How to determine the primary site of gynecological malignancies using Immunohistochemistry (IHC)?

Immunohistochemistry for Determining Primary Site of Gynecological Malignancies

The most effective approach to determine the primary site of gynecological malignancies is to use a panel of immunohistochemical markers including PAX8, WT-1, p53, estrogen receptor (ER), and cytokeratins CK7/CK20, tailored to the specific differential diagnosis. 1

Key Marker Panels Based on Differential Diagnosis

Distinguishing Ovarian vs. Uterine Origin

Serous Carcinomas

• WT-1: Strongly positive in tubo-ovarian serous carcinomas (80-90%), typically negative in uterine serous carcinomas 1
• PAX8: Positive in both ovarian and endometrial carcinomas, but useful as part of panel 1
• p53: Assess pattern rather than simply positive/negative
  • "Mutation-type" staining (diffuse intense or completely absent) in high-grade serous carcinomas
  • "Wild-type" staining (heterogeneous/focal) in low-grade serous carcinomas 1

Endometrioid Carcinomas

• IHC has limited value in distinguishing primary ovarian from uterine endometrioid carcinomas as they share similar immunophenotypes 1
• Look for associated endometriosis in the ovary, which supports primary ovarian origin 1

Distinguishing Serous vs. Mesothelial Proliferations

• BerEP4, ER, PAX8: Typically positive in serous proliferations
• Calretinin, CK5/6: Typically positive in mesothelial proliferations
• WT-1: Usually positive in both serous and mesothelial proliferations (not helpful for this distinction) 1

Distinguishing Serous vs. Endometrioid Carcinomas

• WT-1: Positive in serous carcinomas, negative in endometrioid carcinomas
• p53: "Mutation-type" staining in high-grade serous carcinomas, "wild-type" in most endometrioid carcinomas 1

Distinguishing Sex Cord-Stromal Tumors vs. Epithelial Tumors

• Inhibin, calretinin: Typically positive in sex cord tumors
• EMA (epithelial membrane antigen): Typically negative in sex cord tumors, positive in epithelial tumors 2
• Calretinin: More reliable than inhibin for granulosa cell tumors 2

Distinguishing Primary vs. Metastatic Tumors

Ovarian vs. Colorectal

• CK7: Positive in ~96% of primary ovarian carcinomas, positive in only ~25% of metastatic colorectal carcinomas 3
• CK20: Typically negative in ovarian serous/endometrioid, positive in colorectal carcinomas 3, 2
• CDX2: Typically positive in colorectal carcinomas 2

Cervical vs. Ovarian/Endometrial

• PAX8: Negative in most cervical squamous cell carcinomas (92%), but positive in 70% of usual-type and 83% of endometrioid-type cervical adenocarcinomas 4
• p16: Strong diffuse staining in cervical adenocarcinomas 2

Algorithm for Determining Primary Site

1. First-line panel: CK7, CK20, PAX8, WT-1

   • CK7+/CK20-: Favors gynecologic primary
   • CK7-/CK20+: Favors gastrointestinal primary
   • PAX8+: Supports Müllerian origin (ovarian, fallopian tube, endometrial)
   • WT-1+: Favors ovarian/tubal serous carcinoma or mesothelial origin

2. Second-line markers based on initial results:

   • If suspecting serous carcinoma: p53 pattern (mutation-type vs. wild-type)
   • If suspecting endometrioid vs. serous: ER (typically positive in both but stronger in endometrioid)
   • If suspecting mesothelial vs. serous: BerEP4, calretinin, CK5/6
   • If suspecting cervical origin: p16 (diffusely positive in HPV-related tumors)

3. For mucinous tumors:

   • CK7/CK20 profile
   • CEA, CA19.9, CDX2 (positive in GI primaries)
   • DPC4 (loss suggests pancreatic origin) 1

Common Pitfalls and Caveats

1. No single marker is 100% specific or sensitive - always use panels and interpret in clinical context 1, 2

2. PAX8 reliability issues:

   • Poorly differentiated endometrial carcinomas may have patchy PAX8 expression or lose expression entirely 5
   • Negative PAX8 does not rule out cervical epithelial malignancies 4

3. p53 interpretation errors:

   • Focus on pattern rather than simply positive/negative
   • "Mutation-type" staining: either diffuse intense (>75% of nuclei) OR completely absent
   • "Wild-type" staining: heterogeneous/variable intensity 1

4. Specimen quality factors:

   • Age of specimen can affect staining (specimens ≥19 months old showed significantly different FOLR1 staining patterns) 6
   • Biopsy vs. resection specimens may yield different results 6
   • Metastatic/recurrent sites may show different staining patterns than primary tumors 6

5. Molecular classification should be considered:

   • MMR IHC should be performed as first-line immunotherapy biomarker for dMMR 1
   • TCGA-based molecular classification can be considered for ovarian endometrioid carcinomas 1

By following this systematic approach to IHC marker selection and interpretation, the primary site of gynecological malignancies can be accurately determined in most cases, guiding appropriate treatment decisions and improving patient outcomes.