9 September 2005 Case of the Week #19

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We thank Professor D. Y. Cohen, Department of Pathology, Herzliyah Medical Center, Israel, for contributing this case. We invite you to contribute a Case of the Week by emailing NPernick@PathologyOutlines.com with microscopic images (any size, we will shrink if necessary) in JPG or GIF format, a short clinical history, your diagnosis and any other images (gross, immunostains, EM, etc.) that you have and that may be helpful or interesting. We will write the discussion (unless you want to contribute to it), list you as the contributor, and send you a check for $35 (US) for your time if we use the case. Please only send cases with a definitive diagnosis.

Case of the Week #19

A 40 year old woman had a radical nephrectomy for a large renal mass. The kidney weighed 488 grams after removal of perinephric adipose tissue. The upper portion of the kidney contained a 10 x 9 x 9 cm mass of grey-white solid tissue with central cystic and mucoid degeneration. No golden yellow or hemorrhagic areas were noted. Focally, the tumor formed nodules abutting onto the renal pelvis, 0.5 cm from the renal hilum. The adrenal gland was grossly normal.

Microscopic images: figure 1, figure 2, figure 3, figure 4, figure 5, figure 6

Additional sections showed mitotically active cells, but no neoplastic glandular tissue. No bone or cartilagenous foci were present. The tumor extended into the renal pelvis and within large venous channels. It was adherent to but did not infiltrate into the renal capsule.

The tumor was immunoreactive for vimentin, CD99 (O-13) and Ki-67 (15-20% of cells). The tumor was negative for lymphocyte markers CD45, CD20 and CD3; keratins CK 7, CK20 and pankeratin MNF-118; neuroendocrine markers NSE, chromogranin A and synaptophysin. It was also negative for desmin, smooth muscle actin, CD10 and c-kit/CD 117. Cytogenetics or molecular studies were not available.

What is your diagnosis?

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Diagnosis: Monophasic synovial sarcoma of the kidney

Discussion

Synovial sarcomas are typically found in the extremities of young adults, with less than 30 cases reported in the kidney since 1999. In the kidney, the median patient age is 35 years, range 20-59 years, with no gender preference. They are usually large, partially necrotic masses, with grossly noted cysts in 70%. Histologically, they are usually highly cellular and monomorphic, composed of short intersecting fascicles of plump spindle cells. The tumor cells have indistinct cell borders, minimal cytoplasm, ovoid nuclei and inconspicuous nucleoli. They infiltrate around native, non-neoplastic renal tubules that undergo secondary dilatation, which may simulate a biphasic pattern. They often have a hemangiopericytoma-like vascular pattern with staghorn vessels. Cystic spaces are lined by hobnail-type cells with eosinophilic cytoplasm and apically oriented nuclei. Renal synovial sarcomas may rarely exhibit rhabdoid features (AJSP 2004;28:634), be biphasic (with epithelial cells), or have small cell or high grade spindle cell features. Immunostains are helpful in excluding other spindle cell tumors. Synovial sarcomas are immunoreactive for vimentin and CD56, with variable staining for CD99, bcl2 and EMA. They are typically negative for keratins, S100, CD34, smooth muscle actin and desmin. Over 90% of cases demonstrate t(X;18)(p11.2;q11.2), which produces the SYT-SSX2 (also SSX1 or rarely SSX4) transcript that is specific for synovial sarcoma (figure 3-karyotype #1, #2 (tumor of arm), case report). These tumors appear to have aggressive behavior.

The histologic differential diagnosis includes other spindle cell tumors of the kidney, all of which are negative for t(X;18), although some synovial sarcomas are also negative for this translocation. Of particular concern is the relatively common sarcomatoid renal cell carcinoma. It typically affects older patients, with a mean age of 60 years. Its malignant epithelial component may be focal, so adequate sampling is important. The epithelial component is keratin+ and EMA+, and the sarcomatoid component is often EMA+ and S100+. Another histologic mimic is cellular mesonephric nephroma, although almost all of these patients are newborns (up to age 1 year), and tumor cells have more cytoplasm and are desmin+. The differential also includes two CD34+ tumors: hemangiopericytoma (less cellular pleomorphism and no/rare mitotic activity) and solitary fibrous tumor. Blastemal variants of Wilms tumor are spindled, but often have serpentine or nodular patterns, and are immunoreactive for WT1, in contrast to synovial sarcomas, which are WT1 negative. Finally, Ewings/PNET may appear similar, although they usually have primitive round cells with variable rosette formation, instead of short fascicles of spindled cells, and have a characteristic t(11;22)(q24;q12) translocation.

Additional references: AJSP 2000;24:1087, AJSP 2000;24:1097, Murphy: Tumors of the Kidney Bladder and Related Urinary Structures (AFIP Fascicles, 4th Series, Volume 1, 2004)

Nat Pernick, M.D.
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