# Detecting FR‐⍺ Expression Level in Cytology Effusion Specimens From Ovarian Cancer and Comparing It With Tissue Specimens

## TL;DR

This study shows that FR-⍺ expression can be reliably detected in ovarian cancer effusion samples, matching tissue results and remaining stable after chemotherapy.

## Contribution

Demonstrates the feasibility of using effusion cell blocks for FR-⍺ detection and suggests a reliable cutoff for positivity.

## Key findings

- FR-⍺ expression in effusion cell blocks showed high concordance (96.4%) with tissue specimens using a 25% cutoff.
- A 75% viable tumor cell cutoff with moderate-to-strong staining is recommended for reliable FR-⍺ positivity in cell blocks.
- FR-⍺ expression remained stable in patients before and after chemotherapy, supporting its use for targeted therapy.

## Abstract

Folate receptor‐⍺ (FR‐⍺) is an attractive target for targeted therapy. Mirvetuximab soravtansine (MIRV) has been approved by the United States Food and Drug Administration (FDA) for treating adult patients with FR‐⍺ positive, platinum‐resistant epithelial ovarian cancer, fallopian tube cancer, or primary peritoneal cancer. Therefore, identifying and detecting FR‐⍺ has become an essential part of precision medicine. This study aimed to evaluate the feasibility of detecting FR‐⍺ protein expression in serous cavity effusions using cell blocks (CBs) and compare the results with surgical pathology biopsy or resection specimens. Furthermore, we investigated potential variations in FR‐⍺ expression pre‐ and post‐chemotherapy.

Immunohistochemical (IHC) staining of FR‐⍺ was performed on the serous cavity effusion specimens from 35 patients with epithelial ovarian cancer, along with matched surgical pathological (SP) biopsy or resection specimens, following the manufacturer's guidelines for IHC analysis.

Positive staining was observed in 30 (85.7%) and 32 (91.4%) serous cavity effusion CB and tissue samples, respectively. A total of 31 (88.5%) tissue and 28 (80%) effusion CB samples exhibited moderate‐to‐strong FR‐⍺ expression in at least 25% of tumor cells. A cutoff of 25% for FR‐⍺ expression positivity was used to demonstrate high concordance (96.4%) between cytology CBs and tissue specimens. We recommend a cutoff of 75% viable tumor cells with moderate‐to‐strong membrane staining as “positive” in CBs for greater reliability, which aligns with FDA‐approved criteria. A reasonable consistency was observed in FR‐⍺ expression between the pre‐treatment serous cavity effusion CB specimens and the new biopsy specimens obtained after multiline treatment.

We demonstrated that serous effusion CB specimens may be effective for FR‐⍺ biomarker detection; FR‐⍺ maintains stable expression pre‐ and post‐chemotherapy, and patients can benefit from MIRV.