A Liver Out of Place—and Out of Control

A case of ectopic hepatocellular carcinoma around the common bile duct in a dog

Eren Saito, Naoyuki Fuke, Ryota Okadera, Ryusei Konno, Tatsuyuki Osuga, Mika Tanabe, Takuya Hirai, Kiyokazu Naganobu, Yasuyuki Kaneko

Background
Ectopic liver, defined as hepatic tissue located outside the normal anatomical liver structure, is rare in both humans and animals. While hepatocellular carcinoma (HCC) is common in dogs, the development of HCC in ectopic liver tissue is exceptionally unusual. This report describes only the second known case of ectopic hepatocellular carcinoma in a dog and provides detailed clinical, histopathological, and imaging findings to expand understanding of this rare condition.

Methods
A 10-year-old male neutered pug dog was evaluated for previously diagnosed gallstones. During a follow-up, an incidental mass was found near the liver. Diagnostic evaluation included contrast-enhanced ultrasound and computed tomography, followed by surgical excision and histopathological examination. Immunohistochemical staining for HepPar-1, CK7, Ki-67, and Iba-1 was performed to characterize the tumor.

Results
The mass was located near the caudate lobe but was not continuous with liver tissue and was instead connected by a fibrous membrane to the common bile duct. Imaging showed heterogeneous enhancement patterns and poor Kupffer cell uptake, suggesting malignancy. Histopathology revealed polygonal hepatocyte-like neoplastic cells with mild atypia, no portal triads, and a sheet-like architecture. Tumor cells were positive for HepPar-1 and negative for CK7, with a Ki-67 index of 15%, confirming malignancy. Iba-1 staining showed significantly fewer macrophages in the tumor than in normal liver. The mass was diagnosed as a well-differentiated hepatocellular carcinoma arising from ectopic liver tissue. No metastasis was observed, and the dog remained clinically normal for over 10 months post-surgery.

Limitations
This is a single case report, limiting the generalizability of findings. The origin of the ectopic liver could not be determined due to lack of genetic or developmental history. Imaging limitations might prevent early detection of similar ectopic tissues.

Conclusions
Ectopic liver, although rare in veterinary medicine, has the potential to undergo malignant transformation. This case underscores the diagnostic utility of contrast-enhanced ultrasound and histopathology in distinguishing ectopic HCC from other hepatic masses. Surgical removal is recommended when such tissues are identified, due to their potential for malignancy. Further studies are required to understand the developmental mechanisms and clinical significance of ectopic liver in dogs.

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