerve tumors). Immunohistochemical studies are helpful, PMS are strongly immunoreactivity for HMB-45 (Figure 6. Red Chromogen), S-100 protein (Figure 7. Red chromogen), Vimentin, Laminin, type IV collagen and CD56 (Figure 8). Ultrastructurally, melanosomes and basal lamina formation are readily seen.
Unlike conventional schwannomas, PMS may follow a malignant course after a local recurrence. Although no definite histologic criteria for malignancy exist at this time, the presence of large vesicular nuclei, prominent nucleoli, necrosis and mitoses should raise concern for recurrence and or metastatic potential.
Recently, Stratakis et al have put forward a set of diagnostic criteria for Carney's complex (Stratakis CA, Kirschner LS, Carney JA. Clinical and molecular features of the Carney complex: diagnostic criteria and recommendations for patient evaluation J Clin Endocrinol Metab 2001 Sep;86(9):4041-6.). Be aware that even without all of the other manifestations of Carney's complex, if the patient has a PMS and an inactivating mutation of the PRKR1A gene on chromosome 17, he or she should is best classified as having Carney's Complex.
The initial workup of patients with PMS, includes an echocardiogram (rule out cardiac myxomas), urinary free cortisol levels, serum IGF-I levels, male patients should have testicular USG, thyroid USG at initial evaluation, Dexamethasone-stimulation test, etc.
By the way this patient does not have any of the clinical manifestations of Carney's complex, genetic studies to rule out an inactivating mutation of the PRKAR1A gene are pending.
Rafael Medina-Flores, MD
UPMC Health System
Pittsburgh, PA, USA.