10-year-old spayed female mixed breed dog (Canis familiars).Patient was initially being evaluated for a left mandibular mass. On additional imaging studies, other tumors were found in the left cranial lung lobe and left adrenal gland. The patient then underwent thoracotomy for partial lung lobectomy and the specimen was submitted for histopathological examination. Two weeks later, the dog underwent laparotomy for excision of the left adrenal gland, and an additional mass was noted in the right medial hepatic lobe. The patient never recovered from anesthesia after laparotomy and developed acute hemoabdomen and hematemesis. Samples from the hepatic mass and the left adrenal gland were submitted for histopathologic examination.
The sample from the left cranial lung lobe contained a 7 mm diameter, well demarcated subpleural nodule at the distal border of the lobe (Fig.1). On cut section, this nodule was firm and pale tan. Approximately 3mm cranial to this nodule there was an irregular and poorly demarcated area of parenchymal cavitation immediately adjacent to the inked (yellow) surgical margin.
A section of lung contains an unencapsulated and well-demarcated peripheral subpleural mass. This mass is solid and composed of two distinct cellular populations, characterized by large scattered acinar-like structures and peripheral papilliferous projections (adenocarcinomatous component), amongst tightly packed cords of cells (small cell component), which diffusely obscure a background of fine fibrovascular stroma. The acinar-like structures are lined by a monolayer of tall columnar epithelium with abundant well-delineated acidophilic cytoplasm. The nuclei are round to ovoid and have sparse finely stippled chromatin with small conspicuous nucleoli. There is loss of nuclear polarity. Anisocytosis and anisokaryosis are moderate, up to 2 fold, with 0-1 mitotic figures per high power field (40X). The cells between the acinar structures are smaller, round to polygonal and have moderate to poorly delimited light acidophilic to clear cytoplasm. The nuclei are round to ovoid and have sparse finely stippled chromatin with large conspicuous nucleoli. Anisocytosis and anisokaryosis are moderate to marked, up to 3 fold, with 8-10 mitotic figures per high power field (40X). There is extensive coalescing necrosis of the mass. Abundant neutrophils, macrophages, sloughed neoplastic cells and basophilic mucus fill the lumen of the neoplastic acini. Small numbers of neutrophils, fewer eosinophils, lymphocytes and rare plasma cells infiltrate the mass. The inked surgical margin is clean but narrow and measures less than 1 mm thick. The pulmonary parenchyma adjacent to the mass is moderately atelectatic and displays extensive hemorrhage within alveolar spaces.
With immunohistochemistry, the adenocarcinomatous component of the tumor was strongly positive for pancytokeratin markers AE1 and AE3. Although pancytokeratin expression was negative for the vast majority of the small cell component, scattered individual cells displayed strong cytoplasmic expression for both markers (Fig. 2 and 3). Neither the adenocarcinomatous or small cell component expressed synaptophysin, chromogranin or neuron specific enolase (NSE). Small numbers of scattered cells throughout the tumors expressed CD20 and CD18 and were interpreted in combination with the H&E section as leukocytes (secondary inflammation). Histopathologic examination of the left mandibular mass (not provided) was consistent with a salivary gland adenocarcinoma. Sections from the left adrenal gland (not provided) revealed a malignant pheochromocytoma. The hepatic nodule from the right medial lobe (not provided) displayed the same morphology as observed in the pulmonary nodule and was interpreted as a focus of metastasis from the pulmonary neoplasm.
Lung (left cranial lobe): Combined pulmonary carcinoma.
Combined pulmonary carcinoma
The dog in the present case had three distinct unrelated neoplasms in different organs with no overt clinical evidence of metastatic disease, except for a hepatic nodule, which was later interpreted as a focus of metastasis from the primary pulmonary mass. The cause for initial presentation to our hospital was a left parotid salivary adenocarcinoma, and when a pulmonary nodule was found in subsequent imaging studies, the immediate clinical suspicion was of metastatic disease from the parotid gland to the lungs. Surprisingly, the pulmonary nodule exhibited unique dual microscopic morphology (an adenocarcinomatous and a small-cell component), not observed in the salivary gland mass, and was diagnosed as a combined pulmonary carcinoma.
Combined tumors are neoplasms that display more than one cellular morphology and microscopic architecture, often composed of cells with remarkably different histological, histochemical and immunohistochemical properties.(8) One example of combined tumor in humans is combined pulmonary carcinomas, which have been reported to exhibit combinations of up to 4 distinct morphologies (squamous cell carcinoma, small-cell/neuroendocrine carcinoma, blastoma and adenocarcinoma).(9) Additionally, extra-pulmonary combined tumors with a small cell/ neuroendocrine component associated with a component of squamous cell carcinoma or adenocarcinoma appear to be relatively common in the rare group of small cell carcinomas in people, and have been reported in the esophagus, prostate gland and colon.(10) Aside from combined pulmonary carcinoma of dogs and cats, other examples of combined tumors in domestic animals include carcinosarcomas of mammary gland and combined hepatocellular-cholangiocellular carcinoma.(13)
Overall, primary pulmonary neoplasms are exceedingly common in people and uncommon in dogs.(1) The prevalence of primary lung cancer in the overall canine population has been reported to be of 1% of all diagnosed canine neoplasia.(1,3,12) The most accepted veterinary classification of pulmonary neoplasms to date includes adenomas and papillomas, bronchial gland carcinoma, squamous cell carcinoma, adenocarcinomas of papillary, acinar, solid and mixed types, bronchioloalveolar carcinoma, adenosquamous carcinoma, small and large cell carcinomas, neuroendocrine carcinoma, pulmonary blastoma and combined carcinoma.(4,14) Adenocarcinomas appear to be the most common type of lung cancer in domestic animals.(3) In contrast to the veterinary classification, in people, combined carcinoma is classified as a subvariant of small cell carcinomas, since it has been shown that the small cell component in those tumors has neuroendrocrine properties. The importance of proper tumor classification in this instance lies in the fact that prognostic outcome and therapeutic management largely depend on the leading histomorphological tumor type.(6) The same has not been proven in the rare cases of combined pulmonary tumors of domestic animals(4), where the small cell component has failed to express the most common neuroendocrine immunohistochemical markers, such as chromogranin, synaptophysin and neuron-specific enolase (as in the present case), and accurate definition of histogenesis of this component remains unavailable. To date, the histogenesis of the small cell component of combined pulmonary tumors of domestic animals has only been morphologically and immunohistochemically characterized as a pulmonary basaloid cell. The difficulty in further analyzing these tumors at the molecular level in domestic animals is most likely due to the rarity of the tumor, which has only been reported in cats and less commonly dogs.(4)
Despite the high prevalence of lung cancer in people, combined pulmonary tumors are also rare in this population. In a recent retrospective study of 1,158 pulmonary surgical specimens of people, combined carcinomas represented 1.8% of all pulmonary tumors.(5) Nevertheless, a few studies in human specimens have been able to provide further insight on what seems to be the most intriguing and fascinating aspect of this rare neoplastic entity; the nature of its capability to produce such phenotypically distinct components. The main questions raised around this entity are whether the different components of the tumors are genetically related and why and how differentiation towards such different cellular components occurs.(5,8,9,11) Answer to these questions may shed light on the histogenesis and biologic behavior of the tumor cells. Clonality assays and study of LOH (loss of heterozygosity) have been employed to aid in answering these questions; however, so far, the results raise more questions than definitive results.
To date, a few studies have shown monoclonality and identical genetic alterations in the specific chromosomes analyzed in a few of these tumors, raising the hypothesis that the different cellular components of the tumors are likely derived from a common precursor cell and driven by the same carcinogens.(5,8,9,11) In these cases, it remains to be understood why cells with a common genetic profile display such distinct morphology, histochemical and immunohistochemical properties. It can be hypothesized that during malignant transformation, some of the progeny cells derived from a common precursor cell may convert to a different morphology under influence of different environmental stimuli. Alternatively, the clonal neoplastic cell may retain the ability to differentiate toward other morphologic phenotypes in response to different stimuli.(8)
Contrary to those results, one study has shown a few combined tumors with different genetic profiles, despite such close morphologic relationship of the different cellular components. In those cases, it was hypothesized that the different groups of cells (small cell component with squamous cell differentiation and a separate adenocarcinomatous component) may have undergone separate progression from a pluripotent single clone in a very early stage, resulting in distinct genetic abnormalities that may have developed in a later phase of the tumor progression.(11) Further clonality and LOH studies on this rare neoplastic entity are still necessary and would help to provide additional insight in the field of stem cells, tumor biology and oncologic pathology. The concomitant development of multiple different neoplasms in this dog (salivary gland adenocarcinoma, combined pulmonary carcinoma and pheochromocytoma) is also an interesting point of discussion beyond the scope of this brief review.
Lung: Combined pulmonary carcinoma.
This case provides an exceptional example of a rarely reported tumor in dogs and cats. As the contributor elucidates, the histogenesis of this neoplasm is not definitive which offers an interesting opportunity for discussion and speculation surrounding this case. The additional clinical findings of two other distinct neoplasms and liver metastasis in this dog further adds to the discussion with regard to tumor suppressors and malignant transformation.
As previously outlined, the reactivity of cytokeratin within both cell populations is characteristic to this diagnosis. The conference discussion, however, was focused on the finding of diffuse immunoreactivity among the small cell population with vimentin leading some to consider the diagnosis of carcinosarcoma for this case. Carcinosarcomas usually have areas of osseous or chondrous metaplasia within the mesenchymal cell population4; and the small cell component in this case does not have spindle cell morphology, thus our agreement with the contributors diagnosis.
Participants noted the primitive morphology and loss of polarity within the small cell component. This led to a discussion on the epithelial-mesenchymal transition (EMT) as being a possible cause for the change in cellular morphology as well as the expression of vimentin, which would be consistent with the presence of metastasis observed in this case.
The EMT is a well-defined process of transformation during which neoplastic cells acquire the ability to invade and disseminate. Included in this transformation is a conversion from a polygonal to spindle morphology along with the repression of E-cadherin expression.(7) Loss of E-cadherin, a key cellular adhesion molecule, eliminates the cellular restraint mechanism within the tissue of origin thereby permitting dissemination. This transformation has been characterized histologically by increased expression of vimentin, which also has been found to correlate with grading criteria in some neoplasms.(15) While there have been studies examining cytokeratin and vimentin expression in canine pulmonary tumors, a correlation with histologic grade has not been demonstrated.(2)
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