Soft Tissue Clear Cell Sarcoma

Etiology

While the major cause of these tumors remains unknown, several risks have been described for soft tissue sarcomas, such as genetic predisposition, radiation therapy, lymphedema, gene mutations, and chemotherapy.[3] Clear cell sarcoma was initially described as “malignant melanoma of soft parts” due to their histological similarities. Both share immunohistochemical markers, and their gene expression profiling suggests that CCSST is a genomic subtype of MM.[7] Unlike MM, however, most clear cell sarcomas are associated with a t(12;22)(q13-14;q12) translocation.[8][9]

Epidemiology

The true incidence of this tumor remains unknown. It is more common in Caucasians, and estimated that men and women are equally affected with a median age of 39 years, with rare cases reported in children.[4][9] It typically arises in the lower extremities, particularly around the foot and ankle. However, rare presentations in the kidney, trunk, penis, gastrointestinal tract, head, and neck have been described.[10] The 5- and 10-year overall survival rates for clear cell sarcoma are estimated to be 50% and 38%, respectively, with later stages (III and IV) having worse survival rates.[1][2][3]

Pathophysiology

Clear-cell sarcoma of soft tissue appears as a macroscopically tan-gray firm mass.[4][9] With a size ranging from 0.4 to 14.5 cm, it commonly shows infiltration along with the tendons and aponeuroses. Immunohistochemical studies commonly show neoplastic cells positive with HMB-45, S100 protein, and Melan-A. Most cases exhibit a t(12;22)(q13;q12) translocation causing an EWSR1/ATF1 gene fusion, a finding that helps differentiate these lesions from MM.[4]

Histopathology

On microscopy, the neoplastic cells show abundant cytoplasm containing numerous mitochondria. Cells are commonly polygonal and fusiform, with clear cytoplasm and a centrally located round nucleus. The "clear-cell" appearance represents intracellular glycogen accumulation, as demonstrated by the periodic acid-Schiff stain. Pigmented and unpigmented melanosomes, scattered multinucleated giant cells, and areas of focal necrosis are also commonly present.

CCSST commonly shows highly infiltrative cells, haphazardly organized into compact nests and fascicles that dissect along with dense fibrous connective tissue.[4] Immunohistochemistry shows antigens commonly associated with melanin synthesis, including HMB-45 and S100 within the cytoplasm and nucleus. It may also show positivity for CD99, Melan-A, neuron-specific enolase, and vimentin.[4][8] Cytokeratin, epithelial membrane antigen, carcinoembryonic antigen, desmin, and smooth muscle actin are usually negative.[4][8]

History and Physical

Most patients are present in their thirties and forties. While CCSST commonly appears in the legs or thighs as a small tender spot or swelling, some patients may present involvement of the subcutaneous tissue and adjacent dermis and may present with ulceration of the overlying skin.[4] Rare presentations of CCSST have also been documented in other sites, including the lumbar region, parapharyngeal tissue, penis, and neck.[11][12][13][14][15]

Evaluation

The initial evaluation should include a complete history and physical examination. As previously discussed, most patients present with a small swelling or tender spot in the lower extremities, particularly in the ankle.[16] Patients should be asked about when the mass was noticed and the pattern of growth and evaluated for signs and symptoms that may help in early staging. Physical examination should be focused on determining size, depth, fixation to adjacent tissues, edema, or possible neurovascular compromise.[4]

Soft tissue sarcomas, in general, can be divided into subtypes, and no single imaging modality is ideal for every tumor.[4][17] In addition to plain radiographs, which are useful to evaluate for possible masses arising from bone or to detect intralesional calcifications, cross-sectional imaging with magnetic resonance imaging (MRI) is commonly preferred for masses that arise in the extremities. MRI, unlike CT, delineates different muscle groups and may provide more information regarding the extent of these tumors.[18][19] CT may provide useful information on metastatic disease, particularly if the patient has a large (greater than 5 cm) or deep-seated tumor.[20]

Histology is essential for planning further workup and treatment. Tissue may be obtained through incisional, fine needle aspiration, or core needle biopsy. The latter has a lower incidence of complications, is highly accurate, and is usually preferred.[21] Further pathological workup for CCSST should include immunohistochemistry. Confirmation of CCSST and differentiating this tumor from MM also requires fluorescence in situ hybridization (FISH) or reverse transcription-polymerase chain reaction (RT-PCR).[10] After treatment, discussed below, postoperative imaging is commonly performed 3 months after treatment. For high-grade lesions on presentation, 3- to 6-month interval imaging is preferred, while low-grade lesions may be followed with annual imaging.[19][20][21]

Treatment / Management

For localized lesions, the treatment of choice is wide surgical resection with negative margins.[16][22] If complete excision is achieved, adjuvant therapy is typically unnecessary as there is no significant improvement in survival.[6][23] Intralesional excision has been associated with higher recurrence rates.[23][24](B2)

Radiotherapy may improve outcomes in patients who undergo close resection margins.[23][24] Chemotherapy is primarily employed in patients with metastatic disease. Cisplatin-based and caffeine-assisted chemotherapy with doxorubicin and cisplatin may benefit patients with metastatic disease or large tumors on presentation.[23](B2)

After surgical excision, patients should be followed closely due to the high recurrence rates. Patients with higher risk may benefit from close-interval imaging, while lower-risk patients may be followed with yearly imaging.[2][6][22](B2)

Differential Diagnosis

The differential diagnoses of a tumor located in the proximity of tendons and aponeurosis should include paraganglioma-like dermal melanocytic tumor, MM, malignant peripheral nerve sheath tumor, synovial sarcoma, and clear cell myomelanocytic tumor.[6][23] Compared to MM, CCSST usually occurs in the younger patient population, is located in deeper tissues, is associated with aponeuroses or tendons, and commonly lacks epidermal involvement.[23] Additionally, the t(12;22) translocation in CCSST has not been observed in malignant MM. However, there may be significant histologic overlap. 

Synovial sarcoma commonly shows a t(X;18)(p11.2;q11.2) translocation producing SYT/SSX1 or SYT/sSSX2 gene fusions, not seen in CCSST.[6][23] Paraganglioma-like dermal melanocytic tumors primarily present in the extremities of female patients as a dermal nodule. On microscopy, it shows oval cells arranged in packet-like groups.[6]

Clear cell myomelanocytic tumor belongs to a family of tumors known as the perivascular epithelioid cell family of tumors, which present commonly in the abdomen, usually in the ligamentum teres of the liver of young patients. Malignant peripheral nerve sheath tumor usually arises within large peripheral nerves and is negative for HMB-45.[6]

Prognosis

CCSST is associated with an overall poor prognosis. A study with 75 patients showed a 69% and 21% metastatic disease and local recurrence rate, respectively. The overall 5-year survival rate was 47% and 36% for 10 years. Furthermore, the overall survival was 55% and 41% for patients who presented localized disease at 5 and 10 years, respectively.[22] A case series with 52 patients showed a 59% and 41% survival rate at 5 years and 10 years.[2]

Significant prognostic factors include tumor size, depth, and TNM classification. However, only tumor size has remained a significant prognostic factor after multivariate analysis.[6][22]  Another study showed that 10% to 14% of patients develop regional lymph node metastasis. A majority of all patients with CCSST eventually developed distant metastasis, most commonly to lung and bone tissue, in addition to regional lymph nodes.[23] Overall, the course of CCSST is regarded as relentless and progressive, usually terminating in widespread metastatic disease.[2]

Complications

While many patients present with indolently growing mass in the lower extremity, around 30% to 60% of patients may present pain or tenderness. A minority of patients may present with ulceration of the overlying skin.[6] Untreated CCSST most likely progresses to a widespread, deadly metastatic disease.[2]