Adenocarcinoma accounts for more than 90% of colorectal carcinomas [8]. Adenocarcinoma is a malignant epithelial tumor that shows gland formation, and based on the degree of gland formation, adenocarcinomas can be graded as one of the three differentiation types: well differentiated, moderately differentiated, and poorly differentiated [9]. According to a study conducted in the Netherlands, approximately 20% of patients with colorectal carcinoma present metastatic lesions at diagnosis [10], and a Swiss group revealed that the overall metastasis rate of colorectal carcinoma was approximately 30% [11]. The liver is the most common metastatic site of colorectal carcinoma, and the main differential diagnosis of a liver metastatic lesion of colorectal carcinoma is primary intrahepatic cholangiocarcinoma, as colorectal carcinoma is mostly adenocarcinoma. Histologically, colorectal adenocarcinoma is composed of atypical epithelial cells with various degrees of glandular differentiation and invasion (Fig. 1A) [8,9]. Histologic variants of colorectal adenocarcinomas include mucinous, signet ring cell, medullary, micropapillary, serrated, and cribriform-comedo subtypes, according to the latest World Health Organization (WHO) classification of tumors of the digestive system [9]. Desmoplastic stroma, a type of fibrous proliferation, can be seen around invasive carcinoma cells. Another frequent characteristic is “dirty necrosis,” which is a form of necrotic debris, in glandular lumen. Immunohistochemically, cytokeratin 7 (CK7) negative, cytokeratin 20 (CK20) positive, and caudal-type homeobox 2 (CDX2) positive are the most common immunophenotypes of colorectal carcinoma (Fig. 2A, Table 1) [8,9,12,13]. CDX2 is a highly sensitive and specific marker for the detection of intestinal-origin or intestinal-type epithelial cells, and >90% of adenocarcinomas of the colorectum show diffuse and strong nuclear expression [14-16]. In addition to CDX2, Special AT-rich sequence-binding protein 2 (SATB2) is also a specific immunohistochemical marker for epithelial tumors of the small and large intestines and the appendix [17]. Thus, the combination of CDX2 and SATB2 can be used as a specific marker panel for the diagnosis of metastatic colorectal carcinoma in the liver [17]. A small subset of poorly differentiated carcinomas of the colorectum can show loss of CDX2 and/or CK20 expression [18]. Therefore, it should be noted that CDX2/CK20-negative colorectal carcinoma can occasionally metastasize to the liver.

Breast cancer is a malignant tumor that arises in the breast, and most breast cancers are invasive breast carcinomas [19]. Among the histologic types of breast carcinoma, invasive carcinoma of no special type (NST) is the largest group and is commonly known as invasive ductal carcinoma [20]. Metastasis of breast cancer occurs in about 20-30% of patients [21], and approximately 10-15% of patients develop distant metastasis within three years of primary cancer detection [22]. The common metastatic sites are bone, lung, and liver [23]. As >90% of breast cancers are diagnosed as NST invasive carcinoma, which is characterized by frequent tubule formation that resembles adenocarcinoma, the main differential diagnosis of metastatic breast carcinoma in the liver is primary intrahepatic cholangiocarcinoma. Histologically, breast carcinoma is composed of atypical epithelial cells with uniform or highly pleomorphic nuclei, and tubular, glandular, cribriform, or solid structures can be observed (Fig. 1B) [20]. The tumor border shows infiltrative or pushing margins, and the stromal component shows extreme variability. Invasive lobular carcinoma is the second most common histologic type of breast carcinoma, following ductaltype (usually NST) invasive carcinoma. Invasive lobular carcinoma is morphologically characterized by single-cell or single-file type infiltration of poorly cohesive atypical epithelial cells, which show relatively monotonous nuclei. Immunohistochemically, CK7-positivity and CK20-negativity are the most common breast carcinoma immunophenotypes (Fig. 2B, Table 1) [12,13]. A comparison of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 immuno-profiles between primary breast carcinoma and metastatic carcinoma in the liver aids the differential diagnosis. In addition, immunohistochemical studies for tissue-specific markers, such as gross cystic disease fluid protein 15 (GCDFP-15) and mammaglobin, will be helpful [13,24-26]. However, it should be noted that GCDFP-15 and mammaglobin are highly specific but less sensitive markers for breast-origin carcinomas. The loss of E-cadherin expression in tumor cells is a useful immunohistochemical feature for the confirmation of lobular carcinoma.

Neuroendocrine tumors are defined as neoplasms that are composed of atypical epithelial cells with neuroendocrine differentiation [27]. Neuroendocrine tumors are usually found in the small intestine, appendix, rectum, pancreas, and lung [28,29]. Neuroendocrine tumors have a site-specific staging system and can be graded as grade 1, 2, or 3 based on mitotic count or Ki-67 labelling index [27]. Gastroenteropancreatic neuroendocrine tumors are divided into neuroendocrine tumor grades 1 or 2 and neuroendocrine carcinoma. Neuroendocrine tumors grades 1 and 2 show relatively welldifferentiated neoplasm and are composed of atypical epithelial cells that resemble normal endocrine cells (Fig. 1C). Neuroendocrine carcinomas are poorly differentiated malignant neoplasms that are composed of small or large cells with high mitotic activity and marked nuclear atypia. Neuroendocrine tumor grade 1, formerly called carcinoid tumor, can also metastasize [30]. The metastases of neuroendocrine tumors have been observed in about 20-25% of patients, and the liver is the most common metastatic site [29]. Small intestinal neuroendocrine tumors encompass more than half of the liver metastases of all neuroendocrine tumors. Histologically, neuroendocrine tumors are composed of uniform cells with light eosinophilic cytoplasm, and the tumor cells are arranged in trabeculae and/or pseudorosette patterns [31]. Both hepatocellular carcinoma and intrahepatic cholangiocarcinoma are considered in the differential diagnoses of metastatic neuroendocrine tumors in the liver. Immunohistochemically, neuroendocrine tumors show variable CK7 and CK20 positivity, based on anatomical site (Table 1) [12,13,32]. Characteristically, neuroendocrine tumors show immunoreactivity for neuroendocrine markers, including chromogranin A, synaptophysin, and cluster of differentiation molecule 56 (CD56) (Fig. 2C). Tissue specific markers, such as HepPar1 and Arginase-1 for hepatocellular carcinoma, along with CK7 and CK20 immunophenotypes facilitate the differentiation of metastatic neuroendocrine tumors from hepatocellular carcinoma and intrahepatic cholangiocarcinoma [13]. Primary hepatic neuroendocrine tumors and hepatocellular carcinomas with neuroendocrine differentiation have been reported; however, both cases are extremely rare [33,34].

Lung carcinoma is a malignant epithelial tumor that originates in the lungs. Lung carcinomas are largely classified into two main categories: non-small cell lung carcinoma and small cell lung carcinoma [35]. Non-small cell lung carcinomas are further classified into adenocarcinoma, squamous cell carcinoma, and large cell carcinoma based on histologic features. Metastases of small cell lung carcinomas can occur in about 60% of patients, and the liver is the most common metastatic site, after pleural and/or pericardial fluids [36]. In patients with non-small cell lung carcinoma, metastases occur in approximately 40-50% of cases, and liver metastases account for 10-14% of the total metastases [37,38]. Adenocarcinoma and squamous cell carcinoma account for most non-small cell lung carcinomas [35]. Adenocarcinoma shows acinar, papillary, micropapillary, lepidic, or solid growth patterns. Intrahepatic cholangiocarcinoma is the main differential diagnosis of metastatic pulmonary adenocarcinoma of the liver (Fig. 1D). Squamous cell carcinoma shows keratinization and/or intercellular bridges, and hepatocellular carcinoma can be included as a differential diagnosis of metastatic squamous cell carcinoma in the liver. Small cell lung carcinoma is composed of atypical, small, and round epithelial cells with scant cytoplasm and finely-dispersed and granular nuclear chromatin (Fig. 1E). Combined hepatocellular-cholangiocarcinoma with stem cell features as well as poorly-differentiated hepatocellular carcinoma or intrahepatic cholangiocarcinoma can be considered as differential diagnoses. Immunohistochemically, lung adenocarcinoma usually shows immunoreactivity for CK7, thyroid transcription factor 1 (TTF-1), and napsin A, whereas lung squamous cell carcinoma shows immunoreactivity for cytokeratin 5/6 (CK5/6), p63, and p40 (Fig. 2D, Table 1) [12,13,35,39-41]. Small cell lung carcinoma shows positivity for at least one neuroendocrine marker, including CD56, synaptophysin, and chromogranin A (Fig. 2E, Table 1). TTF-1 positivity has also been reported in 85-95% of small cell lung carcinomas [35,42-45].

Gastric carcinoma is defined as a malignant epithelial neoplasm that arises in the stomach [46]. According to a study of Swedish patients, 26% of gastric carcinomas develop distant metastases, and the liver (48% of all metastases) is the most common metastatic site [47]. Histologically, tubular adenocarcinoma is the most common histological subtype of gastric carcinoma (Fig. 1F). Other histological gastric carcinoma variants include poorly cohesive carcinoma (including signet ring cell carcinoma), papillary adenocarcinoma, and mucinous adenocarcinoma [48]. As adenocarcinoma is the most common histologic subtype of gastric carcinoma, the main differential diagnosis of metastatic gastric carcinoma in the liver is intrahepatic cholangiocarcinoma. Immunohistochemically, CK7-positivity and CK20-negativity are the most common immunophenotype of gastric carcinoma. However, a substantial number of patients with gastric carcinoma can show CK7-positive and CK20-positive immunophenotypes (Table 1) [12,13,49-52]. Both gastric carcinoma and intrahepatic cholangiocarcinoma show similar cytokeratin 19 (CK19) positivity [50,52,53]. CDX2 might facilitate the differentiation of metastatic gastric carcinoma in the liver from intrahepatic cholangiocarcinoma because 60.9% of patients with gastric carcinoma have shown immunoreactivity, compared to 13% of those with biliary cancers [13,16,54,55]. Rarely, hepatoid adenocarcinoma can occur in less than 5% of patients with gastric carcinoma [56-58]. The metastasis of hepatoid adenocarcinoma to the liver needs to be differentially diagnosed from hepatocellular carcinoma. Both carcinomas are AFP-positive, but CK19 positivity is more frequent in metastatic hepatoid adenocarcinoma than in primary hepatocellular carcinoma [59]. Mucin 5AC (MUC5AC), a gastric-type mucin, is detected in 38-85% of gastric cancers (GCs) (Fig. 2F) [55,60,61]. However, MUC5AC should be used with caution for the differential diagnosis of metastatic GC in the liver because MUC5AC positivity can be observed in 42-61.1% of intrahepatic cholangiocarcinomas and is rarely observed in adenocarcinomas from other sites (Table 1) [61-89].