Retroperitoneal melanotic schwannoma: Ultrasonographic features

Case Report

J Clin Ultrasound 23:42-48, January 1995 0 1995 John Wiley & Sons, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America. CCC 0091-27511951010042-07

Retroperitoneal Melanotic Schwannoma: Ultrasonographic Features Daniel D. Do-Dai, MD,* Vincent B. Ho, MD,*§II Miguel J. Rovira, MD,*§ Richard W. Knight, MD,t and Patrick A. Twomey, MDS

In 1932, Millar reported the first case of melanotic schwannoma. Since then, 72 additional cases have been reported, mainly in the pathological and surgical 1iterature.'-l8 Melanotic schwannoma is an extremely rare variant of neurilemoma capable of melanogenesis. Although the computed tomography (CT) and magnetic resonance imaging (MRI) findings have been shown in sevto our knowledge, the ultraera1 sonographic (US) findings of melanotic schwannoma have not yet been reported. We describe the US features of melanotic schwannoma of the iliohypogastric nerve with CT, MRI, and pathologic correlation. A review of the 73 previously reported cases of melanotic schwannomas is also included.

CASE REPORT

A 73-year-old white man was initially seen at the urologic clinic with a history of urinary frequency and vague abdominal pain. He had no history of fever, hematuria, or flank pain. His medical history was significant for atrial fibrillation, hypertension, hypothyroidism, and non-insulindependent diabetes mellitus. His family histories revealed no history of genitourinary, congenital, or neurocutaneous disease. Physical examination did not demonstrate a skin abnormality or a palpable mass. Laboratory studies, including urinal-

From the Departments of *Radiology, ?Urology, and SPathology, Madigan Army Medical Center, Tacoma, Washington; the Department of §Radiology, School of Medicine, University of Washington, Seattle, Washington; and the "Department of Radiology and Nuclear Medicine, Uniformed Services University of the Health Sciences, Bethesda, Maryland. For reprints contact Daniel D. Do-Dai, MD, Department of Radiology, Madigan Army Medical Center, Tacoma, WA 98431-5419. The expressed opinions or assertions are the private views of the authors and do not reflect the views of the United States Army or the Department of Defense.

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ysis, were unremarkable except for an elevated serum glucose of 243 mg/dL, blood urine nitrogen of 29 mg/dL, and creatinine of 1.5 mg/dL. A screening right upper-quadrant US was performed with a 3.5-MHz sector transducer, revealing multiple gallstones. Incidentally noted was an oval 2.9-cm x 2.7-cm sharply demarcated hypoechoic mass abutting the posterior lower pole of the right kidney (Figure 1A). A thin, echogenic plane separated the mass from the kidney, suggesting an extrarenal origin of the mass. Further evaluation with a high-resolution 10-MHz transducer confirmed the extrarenal origin and better characterized the hypoechogenicity, with a slight distal acoustic enhancement, of the mass (Figure 1B). CT (Figure 2) and MRI (Figure 3) of the abdomen confirmed an extrarenal mass in the posterior pararenal space. The tumor abutted the transversalis fascia of the posterior abdominal wall and displaced the Gerota's fascia anteriorly (Figure 2). A mild heterogeneous enhancement of the mass was noted with contrast-enhanced CT. A CT-guided core biopsy of the mass was performed. Histologic studies yielded a diagnosis of benign melanotic schwannoma. At surgery, a 5.6-cm x 3.5-cm X 2.9-cm wellcircumscribed soft-tissue mass was found, and noted to surround the iliohypogastric nerve eccentrically. The tumor was completely excised following transection of the iliohypogastric nerve proximally and distally. On cross-section, the tumor was encapsulated with a thin, white, fibrous membrane and appeared tan-yellow with areas of brown pigmentation (Figure 4A). Microscopically, hematoxylin and eosin stained sections showed irregularly interlacing fascicles of spindle-shaped cells in association with a moderate number of small, thick-walled blood vessels (Figure 4B). Many of the spindle-shaped cells contained brownish melanin pigment in their cytoplasm. Mallory's iron stain revealed that 30% to 40% of the pigment was hemosiderin and the remaining 60% to 70% was melanin. JOURNAL OF CLINICAL ULTRASOUND

RETROPERITONEAL MELANOTIC SCHWANNOMA

DISCUSSION

FIGURE 1. (A) Transverse scan with a 3.5-MHz sector transducer reveals a round, hypoechoic mass (M) abutting the posterior inferior pole of the right kidney (K). A thin, discrete echogenic plane (arrowheads) was identified separating the mass from the kidney. (B) Longitudinal scan with a high-resolution linear-array transducer better delineates the well-circumscribed oval shape and the hypoechogenicity with a slight distal acoustic enhancement of the mass (M). The echogenic fat plane (arrowheads) between the mass and the kidney (K) was also seen.

VOL. 23, NO. 1, JANUARY 1995

Right upper-quadrant US is frequently requested by clinicians as a screening method for vague abdominal symptoms. Invariably, incidental retroperitoneal abnormalities are identified. Most of these “incidentalomas” are commonly of renal origin. Infrequently, a retroperitoneal mass abutting the kidney may mimic an intrarenal mass. The use of a high-resolution transducer can often delineate the true origin of the mass, especially when the mass is superficial in location. Furthermore, a high-resolution transducer may also better characterize the true echotexture of the mass and also the echogenic fat plane between the mass and the kidney (Figure 1B). Reports of imaging studies on melanotic schwannomas in the literature are scant. Ultrasound has proven to be a useful method in the evaluation of suspected nerve sheath turn or^.'^*^^ Normal nerves appear as hyperechoic tubular structures with fibrillar echotexture in longitudinal scans and rounded shapes with a coarsely dotted pattern in transverse scans. US findings of melanotic schwannomas have not yet been reported. In our case, the melanotic schwannoma was seen as a well-circumscribed, oval, hypoechoic mass with a slight distal acoustic enhancement similar t o the US features of a benign s ~ h w a n n o m a Differentiation .~~~~~ of melanotic from nonmelanotic schwannomas cannot be made 43

CASE REPORT: DO-DAI ET AL.

FIGURE 2. (A) Unenhanced CT scan reveals a well-defined, round, soft-tissue mass of muscle attenuation in the enlarged right posterior pararenal space. The tumor abuts the transversalis fascia of the posterior abdominal wall and displacesthe posterior Gerota’s fascia (arrowheads) anteriorly. Note the normal, small, left posterior pararenal space (arrow). (B) Contrast-enhanced CT scan demonstrates a mild heterogeneous enhancement of the posterior pararenal mass.

sonographically. Cystic and necrotic changes were not demonstrated in this case, but have been reported in 57% to 63%of benign retroperitoneal neurilemoma.21The distal acoustic enhancement, which is most commonly associated with simple cysts, is seen in 50% of sch~annomas.~’ It can also be seen in an abscess or a variety of masses, including hemangioma, fibroadenoma, and lymphoma.22*23 The distal acoustic enhancement results from better sound penetration in the homogeneous mass than in the adjacent surrounding structures, secondary to the relatively lower acoustic attenuation coefficient of the mass.24 CT and MRI are often useful in preoperative evaluation of retroperitoneal masses, especially 44

differentiating a renal from an extrarenal mass. Both CT and MRI are very sensitive in excluding fat-containing tumors, such as angiomyolipomas, myelolipoma, and lipomas. MRI also has an advantage in the evaluation of melanotic lesions by demonstrating the T1 shortening of melanin on T1-weighted images.10311’13,14 The melanotic schwannoma in our case did not demonstrate this T1 shortening property, and appeared relatively isointense to muscle on T1-weighted images (Figure 3). Only one previous case of melanotic schwannoma with low T1 signal intensity has been r e p ~ r t e d . ’The ~ reason for this behavior is not clear, however. The absence of a high signal intensity in pre-Gadolinium T1-weighted images JOURNAL OF CLINICAL ULTRASOUND

RETROPERITONEAL MELANOTIC SCHWANNOMA

FIGURE 3. TI-weighted coronal spin echo 30011612 (repetition timeiecho tirneiexcitations) image shows a homogeneous muscle-isointense elliptical mass (arrow) lying obliquely and following the expected course of the right iliohypogastric nerve.

cannot be used as the criterion to differentiate melanotic from nonmelanotic schwannomas. Heterogeneous enhancement of the schwannoma on contrast-enhanced CT and MRI is also nonspecific for malignant versus benign nature of the sch~annomas.'~~'~ The differential diagnosis of a mass in the posterior pararenal space is limited, as this space contains no visceral organs. Tumors arising in the posterior pararenal space can originate from the supporting or adjacent tissues. They may be benign and arise from fibrous tissue (fibroma), vessel (hemangioma), fat (lipoma), muscle (leiomyoma or rhabdomyoma), or nerve (schwannoma, neurofibroma, or paragangli~ma).'~ Or, they may be malignant like malignant fibrous histiocytoma, fibrosarcoma, angiosarcoma, liposarcoma, rhabdomyosarcoma, malignant schwannoma, lymphoma, and metastase~.'~Of course, pseudomasses like loculated fluid collections, abscess, and hematoma should be included in the differential diagn~sis.'~ A simple cyst can be excluded based on the hypoechoic appearance of the mass. Abscesses and hematomas can be difficult to differentiate VOL. 23, NO. 1, JANUARY 1995

from solid masses sonographically, especially when internal debris is present, but are unlikely when the patient has no supporting clinical history of fever to suggest infection, inflammatory processes like pancreatitis, bleeding diathesis, trauma, or interventional procedure. The homogeneous hypoechoic texture of the mass makes lipoma and hemangioma less likely, as these tumors tend to be e~hogenic.'~ Tumors of neurogenic and muscle origins cannot usually be differentiated sonographically. A diagnosis of schwannoma can be suggested by identifying the exact junction between the hypoechoic tumor and the normal hyperechoic n e r ~ e , ~although ~ , ' ~ a normal nerve may be difficult to evaluate with US if it is located within the echogenic retroperitoneal fat. The diffuse heterogeneous enhancement on CT confirms the solid nature of the mass and also excludes fibroma and lipoma. The well-circumscribed appearance and the lack of local invasion of the mass suggest a benign lesion. However, with the presence of heterogeneous enhancement on CT, a malignant neoplasm cannot be excluded. Thus, an US- or CT-guided biopsy or even open biopsy is required to make a definitive diagnosis. 45

CASE REPORT: DO-DAI ET AL.

FIGURE 4. (A) Surgically excised mass had a tan-yellow parenchyma with areas of dark-brown pigmentation (arrowheads) and a thin fibrous capsule (arrows). (B) High-power photomicrograph (H & E stain, x40) revealed spindle-shaped cells with brown melanin pigment (arrows) deposition throughout the lesion.

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RETROPERITONEAL MELANOTIC SCHWANNOMA

Our review of the literature revealed a wide range of presentation age: from 10-84 years, with the highest peak in the 4th and 5th decades and a mean age of 38 years.'-'' There was a slight female predominance for melanotic schwannomas: 40 women versus 34 men. The symptoms and signs of melanotic schwannomas varied according to the anatomic sites. Patients with tumors involving the spinal cord or nerve roots usually were seen initially with spinal cord compression symptoms or radiculopathy; however, those with subcutaneous or soft-tissue melanotic schwannomas were first seen with a painless mass. Of a total of 74 documented cases of melanotic schwannoma, the peripheral nerves were most frequently involved (33 of 74, 45%), followed by spinal nerve roots (24 of 74, 32%), sympathetic ganglia (12 of 74, 16%),spinal cord (3 of 74, 4%), and cranial nerves (2 of 74, 3%). Melanotic schwannomas are round or oval in shape, with varied sizes ranging from 8 mm to 19 ern.'-'' Most of the tumors are well-encapsulated or sharply circumscribed, and all have a dark brown or black pigmented appearance on gross inspection. The histological features of melanotic schwannoma include hypercellularity, hypervascularity , cellular pleomorphism, and spindleshaped cells containing brownish melanin pigment in the cytoplasm as demonstrated in this case. Complete resection is the treatment of choice and is often curative. Chemotherapy and radiotherapy can be used as adjuvant treatment for patients with local invasiveness or incomplete excision; however, no specific protocol has been defined. Mixed results have been obtained thus far.2,4,10,13 The overall rate of local recurrence and metastasis of melanotic schwannomas was 24 of 74 (32%).Benign nonmelanotic schwannomas, however, have been shown to have no recurrence after simple or even incomplete exci~ion.~',~' Complete surgical excision is, therefore, recommended for all melanotic schwannomas-even in benign cases. REFERENCES 1. Millar W G A malignant melanotic tumor of ganglion cells arising from the thoracic sympathetic ganglion. J Pathol 35:351-357, 1932. 2. Krausz T, Azzopardi JG, Pearse E: Malignant melanoma of sympathetic chain: with a consideration of pigmented nerve sheath tumors. Histopathology 8:881-894, 1984. 3. Iizuka H, Nakamura T, Kadoya S: Spinal melanotic schwannoma: report of a case. Neurol Surg 16: 1199-1205, 1988. VOL. 23, NO. 1, JANUARY 1995

4. Killeen RM, Davy CL, Bauserman SC: Melanocytic schwannoma. Cancer 62:174-183, 1988. 5. Roytta M, Elfversson J , Kalimo H: Intraspinal pigmented schwannoma with malignant progression. Acta Neurochir 95:147- 154, 1988. 6. Terzakis JA, Melamed J, Santagada E, et al: Pigmented melanocytic schwannoma of the uterine cervix. Ultra PathoE 14:357-366, 1990. 7. Cras P, Ceuterick-de Groote C, Van Vyve M, et al: Malignant pigmented spinal nerve root schwannoma metastasizing in the brain and viscera. Clin Neuropathol 9:290-294, 1990. 8. Jensen OA, Bretlau P: Melanotic schwannoma of the orbit: immunohistochemical and ultrastructural study of a case and survey of the literature. Acta Pathol Microb Immun Scand 98:713-723, 1990. 9. Jones H, Theaker JM, Kelly PM, et al: Melanocytic schwannomas arising within localized pigmented neurofibromatosis of the sympathetic chain. Histopathology 17:567-569, 1990. 10. Liessi G, Barbazza R, Sartori F, et al: CT and MR imaging of melanocytic schwannomas: report of three cases. Eur J Radiol 11:138-142, 1990. 11. Marchese MJ, McDonald JV: Intramedullary melanotic schwannoma of the cervical spinal cord: report of a case. Surg Neurol33:353-355, 1990. 12. Myers JL, Bernreuter W, Dunham W. Melanotic schwannoma of bone: clinicopathologic immunohistochemical, and ultrastructural features of a rare primary bone tumor. A m J Clin Pathol 93: 424-429, 1990. 13. Abbott AE Jr, Hill RC, et al: Melanotic schwannoma of the sympathetic ganglia: pathological and clinical characteristics. A n n Thorac Surg 49:10061008,1990. 14. Martin-Reay DG, Shattuck MC, Guthrie FW: Psammomatous melanotic schwannoma: an additional component of Carney's complex. Am J Clin Pathol 95:484-489, 1991. 15. Hisaoka M, Ohta H, Haratake J , et al: Melanocytic schwannoma in the spinal canal. Acta Pathol Jpn 41:685-688, 1991. 16. Imoto K, Yamazaki Y, Kawahara E, et al: Malignant melanotic schwannoma of the nasopharynx. J Oto Rhino Laryngo 53:48-51, 1991. 17. Gratz KW, Makek M, Sailer HF: Malignant melanotic schwannoma of the oral cavity. Znt J Oral Maxillofac Surg 201236-238, 1991. 18. Thornton CM, Handley J, Bingham EA, et al: Psammomatous melanotic schwannoma arising in the dermis in a patient with Carney's complex. Histopathology 20:71- 73, 1992. 19. Fornage BD: Peripheral nerves of the extremities: imaging with US. Radiology 167:179-182, 1988. 20. Hoddick WK, Callen PW, Filly RA, et al: Ultrasound evaluation of benign sciatic nerve sheath tumors. J Ultrasound Med 3:505-507, 1984. 21. Kuo C, Changchien C: Sonographic features of retroperitoneal neurilemoma. J Clin Ultrasound 21: 309-312, 1993. 47

CASE REPORT: DO-DAI ET AL. 22. Cooperberg PL: Artifacts in US, in Rifkin MD, Charboneau JW, Laing FC (eds): Syllabus Special Course: Ultrasound 1991. Oak Brook, IL, RSNA Publications, 1991, pp 57-72. 23. Kriegshauser JS, Carroll BA: The adrenal glands and retroperitoneum, in Rumack CM, Wilson SR, Charboneau JW (eds): Diagnostic Ultrasound. St Louis, Mosby-Year Book, 1991, pp 289-314, 24. Burns PN: Principles of US, in Rifkin MD, Charboneau JW, Laing FC (eds): Syllabus Special Course: Ultrasound 1991. Oak Brook, IL, RSNA Publications, 1991, pp 33-55.

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25. Cohen LM, Schwartz AM, Rockoff SD: Benign schwannomas: pathologic basis for CT inhomogeneities. AJR 147:141-143, 1986. 26. Stull MA, Moser RP J r , Kransdorf MJ, et al: Magnetic resonance appearance of peripheral nerve sheath tumors. Skeletal Radio1 20:9-14, 1991. 27. Stout AP: The peripheral manifestations of specific nerve sheath tumor (neurilemoma). A m J Cancer 24:751, 1935. 28. Guz BV, Wood DP, Montie JE, et al: Retroperitoneal neural sheath tumors: Cleveland clinic experience. J Urol 142:1434-1437, 1989.

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