Metastatic Colorectal Carcinoma

Case Overview

By Alison Messer, MD
Educational dermpath case series for dermatology residents

Patient: 73-year-old woman
Lesion Location: Groin

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A biopsy of pink papules and nodules from the groin was performed. Take a moment to review the histologic images below — what’s your diagnosis?

 

Key Histologic Findings

On examination, several defining features stand out:

• Basaloid Cells
• Duct Formation
• “Dirty” Necrosis with intraluminal neutrophilic debris

 

(CDX-2)

(CK-20)

 

Differential Diagnosis: Three Common Mimickers

When evaluating an atypical basaloid lesion in the skin, the differential diagnosis includes both primary cutaneous tumors and metastatic lesions. Let’s walk through the three most likely contenders and highlight what sets them apart.

1️⃣ Merkel Cell Carcinoma

Etiology: Merkel cell polyomavirus (MCPyV) or UV-induced carcinogenesis; neuroendocrine origin

Histology:

• Basaloid cells with scant cytoplasm and vesicular nuclei
• CK20 positive (perinuclear dot pattern) — a key overlap with colorectal carcinoma
• Negative for CDX2
• Lacks duct formation

Key Distinction:

Although Merkel cell carcinoma and metastatic colorectal carcinoma both exhibit CK20 positivity and a basaloid appearance, Merkel cell carcinoma lacks duct formation and is CDX2 negative. The perinuclear dot-like pattern of CK20 staining and positivity for neuroendocrine markers (synaptophysin, chromogranin) confirm a diagnosis of Merkel cell carcinoma.

(Merkel cell carcinoma)

2️⃣ Basal Cell Carcinoma

Etiology: UV-induced mutations in the hedgehog signaling pathway (PTCH1); most common skin malignancy

Histology:

• Nests of basaloid cells with peripheral palisading
• Retraction artifact between tumor nests and stroma
• Negative for CK20 and CDX2
• Duct formation is uncommon

 

Key Distinction:

While basal cell carcinoma shares the basaloid cell morphology, it is readily distinguished by its negative CK20 and CDX2 immunoprofile. Peripheral palisading and stromal retraction artifact are hallmarks of BCC not seen in metastatic colorectal carcinoma. The absence of “dirty” necrosis and duct formation further argues against a colorectal primary.

(Basal cell carcinoma: peripheral palisading and stromal retraction)

 

3️⃣ Metastatic Breast Carcinoma

Etiology: Invasive breast carcinoma with cutaneous metastasis; most commonly invasive ductal carcinoma

Histology:

• Duct formation can be prominent — a key overlapping feature
• Often CK7 positive
• CK20 and CDX2 negative
• Necrosis may be present but lacks the “dirty” quality of colonic origin

 

Key Distinction:

Metastatic breast carcinoma can closely mimic colorectal carcinoma when ductal structures are present. The IHC profile is the key separator: breast carcinoma is typically CK7 positive, and negative for CK20 and CDX2. Metastatic colorectal carcinoma shows the reverse pattern, making a targeted IHC panel essential.

 

(Metastatic breast carcinoma: basaloid cells infiltrating the dermis)

The Final Diagnosis:

Metastatic Colorectal Carcinoma

The combination of basaloid cells, duct formation, and “dirty” necrosis is highly characteristic of colorectal origin. Immunohistochemistry confirms CK20 and CDX2 positivity, supporting gastrointestinal origin and ruling out other entities in the differential diagnosis. In the context of a cutaneous metastasis from the groin in a 73-year-old woman, these findings together are diagnostic.

Key Takeaways for Residents:

• Always evaluate basaloid cutaneous tumors at high power — duct formation and dirty necrosis are the diagnostic keys.
• CK20 positivity is shared between colorectal carcinoma and Merkel cell carcinoma; CDX2 is the differentiating stain — positive only in tumors of GI origin.
• Correlate with clinical history. A cutaneous metastasis in a 73-year-old should prompt evaluation for a GI or GYN primary.
• A targeted IHC panel (CK7, CK20, CDX2, synaptophysin, chromogranin) resolves the vast majority of basaloid cutaneous tumors in the differential.

 

(Diagnostic high-power view: duct formation with “dirty” necrosis)

 

📚 Quick Summary

Feature	Metastatic Colorectal Carcinoma	Merkel Cell Carcinoma	Basal Cell Carcinoma	Metastatic Breast Carcinoma
HPV-related	❌ No	❌ No	❌ No	❌ No
Basaloid cells	✅ Present	✅ Present	✅ Present	❌ Variable
Duct formation	✅ Present	❌ Absent	❌ Uncommon	✅ Present
“Dirty” necrosis	✅ Present	❌ Absent	❌ Absent	❌ Absent
CK-20 positive	✅ Yes	✅ Yes	❌ No	❌ Typically No
CDX-2 positive	✅ Yes	❌ No	❌ No	❌ No
CK-7 positive	❌ Typically No	❌ No	❌ No	✅ Typically Yes
Behavior	Metastatic	Aggressive / metastatic	Locally aggressive	Aggressive / metastatic

 

💬 Final Thought

Metastatic colorectal carcinoma is a classic dermatopathology “look-alike” that rewards careful microscopic examination and a disciplined IHC approach. Remember: the dirty necrosis and CDX2 tell the story.

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