Diagnostic Approach, Molecular Abnormalities, and IHC in Soft Tissue Tumors / Sarcomas / PEComas

1. Diagnostic Approach

• Age, site, depth (superficial vs deep), size, and growth rate narrow the differential (e.g., myxoid liposarcoma - thigh; synovial sarcoma - near joints in young adults; GIST - stomach/small bowel).
• MRI is preferred for extent/relation to neurovascular bundle; CT for retroperitoneal/visceral lesions and calcification.

• Core needle biopsy is the standard first step for a soft tissue mass >5 cm or deep-seated; incisional biopsy if core is non-diagnostic. Avoid enucleation ("shell-out") biopsy which risks tumor seeding and compromises later margins.

• Pattern recognition: spindle cell, round cell, pleomorphic, epithelioid, or myxoid morphology.
• Grading by FNCLCC (Fédération Nationale des Centres de Lutte Contre le Cancer) system - based on tumor differentiation score, mitotic count, and percentage of necrosis, summed to Grade 1-3. This has been shown to be superior to the older NIH system for predicting metastasis and survival - Sabiston Textbook of Surgery.

1. Immunohistochemistry (IHC) - lineage-specific markers to categorize otherwise undifferentiated tumors (as in any anaplastic neoplasm, IHC distinguishes carcinoma, lymphoma, melanoma, and sarcoma) - Robbins, Cotran & Kumar Pathologic Basis of Disease.
2. Molecular/cytogenetic testing - FISH (break-apart probes), RT-PCR, and next-generation sequencing to detect diagnostic fusion transcripts or mutations, especially in "small round blue cell" and translocation-associated sarcomas where morphology and IHC overlap - Henry's Clinical Diagnosis and Management by Laboratory Methods.
3. Correlation with WHO classification of soft tissue and bone tumors, which increasingly incorporates molecular findings into diagnostic categories.

2. Molecular Abnormalities in Sarcomas

• Sarcomas with specific/simple genetic alterations (~15-20%): reciprocal translocations producing fusion oncoproteins, or specific activating mutations.
• Sarcomas with complex, non-specific karyotypes (e.g., leiomyosarcoma, undifferentiated pleomorphic sarcoma/MFH, most osteosarcomas) - these lack a single diagnostic marker and rely more heavily on morphology plus exclusionary IHC.

3. IHC in Soft Tissue Tumors/Sarcomas

4. PEComas (Perivascular Epithelioid Cell Tumors)

• HMB-45 and Melan-A - characteristic melanocytic markers, brown cytoplasmic staining, considered defining for the diagnosis - Current Surgical Therapy 14e; Campbell-Walsh-Wein Urology.
• Smooth muscle actin (SMA), desmin (variable) - reflect the myoid component.
• Cathepsin K - increasingly used, sensitive marker.
• TFE3 - positive in the TFE3-rearranged molecular subgroup (often HMB-45 weaker/negative in this subgroup).

• Most PEComas show biallelic inactivation/deletions of TSC1 or TSC2, leading to constitutive activation of the mTOR pathway - this is the basis for treatment with mTOR inhibitors (sirolimus/everolimus) - Campbell-Walsh-Wein Urology, Table 100.1.
• A distinct molecular subset harbors TFE3 gene rearrangements (t(X;...) fusions such as SFPQ-TFE3), which occurs independent of TSC alteration and defines a morphologically and immunophenotypically distinct group (TFE3-positive, often HMB-45 dim).
• Sporadic PEComas may occur in the setting of Tuberous Sclerosis Complex (TSC1/TSC2 germline mutation), linking renal AML and pulmonary LAM to the same pathway.

1. Recognize the characteristic epithelioid perivascular morphology on H&E.
2. Confirm with melanocytic/myoid IHC panel (HMB-45, Melan-A, SMA, Cathepsin K).
3. If HMB-45 is weak/negative but morphology suggests PEComa, test TFE3 IHC/FISH to identify the TFE3-rearranged subgroup.
4. Assess for TSC association clinically (renal AML, LAM, other TSC stigmata) since this has implications for mTOR-inhibitor therapy in malignant/unresectable disease.
5. Malignant potential is assessed using criteria such as size >5 cm, high-grade nuclei/pleomorphism, high mitotic rate, necrosis, and vascular invasion (the "PEComa malignancy criteria").

Summary (Exam Point Form)

• Diagnosis = morphology (biopsy/FNCLCC grading) + IHC (lineage markers) + molecular cytogenetics (FISH/RT-PCR/NGS for fusion genes or mutations).
• Molecular abnormalities: translocation-driven fusion oncogenes (EWSR1-FLI1, SS18-SSX, FUS-DDIT3, PAX3-FOXO1, COL1A1-PDGFB, NAB2-STAT6) or point mutations (KIT/PDGFRA in GIST) define specific entities; complex karyotype sarcomas (leiomyosarcoma, UPS) lack a single marker.
• IHC narrows differential between sarcoma vs carcinoma/lymphoma/melanoma and subtypes sarcoma by lineage (desmin/myogenin for muscle, S100 for nerve/cartilage, CD117/DOG1 for GIST, STAT6 for SFT, TLE1 for synovial sarcoma).
• PEComa: HMB-45/Melan-A/SMA/Cathepsin K positive; driven by TSC1/TSC2 loss (mTOR activation) or TFE3 rearrangement; linked to Tuberous Sclerosis Complex.