Case 82

A 78-year-old male with a cervical lesion.

1. If the tumor is positive for CD99, what's the diagnosis?

   A: Ewing sarcoma

   B: Neuroendocrine carcinoma

   C: Merkel cell carcinoma

   D: HPV-related poorly differentiated carcinoma

Answer

This is a case of Ewing sarcoma. NKX 2.2 is the best sensitive and specific marker for Ewing sarcoma.

Key Features Supporting Ewing Sarcoma Diagnosis:

Histologic Features:

• Monotonous small round blue cells arranged in sheets or solid aggregates.

• Fibrous septa separating tumor cells.

• Focal rosettes may be present.

• Round nuclei, coarse chromatin, small nucleoli.

• Cytoplasm ranges from scant to abundant and may be eosinophilic or clear.

• Brisk mitotic activity.

• Common necrosis.

Immunohistochemical Profile:

• Strong, diffuse membranous CD99 expression (hallmark feature of Ewing sarcoma).

• Nuclear FLI1 expression (consistent with Ewing sarcoma family tumors).

• Focal positivity for broad-spectrum cytokeratins (seen in some cases).

• Negative for desmin, myogenin, and MYOD1 (excludes rhabdomyosarcoma).

Molecular Features:

• EWSR1::FLI1 fusion (t(11;22)(q24;q12)) is the most common translocation.

• Other less common translocations include EWSR1::ERG (t(21;22)(q22;q12)).

Differential Diagnosis:

1. Lymphoblastic lymphoma → Negative for CD99, positive for TdT.

2. Rhabdomyosarcoma → Positive for desmin, myogenin, MYOD1.

3. Neuroblastoma → Positive for synaptophysin, negative for CD99.

4. Synovial sarcoma (monophasic type) → May be positive for CD99 but shows SYT-SSX translocation.

Reference: https://tumourclassification.iarc.who.int/chaptercontent/34/447

Case credit: https://qp.91360.com/9817.html?lang=en

Author: Wangpan Jackson Shi, MD