Synovial Sarcoma



Synovial Sarcoma







Key Facts


Clinical Issues



  • Predilection for young adults (median: 35 years)


  • Female predilection (2:1)


  • Aggressive behavior similar to soft tissue counterpart


Microscopic Pathology



  • Uniform and monotonous proliferation of atypical oval to spindle cells with minimal stroma


  • Glandular structures may be very focal or may predominate


  • Fascicular growth pattern with “herringbone” appearance is most common histologic pattern of growth


  • Many tumors show, at least focally, a prominent hemangiopericytomatous growth pattern


  • Other growth patterns include storiform, palisading, and sclerosing


  • Stromal changes may include fibrosis, calcifications, and myxoid changes


Ancillary Tests



  • Pattern of cytokeratin positivity in monophasic spindle cell tumors is usually weak and focal (patchy)


  • Spindle cells



    • Strongly and diffusely stain for Bcl-2


    • Usually stain for CD99


    • May be positive for S100 protein in 30% of cases


    • Are negative for actin, desmin, CD34, HMB-45, CEA, MOC-31, and p63


  • (X;18)(p11;q11) translocation is characteristically present in > 80% of cases






Monophasic spindle cell variant of synovial sarcoma of the mediastinum shows a monotonous population of atypical spindle cells with scant stroma.






Biphasic variant of synovial sarcoma of the mediastinum shows admixture of well-formed glandular structures with a monotonous population of atypical spindle cells.


TERMINOLOGY


Abbreviations



  • Synovial sarcoma (SS)


Synonyms



  • Primary carcinoma of soft tissue


Definitions



  • Primary malignant epithelial neoplasm arising from soft tissues


ETIOLOGY/PATHOGENESIS


Pathogenesis



  • Unknown; unrelated to synovium or synovial cells


  • Pluripotential mesenchymal cells capable of aberrant epithelial differentiation


CLINICAL ISSUES


Epidemiology



  • Incidence



    • Rare tumor


  • Age



    • Broad age range: 3-83 years


    • Predilection for young adults (median: 35 years)


  • Gender



    • Female predilection (2:1)


Site



  • May involve all mediastinal compartments: Anterior, middle, and posterior


Presentation



  • Cough


  • Chest pain


  • Pleural effusion


  • Shortness of breath


  • Neck or back pain


Treatment



  • Surgical excision


Prognosis



  • Aggressive behavior similar to soft tissue counterpart


  • Local recurrences and distant metastases to lung, lymph nodes, liver, and epidural space


IMAGE FINDINGS


General Features



  • Tumors may present as well circumscribed or infiltrative masses with focal calcifications


CT Findings



  • Well-circumscribed mediastinal mass with low attenuation


  • Pleural effusion


MACROSCOPIC FEATURES


General Features



  • Usually well-circumscribed, fleshy tumors surrounded by thin, fibrous capsule


  • May be infiltrative and extend into surrounding structures, such as lung, pleura, pericardium, chest wall, and ribs


  • Cut section shows gray-white to tan, homogeneous soft tissue with gelatinous consistency


  • Some cases may undergo extensive cystic changes


  • May contain scattered calcifications


  • Can show focal hemorrhage and necrosis


Sections to Be Submitted



  • 1 section per each centimeter of largest tumor diameter



Size



  • 5-20 cm


MICROSCOPIC PATHOLOGY


Histologic Features



  • Monophasic (spindle cell) synovial sarcoma



    • Uniform and monotonous proliferation of atypical oval to spindle cells with minimal stroma


    • Spindle cells show vesicular or hyperchromatic nuclei with coarse chromatin pattern and inconspicuous nucleoli


    • Mitotic activity may vary from low (1 mitosis per 10 HPF) to high (> 10 mitoses per 10 HPF)


    • Fascicular growth pattern with “herringbone” appearance is most common histologic pattern of growth


    • Many tumors show, at least focally, a prominent hemangiopericytomatous growth pattern


    • Other growth patterns include storiform, palisading, sclerosing, nesting, and rhabdoid


    • Stromal changes may include fibrosis, calcifications, and myxoid changes


    • “Poorly differentiated” variant characterized by round cell, epithelioid morphology rather than spindle cells


    • Some tumors can show prominent hemorrhage and necrosis


    • Secondary cystic changes are often seen in monophasic tumors


  • Biphasic synovial sarcoma



    • Shows admixture of well-formed glandular structures and spindle cells


    • Spindle cell component surrounding glands has identical features to those seen in monophasic variant


    • Glandular structures may be very focal or may predominate


    • Glandular structures are lined by large, round to polygonal or columnar epithelial cells


    • Glandular cells are round to oval nuclei with abundant eosinophilic cytoplasm


    • Glands may contain dense, homogeneous eosinophilic secretions in lumen


    • Glandular structures may adopt tubulo-papillary pattern with intraluminal papillary infoldings


    • Glandular structures can display prominent cytoplasmic clearing of tumor cells


ANCILLARY TESTS


Immunohistochemistry



  • All biphasic tumors show strong keratin positivity in the glandular component


  • Monophasic tumors are frequently, but not always, positive for cytokeratin stains


  • Pattern of cytokeratin positivity in monophasic spindle cell tumors is usually weak and focal (patchy)


  • Epithelial membrane antigen (EMA) shows similar pattern and distribution as cytokeratin


  • Low molecular weight cytokeratins are more sensitive than broad-spectrum cytokeratins for monophasic tumors


  • Spindle cells



    • Strongly express vimentin


    • Strongly and diffusely stain for Bcl-2


    • Usually stain for CD99


    • May be positive for calretinin and calponin


    • May be positive for S100 protein in 30% of cases


    • Negative for actin, desmin, CD34, HMB-45, CEA, MOC-31, and p63


Cytogenetics



  • (X;18)(p11;q11) translocation characteristically present in > 80% of cases


  • Other complex translocations may be found infrequently


  • FISH and Rt-PCR are useful for detecting translocations in paraffin-embedded tissues


  • 2/3 contain an SYT/SSX1 fusion


  • 1/3 contain an SYT/SSX2 fusion


  • Rare cases may show an SYT/SSX4 fusion



  • Cases with SYT/SSX2 fusion are associated with better prognosis


Electron Microscopy



  • Glandular epithelial component shows similar features to adenocarcinoma


  • Monophasic spindle cell component shows closely apposed cell membranes, frequent desmosomal junctions, and occasional surface microvilli


  • Monophasic spindle cell component does not contain abundant rough endoplasmic reticulum or other features of fibroblastic cells


DIFFERENTIAL DIAGNOSIS


Metastatic Adenocarcinoma



  • Clinical history is helpful for diagnosis


  • Metastatic adenocarcinoma will usually not contain stromal, atypical spindle cell component


Pulmonary Blastoma



  • Only rarely will secondarily involve mediastinum, but bulk of tumor will be centered in lung


  • Cells lining glands in pulmonary blastoma show subnuclear clearing simulating fetal lung


  • Most biphasic pulmonary blastomas are characterized by squamoid morules located at base of glands


  • Spindle cell component in pulmonary blastoma is negative for epithelial markers (keratin, EMA)


Thymic Carcinosarcoma



  • Spindle cell component in thymic carcinosarcoma is negative for epithelial markers (keratin, EMA)


  • Glandular component in thymic carcinosarcoma displays pronounced nuclear atypia


  • Negative for t(X;18) translocation


Biphasic Pleural Mesothelioma



  • Usually grows as diffuse, plaque-like process covering pleural surface, rather than discrete, well-circumscribed tumor mass


  • Spindle cell component in pleural mesothelioma shows marked cytologic atypia and nuclear pleomorphism


  • Spindle cells in pleural mesothelioma are negative for Bcl-2 and CD99


Spindle Cell Thymoma



  • Slow-growing tumor that is usually encapsulated and noninvasive


  • Absence of cytologic atypia or mitotic activity in spindle cells


  • Lobular architecture with fibrous bands separating lobules


  • Presence of immature T lymphocytes


  • Spindle cells are negative for vimentin, CD99, and Bcl-2


Spindle Cell Thymic Carcinoma



  • Marked cytologic atypia, nuclear pleomorphism, and mitotic activity


  • Spindle cells are negative for Bcl-2 and CD99


  • Absence of t(X;18) translocation


Malignant Peripheral Nerve Sheath Tumor



  • Commonly arises in background of neurofibromatosis


  • Grossly associated with nerve trunks


  • Spindle cells are negative for cytokeratins and CD99


  • Electron microscopy shows complex interdigitating cytoplasmic processes invested by basal lamina material


  • Absence of t(X;18) translocation


Solitary Fibrous Tumor



  • Usually shows variegation in growth patterns within same tumor and lack of uniformity of cell population


  • Commonly shows variable degrees of stromal fibrosis, including characteristic, rope-like deposition of keloidal collagen


  • Spindle cells are positive for CD34


  • Spindle cells are negative for cytokeratins and EMA


  • Absence of t(X;18) translocation

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Jul 9, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Synovial Sarcoma

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