What Do Histopathology, Immunohistochemistry and Molecular Diagnosis Reveal Together in Sarcoma?

Sarcomas arise from connective and supporting tissues and are rare but aggressive cancers. Originating in muscle, fat, bone, nerves, cartilage or vessels, these tumors comprise more than 100 subtypes .

This diversity complicates diagnosis, which should be performed by experienced pathologists. An incorrect or incomplete diagnosis may directly affect treatment success and prognosis.

A soft tissue mass may be not only sarcoma but also lipoma or another benign lesion, or metastatic carcinoma, melanoma or lymphoma , among other malignancies.

We examine the methods used for accurate diagnosis: histopathology, immunohistochemistry, molecular diagnosis and tumor grading, explaining how accuracy is achieved and what it means for patients.

The foundation of sarcoma diagnosis is histopathological examination. Tissue obtained by biopsy or surgery is assessed microscopically.

WHO classifies soft tissue neoplasms into more than 100 histological subtypes . Common adult types include:

• liposarcoma,
• synovial sarcoma,
• leiomyosarcoma,
• rhabdomyosarcoma,
• fibrosarcoma and
• angiosarcoma.

Classification generally follows the normal tissue most closely resembled. For example, liposarcoma → fat, leiomyosarcoma → smooth muscle. However, alveolar soft part sarcoma or epithelioid sarcoma are named for tumor architecture.

Beyond cellular morphology, pathologists use additional immunohistochemical and molecular tests to confirm diagnosis.

Below is the histological distribution in Memorial Sloan Kettering Cancer Center's (MSKCC) database of 10,000 adult soft tissue sarcoma cases.

A key diagnostic tool, immunohistochemical (IHC) stainingprovides clues to tumor origin. Antibodies recognizing specific cellular proteins reveal the tissue's likely differentiation.

Although histology is often distinctive, IHC helps distinguish subtypes with similar morphology. Each marker is assessed within a “marker panel” , rather than alone.

Common sarcoma IHC markers include:

• 🟣 Desmin: Indicates muscle origin, particularly important for rhabdomyosarcoma (RMS) and, less frequently, leiomyosarcoma . Positivity supports myogenic differentiation.
• 🔵 S100 and Neurofilaments: Often positive in nerve sheath tumors , these markers may also appear with melanocytic differentiation . Among sarcomas, clear cell sarcoma and PEComa may stain positively.
• 🟠 Cytokeratin: Absent from most sarcomas, making positivity significant. Synovial sarcoma and epithelioid sarcoma may be cytokeratin-positive, while fibrosarcoma is generally negative. This helps differential diagnosis.
• 🔴 Factor VIII-Related Antigen and CD31: These markers identify tumors arising from endothelial cells , particularly confirming angiosarcoma and similar tumors.

Interpreted with morphology, these markers identify subtypes and provide biological insights affecting treatment decisions .

Many sarcoma subtypes are associated with specific chromosomal translocations . Resulting fusion genesmatter for both diagnosis and targeted treatment.

These changes may be detected through:

• FISH (Fluorescence In Situ Hybridization): Visualizes specific chromosomal translocations, particularly to confirm subtypes such as synovial sarcoma.
• RT-PCR: Detects fusion-gene mRNA.
• NGS-Based Fusion Panels: Analyze many fusion genes simultaneously. Clinical suspicion should prompt multidisciplinary assessment.

Prospective studies show that molecular methods can change the histopathological diagnosis . Tests should therefore be interpreted under experienced sarcoma pathologists' guidance alongside clinical information.

Pathological assessment of most sarcomas includes histological grading based on differentiation, mitotic activity and necrosis, reflecting aggressive potential.

• Grade 1 (Low): Well-differentiated cells resembling normal tissue.
• Grade 2 (Intermediate): Moderately differentiated cells.
• Grade 3 (High): Poorly differentiated cells markedly unlike normal tissue, with aggressive behavior.

This assessment aligns with CAP, AJCC/UICC and FNCLCC and similar international frameworks.

However, grading is unsuitable for some subtypes, including malignant peripheral nerve sheath tumors, alveolar soft part sarcoma and epithelioid sarcoma, where grading may lack prognostic meaning.

🧩 Why Is Sarcoma Diagnosis Challenging?
The image illustrates the diversity of sarcomas with uncertain histogenesis. Differences in age, location and microscopic features may create substantial diagnostic uncertainty.

Sarcomas require comprehensive analysis at histopathological, immunohistochemical and molecular levels for accurate subtype classification.

Identifying translocations and fusion genes has improved diagnostic accuracy and opened the way for targeted treatment strategies .

Sarcoma diagnosis therefore requires a multidisciplinary approach . Collaboration among pathology, oncology, surgery and radiology identifies optimal personalized treatment , improving survival and quality of life.