**Question:** Sections from a solid-cystic unilaterally located ovarian tumor in a 30-year-old female show a tumor composed of diffuse sheets of small cells with doubtful nuclear grooving and scanty cytoplasm. No Call-Exner bodies are seen. The ideal immunohistochemistry panel would include:

A. PAX8
B. CD99
C. CD10
D. CAM5.2

**Correct Answer:**

**Core Concept:** Ovarian tumors can be classified into various types based on histopathological and immunohistochemical features. Some common types include serous cystadenomas, mucinous cystadenomas, endometrioid adenocarcinomas, clear cell carcinomas, and germ cell tumors. In this case, we are dealing with a solid-cystic unilaterally located ovarian tumor.

**Why the Correct Answer is Right:**

The tumor in question shows features of a small cell carcinoma, which is a rare and aggressive type of ovarian cancer. Small cell carcinomas are often negative for Call-Exner bodies, which are seen in mucinous cystadenomas and endometrioid adenocarcinomas. The tumor has doubtful nuclear grooving and scanty cytoplasm, characteristics of small cell carcinoma.

These tumors are typically immunohistochemically positive for PAX8, CD99, and CD10. PAX8 is a transcription factor expressed in various neoplastic and non-neoplastic cells of the female reproductive system. CD99 is a membrane-associated protein expressed in various malignant tumors, including small cell carcinoma of the ovary. CD10 is a protease enzyme expressed in various malignancies, including small cell carcinoma of the ovary.

**Why Each Wrong Option is Incorrect:**

Option A (PAX8) is correct, as mentioned above, but lacks specificity for small cell carcinoma. Option B (CD99) is also correct as it is membrane-associated protein expressed in small cell carcinoma. However, CD99 is not specific to ovarian tumors and can be expressed in other malignancies as well. Option C (CD10) is correct as it is a protease enzyme expressed in various malignancies, including small cell carcinoma of the ovary. Option D (CAM5.2) is a pan-cytokeratin marker that is not specific for small cell carcinoma and can be expressed in various other malignancies.

**Clinical Pearls:**

Small cell carcinoma of the ovary is a rare subtype of ovarian cancer that presents with cystic and solid components. These tumors are typically associated with poor prognosis and a high rate of relapse. They occur in young females, often below the age of 40, and are commonly seen in nulliparous and steroid hormone receptor-negative patients.

**Why the Correct Answer is Right:**

The correct answer is D (CAM5.2) because it is a pan-cytokeratin marker that stains for cytokeratins, a family of structural proteins that are expressed in various epithelial tumors, including small cell carcinoma of the ovary. These tumors often express cytokeratins and CAM5.2 is a reliable marker for cytokeratin staining