When signet‑ring cells are identified in ascitic fluid cytology but imaging shows no tumor, should this be considered a false‑positive result, and what diagnostic steps are recommended?

Signet-Ring Cells in Ascitic Fluid Without Visible Tumor: Diagnostic Approach

When signet-ring cells are identified in ascitic fluid cytology but no tumor is visible on imaging, this should NOT be automatically dismissed as false-positive—malignancy remains the most likely diagnosis (78% of cases), but critical mimics must be systematically excluded through immunohistochemistry and clinical correlation before proceeding with cancer treatment. 1

The Core Diagnostic Challenge

You are correct that signet-ring cell morphology can be mimicked by benign entities, creating a genuine diagnostic pitfall. The cytomorphology alone is insufficient for definitive diagnosis when imaging is negative.

Malignancy Probability

• Signet-ring cells in ascitic fluid indicate malignancy in 78% of cases 1
• Of malignant cases, 72% are metastatic and 51% are from unknown primaries 1
• However, 16% of cases with signet-ring cells are benign 1

Critical Benign Mimics to Exclude

1. Benign Histiocytes (Most Common Benign Mimic)

The most recent evidence (2024) identifies benign histiocytic signet-ring cells as a major diagnostic pitfall 2:

Key distinguishing features:

• Cells arranged in sheets or scattered individually—NOT in 2D/3D clusters
• Intermediate size with normal nuclear-to-cytoplasmic ratio
• Eccentric nuclei with fine granular chromatin (no hyperchromasia or coarse chromatin)
• No nuclear atypia
• CD68 strongly positive, but CK7/CK20/epithelial markers negative 2

2. Reactive Mesothelial Cells

Account for 69% of benign cases with signet-ring morphology 1

3. Peritoneal Mesothelioma

Multiple recent reports (2022) document mesothelioma being misdiagnosed as adenocarcinoma due to signet-ring cells 3, 4:

• Can present with peritoneal carcinomatosis-like imaging
• Requires calretinin, WT-1, D2-40 positivity and CK20/CDX2 negativity
• Often lacks asbestos exposure history

Mandatory Diagnostic Algorithm

Step 1: Perform Immunohistochemistry on Cell Block

This is non-negotiable when imaging is negative:

For adenocarcinoma confirmation:

• CK7, CK20, CDX2, SATB-2 (GI origin)
• PAX-8, WT-1 (gynecologic origin)
• Sensitivity for cancer diagnosis: 97%, Specificity: 100% 1

To exclude benign mimics:

• CD68 (histiocytes—if positive, likely benign) 2
• Calretinin, D2-40 (mesothelioma)

Step 2: Correlate with Clinical Context

Specific features to assess:

• Age and sex (young patients: consider embryonal carcinoma 5)
• GI symptoms or occult GI bleeding (appendiceal SRCC can present this way 6)
• Prior malignancy history
• Asbestos exposure (though absence doesn't exclude mesothelioma 3, 4)

Step 3: Advanced Imaging if IHC Suggests Malignancy

• Laparoscopy with direct visualization and biopsy is recommended for peritoneal carcinomatosis workup 7
• Consider PET-CT for occult primary detection
• Upper endoscopy and colonoscopy (gastric and appendiceal primaries are common 6)

Step 4: If Malignancy Confirmed but Primary Unknown

Follow ESMO 2023 CUP guidelines 8:

• Women with peritoneal carcinomatosis and serous papillary adenocarcinoma = "ovary-like CUP" (favorable subtype)
• Treat as presumed ovarian cancer with platinum-based chemotherapy
• Adenocarcinoma with colorectal IHC profile (CK7-negative, CK20-positive, CDX2-positive) = "colon-like CUP"

Common Pitfalls to Avoid

1. Never diagnose adenocarcinoma on morphology alone when imaging is negative—IHC is mandatory 2, 3, 4

2. Don't assume negative imaging excludes malignancy—peritoneal carcinomatosis can be microscopic initially, and 78% of signet-ring cells in ascites are malignant 1

3. Beware of "pseudo-signet ring cells" in inflammatory conditions—these are increasingly recognized 9

4. Don't overlook mesothelioma—it frequently masquerades as adenocarcinoma with signet-ring morphology 3, 4

When to Proceed with Cancer Treatment

Only proceed with cancer-directed therapy when:

• IHC confirms epithelial malignancy (CK7/CK20/CDX2 pattern consistent with adenocarcinoma)
• CD68 is negative (excludes histiocytes)
• Mesothelial markers are negative (excludes mesothelioma)
• Clinical context supports malignancy

If IHC is equivocal or unavailable:

• Pursue tissue biopsy via laparoscopy 7
• Repeat cytology with fresh specimen for cell block
• Do NOT initiate chemotherapy based on cytomorphology alone

The pathologist may indeed be seeing a benign mimic, but systematic exclusion through IHC is the only way to know with certainty. The 97% sensitivity and 100% specificity of cytologic diagnosis with appropriate ancillary studies supports that proper workup can reliably distinguish true malignancy from mimics 1.