Hepatoid malignancy of unknown origin--a diagnostic conundrum: review of literature and case report of collision with adenocarcinoma.

UNLABELLED: Hepatoid carcinomas are a group of neoplasms with features resembling hepatocellular carcinomas. Although rare, more cases have been noted to arise from various organs within the last decade. Differentiating these tumors is not only a challenge but also critical, since treatment modalities and operative strategies are dependent upon the exact nature of the hepatoid cancer. Based on a review of literature, we discuss the guidelines for differentiating these tumors and utilize these criteria to differentiate these tumors irrespective of their primary tissue of origin. We also describe an unusual case of hepatoid variant of primary peritoneal yolk sac tumor presenting with extensive carcinomatosis and as a collision with two synchronous primary colonic adenocarcinomas, neither of which has been reported to our knowledge to date, thereby falsely mimicking metastatic dedifferentiated colonic adenocarcinoma.

CASE DESCRIPTION: A septuagenarian female presented with weight loss, chronic anemia and ascites associated with bloating. Her past history was significant for COPD, hysterectomy and a bilateral oophorectomy 40 years previously. A work up which included computed tomography (CT) and contrast enema revealed significant ascites with multiple peritoneal nodules causing ascending colonic obstruction. Celiotomy revealed the entire peritoneal surface to be studded with nodules, with the largest measuring 6.0 cm. Intraoperative ultrasound, prompted by the frozen section of nodules resembling liver tissue, revealed no intrahepatic nodules. Right colectomy, omentectomy and tumor debulking were performed. The postoperative course was uneventful except for ascitic leak that spontaneously resolved.

DISCUSSION: The peritoneal nodules consisted of malignant cells arranged in cords that resembled liver tissue. In the absence of a primary identifiable liver disease, this is consistent with either hepatoid variant of primary yolk sac tumor or hepatoid carcinoma arising from the peritoneum. The right colectomy specimen revealed two mucosal ulcers consistent with colonic adenocarcinoma abutting two large tumor nodules on the serosal surface. Although grossly appearing to be contiguous, low power examination showed that the serosal nodule and the mucosal lesion were entirely different, and separated by a definite fibrous band. Immunohistochemical stains and patterns were used to differentiate the type of tumor. This article also discusses the criteria used to differentiate hepatoid yolk sac tumors (hepatoid-YSTs), hepatoid carcinomas and metastatic hepatocellular carcinomas and the phenomenon of opisthoplasia observed in certain malignancies. The difficulties encountered in identifying hepatoid carcinomas and current modalities used to differentiate these tumors are highlighted. The phenomenon of opisthoplasia and the challenges posed by this phenomenon in certain metastatic lesions are reviewed and explanations for the possible origins of hepatoid tumors are considered.

CONCLUSION: The most likely explanation in our case is that the hepatoid tumor either originated from microscopic remnant ovarian tissue left behind or primarily from the peritoneum. With no evidence of yolk sac component within the colonic tumor or in the draining lymphatics, this essentially excludes the commonly observed metastatic dedifferentiation (opisthoplasia) of adenocarcinoma to primitive forms (also known as combination tumors). Based on analysis of various factors, including tumor behavior and response to chemotherapy, we conclude that our case was a hepatoid variant of yolk sac tumor presenting in an elderly woman with carcinomatosis. This unusual presentation of two entirely different primary malignancies in close proximity is defined as "collision tumor". This is the first reported case of collision tumors involving dual colonic and primary peritoneal hepatoid-YST. Identifying the exact type still remains to be the most challenging aspect in the diagnosis of hepatoid tumors.