FINAL DIAGNOSIS CYSTIC SCHWANNOMA
Schwannoma, also known as neurilemmoma or just neurinoma, is a benign tumor of neuro-ectodermal origin arising sporadically or in association with Neurofibromatosis type II from the Schwann cells of the peripheral nervous system. Males and females, typically ages 20-50, are about equally affected. It occurs most frequently in the head/neck and extremities but can occur in the retroperitoneum 3% of the time 1. Sporadic growth in the pancreas is even rarer with fewer than 40 reported cases in the literature as of the time of writing.
In this scenario, diagnosis is often delayed and difficult for several reasons. Schwannomas are relatively non-specific radiographically. On sonography they are typically hypoechoeic while on computed tomography they appear well-demarcated and homogenous 2. Assumption of a cystic growth pattern, as in this case, makes proper identification considerably more difficult. Likewise, no specific laboratory abnormality is able to provide diagnostic information. Patients typically present when expansion of the tumor causes compression of nearby organs or structures sufficient to cause symptoms.
Grossly schwannomas are usually solitary, well demarcated, encapsulated masses, with a solid, pale yellow, rubbery parenchyma. They are frequently small but can grow quite large depending upon the site. On occasion they can show areas of calcification or cystic change with hemorrhage or clear fluid. Cytologic specimens frequently show cohesive groups of bland spindle-cells sometimes with palisading nuclei. The background can appear mucinous in this location resulting in a misdiagnosis of mucinous tumor or pseudocyst.
On histology, schwannomas are traditionally described as biphasic, having alternating hypercellular and hypocellular areas, referred to as Antoni A and Antoni B, respectively. Antoni A areas contain many spindle-shaped cells with nuclear palisading and Verocay bodies. Antoni B areas are relatively hypocellular with a myxoid background and more prominent vessels. Calcifications and mitotic figures are rare but nuclear atypia (ancient change) is not unusual and should not be mistaken for malignancy.
S100 is near universally positive with GFAP, type 4 collagen, and vimentin also frequently positive. Actin and EMA usually stain the capsule. The differential diagnosis depends upon the site, but in this location would include a gastrointestinal stromal tumor, neurofibroma, leiomyoma, or solitary fibrous tumour. Helpful negative stains include keratins, neurofilament, c-kit, CD34 and desmin. Complicating factors include the many schwannoma variants including epithelioid, cellular, ancient, microcystic-reticular, pigmented and others in addition to rare malignant transformation.
Proper identification of the cytologic or surgical specimen and correlation with the patient's symptoms is essential for guiding treatment. Surgical excision is usually curative, although rarely recurrence occurs requiring a subsequent procedure. In the head of the pancreas, pancreaticoduodenectomy is commonly preferred and barring complications results in an excellent outcome 3.
REFERENCES & ADDITIONAL RESOURCES
Contributed by Chris Gilbert, MD and Scott Kulich, MD.