Adenocarcinoma is a cancer that originates in glandular tissue. This tissue is also part of a larger tissue category known as epithelial tissue. Epithelial tissue includes skin, glands and a variety of other tissue that lines the cavities and organs of the body. Epithelium is derived embryologically from ectoderm, endoderm and mesoderm. To be classified as adenocarcinoma, the cells do not necessarily need to be part of a gland, as long as they have secretory properties. This form of carcinoma can occur in some higher mammals, including humans. Well differentiated adenocarcinomas tend to resemble the glandular tissue that they are derived from, while poorly differentiated may not. By staining the cells from a biopsy, a pathologist will determine whether the tumor is an adenocarcinoma or some other type of cancer. Adenocarcinomas can arise in many tissues of the body due to the ubiquitous nature of glands within the body. While each gland may not be secreting the same substance, as long as there is an exocrine function to the cell, it is considered glandular and its malignant form is therefore named adenocarcinoma. Endocrine gland tumors, such as a VIPoma, an insulinoma, a pheochromocytoma, etc, are typically not referred to as adenocarcinomas, but rather, are often called neuroendocrine tumors. If the glandular tissue is abnormal, but benign, it is said to be an adenoma. Benign adenomas typically do not invade other tissue and rarely metastasize. Malignant adenocarcinomas invade other tissues and often metastasize given enough time to do so.
Cancer for which a primary site cannot be found is called cancer of unknown primary.
Examples of tissues where adenocarcinomas may arise:
When these glands undergo a number of changes as the genetic level, they proceed in a normal and predictable manner as they move from benign to an invasive, malignant colon cancer. In the research paper, Vogelstein, et al, suggested that colon cells lose the APC tumor suppressor gene and become a small polyp. Next, they suggested that k-Ras becomes activated and the polyp becomes a small, benign, adenoma. The adenoma, lacking the "carcinoma" attached to the end of it, suggests that it is a benign version of the malignant adenocarcinoma. The gastroenterologist uses a colonoscopy to find and remove these adenomas and polyps to prevent them from continuing to acquire genetic changes that will lead to an invasive adenocarcinoma. Volgelstein et al went on to suggest that loss of the DCC gene and of p53 result in a malignant adenocarcinoma.
Grossly, one will see a mass that looks of a different color than the surrounding tissue. Bleeding from the tumor is often apparent as the tumor tends to grow blood vessels into it in a haphazard manner via secretion of a number of angiogenesis promoting factors such as VEGF. Histologically, a glandular structure, similar to the healthy normal surrounding glands may be seen. If they look very similar, this is a low grade, well differentiated tumor. Often these glands will be disorganized and they will be seen growing back to back. However, if the tumor does not look like a gland anymore, it is a high grade tumor with poor differentiation. Regardless of the grade, malignant tumors tend to have a large nucleus with prominent nucleoli. There will also be a noticeable increase in the incidence of mitoses, or cell divisions.
Currently, the most common type of lung cancer is the adenocarcinoma. This cancer usually is seen peripherally in the lungs, as opposed to small cell lung cancer and squamous cell lung cancer, which both tend to be more centrally located. The adenocarcinoma has an increased incidence in smokers, but is also the most common type of lung cancer seen in non-smokers. Adenocarcinoma of the lung tends to stain mucin positive as it is derived from the mucus producing glands of the lungs. Similar to other adenocarcinoma, if this tumor is well differentiated (low grade) it will resemble the normal glandular structure. Poorly differentiated adenocarcinoma will not resemble the normal glands (high grade) and will be detected by seeing that they stain positive for mucin (which the glands produce).