Gynecologic Biomarker Reporting

Table 1 — ER / PgR reporting

Percentage may be visually estimated or quantitated (percentage of positive cells in the entire section). Significant regional variation should be reported. Staining intensity (weak to strong nuclear positivity) should also be reported — intensity can be affected by protein amount, antibody used, and antigen retrieval; most cancers show heterogeneous immunoreactivity.

Table 2 — HER2 IHC for Trastuzumab eligibility (NCT01367002 criteria)

HER2 staining in endometrial carcinoma is often lateral or basolateral and spares the apical portion of tumor cells — unlike the pattern typically seen in breast carcinoma. Heterogeneous staining (≥2-degree intensity difference in >5% of tumor) is frequently seen.

Table 3 — HER2 IHC for Trastuzumab-Deruxtecan (T-DXd) eligibility (DESTINY-PanTumor02 criteria, gastric-cancer-based algorithm)

FISH is not indicated for this pathway. The DESTINY-PanTumor02 trial was based solely on HER2 IHC status and did not include FISH results for 2+ cases.

Eligibility nuance. The trial enrolled tumors with 3+ or 2+ HER2 IHC scores. The FDA's April 2024 tumor-agnostic accelerated approval for T-DXd is specifically for 3+ HER2 IHC expression. Reporting low levels of HER2 expression in gynecologic tumors is not currently required (unlike some breast cancer contexts) given limited gynecologic-specific data to date.

Table 4 — HER2 dual-probe FISH for Trastuzumab eligibility

Count signals in ≥20 contiguous tumor cells, at the site of highest IHC protein expression. Heterogeneity (cluster amplification in groups of ≥20 cells, or mosaic/isolated amplified cells) should be reported when present.

Mismatch repair (MMR) IHC

Reported as intact expression, loss of expression, or subclonal loss of expression, for each of MLH1, PMS2, MSH2, MSH6 (or PMS2/MSH6 only in the 2-antibody algorithm). MMR proteins work as dimers: MLH1 loss leads to loss of MLH1 and PMS2; MSH2 loss leads to loss of MSH2 and MSH6.

Loss should only be reported if internal control cells are positive.

Subclonal loss (discrete areas of complete loss adjacent to retained expression) should not be regarded as intact — distinguish from patchy staining, which can occur with intact expression.

Aberrant patterns (cytoplasmic, nucleolar, or punctate nuclear staining) should not be interpreted as intact.

Loss of MLH1/PMS2 → reflex MLH1 promoter methylation testing. Methylated → sporadic, germline testing generally not indicated. Unmethylated → suggests Lynch syndrome, germline MLH1 testing indicated.

Loss of MSH2/MSH6, or isolated loss of MSH6 or PMS2 → high probability of Lynch syndrome; genetic counseling should be considered.

Table 5 — p53 IHC

Serves as a surrogate for TP53 mutation status; 4% of high-grade serous carcinomas show a wild-type IHC pattern despite harboring a loss-of-function TP53 mutation. Subclonal abnormal pattern (up to 21% of endometrial carcinomas) often suggests a secondary mutational event with MMR-deficiency or POLE mutation, or may indicate a mixed carcinoma (correlate with morphology).

Ovarian mucinous tumors may show a "terminal differentiation" pattern — basal overexpression sparing superficial areas. p53 IHC accuracy can be affected by antigen retrieval, archival tissue, fixation quality, and interlaboratory protocol differences (D07 clone showed best performance in one study); ambiguous cases may warrant testing both biopsy and hysterectomy specimens, with TP53 sequencing considered.

Vulvar lesions have their own separate interpretation guidance in the CAP Vulvar Carcinoma protocol.

Table 6 — PD-L1 IHC (Combined Positive Score)

CPS = (PD-L1-positive tumor cells + tumor-associated immune cells) ÷ total viable tumor cells × 100, capped at 100. Requires ≥100 viable tumor cells; evaluate the entire tumor area (not hot spots) at 20× objective. Count tumor cells with any-intensity membrane staining and tumor-associated immune cells (intra- or peritumoral, including lymphoid aggregates) with membrane or cytoplasmic staining; exclude stromal cells, neutrophils, and plasma cells.

FDA-approved (2018) for pembrolizumab eligibility in PD-L1-expressing recurrent/metastatic cervical cancer (22C3 pharmDx assay). Up to 82% of cervical carcinomas are CPS≥1. For vulvar cancer, up to 50% express PD-L1; no current FDA approval, but NCCN and SITC guidelines list pembrolizumab as a second-line option for PD-L1-expressing or MMR-deficient/MSI-H disease. For endometrial cancer, PD-L1 testing is not required — eligibility is determined by MMR/MSI status; PD-L1 can be considered when MMR/MSI is normal.

Table 7 — Folate Receptor Alpha (FOLR1) IHC

Companion diagnostic (VENTANA FOLR1 [FOLR-2.1] RxDx Assay) for mirvetuximab soravtansine (MIRV) eligibility in FOLR1-positive platinum-resistant epithelial ovarian, fallopian tube, or primary peritoneal cancer (FDA accelerated approval, November 2022). Normal fallopian tube serves as both positive and negative control. Re-reading by an additional pathologist is recommended for both positive and negative results.

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