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National Cancer Institute U.S. National Institutes of Health www.cancer.gov
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Malignant tumors

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Main criteria for malignancy include size over 5 mm in diameter, invasion of airways, blood or lymphatic vessels, regional and distant metastasis, and ability to grow upon transplantation. Nuclear and cellular atypia, and loss of architecture should be considered as ancillary criteria for defining malignancy.

Squamous cell carcinoma
The hallmarks of squamous cell carcinoma are the differentiation features of the squamous epithelium: keratinization and intercellular bridges. Large central masses of keratin, individual cell keratinization, and/or keratin pearls may form. Necrosis of tumor cell nests and accumulation of acute inflammatory cells are frequent features of poorly differentiated squamous cell carcinoma.
Adenocarcinoma
Compared to adenomas, adenocarcinomas show greater cytological atypia, increased frequency of mitoses, regional variation in growth pattern, more papillary structures, have size over 5 mm in diameter, show invasion of vessels, large airways or pleura, as well as lymphatic and hematogenous metastases.
Adenosquamous carcinoma
Composed of both adenocarcinoma and malignant squamous components with the presence of at least 10% of each component. Keratinization is common.
Neuroendocrine carcinoma
This is a group consisting of tumors, in which neuroendocrine differentiation has been confirmed by immunohistochemical detection of such proteins as synaptophysin, CGRP and chromogranin. These tumors have only recently been described in mice. Their morphology varies from acinar pattern formation (LW004) to more palisading structures (LW011) to poorly differentiated neoplasms consisting of small hyperchromatic cells with minimal cytoplasm (LW009). Their relevance to human small cell carcinomas and other neuroendocrine carcinomas remains to be determined.
Carcinoma, other
Usually, this category includes carcinomas without definitive diagnoses due to a small amount of material, or its low quality. Novel tumors, which are not completely characterized, can be temporarily allocated to this category. Caution needs to be taken to differentiate primary lung tumors from pulmonary metastases from other tumor sites.