The term adenocarcinoma (AC) generally applies to skin surface (epithelial) cancer that originates from abnormal gland cells that are on the lining or inner surface of an organ. Adenomas are benign tumors of gland cells, which over time, may transition into cancerous (malignant) tumors. AC’s and adenomas may originate in any part of the body.
The skin and delicate mucous membranes are commonly affected in senior pets.  Dogs are more likely to develop sebaceous (fatty), apocrine (anal sac), perianal (anal area), ceruminous (ear wax), salivary and sweat gland tumors, while cats are more likely to have basal cell tumors. Breast cancer (which is sex hormone related) is commonly encountered in intact female dogs or in dogs spayed after 2-1/2 years of age. It is less common in cats but much more malignant. (See section on Breast Cancer)
Skin cancer or squamous cell carcinoma (SCC) appears in lightly pigmented facial skin of cats and in the underbelly skin of lightly pigmented dogs. It is solar induced, which means exposure to sunlight causes the cancer by mutations of the squamous cells. However, SCC also appears in the oral cavity, tongue, tonsils, esophagus, nasal and paranasal sinuses, respiratory tract and nail beds. Some reports have associated tonsilar SCC in dogs with environmental pollution and oral SCC and GI lymphoma in cats with exposure to coat-associated carcinogens from cigarette smoke.
AC’s in the abdomen may originate from glands such as: liver, colon, intestine, stomach (gastrinoma), kidney, bladder, pancreas, prostate and the adrenal glands. Widespread scattering of cancer in the abdomen and throughout the body is often termed as carcinomatosis, but the term may include other tissue types.
AC may also originate from any gland in the neuroendocrine or reproductive system such as the thyroid glands, pancreas, adrenal glands, pituitary gland, ovaries, uterus, testicles and prostate. These tumors often cause paraneoplasic syndromes related to their cell products. (See section on Problems Associated with Cancer (paraneoplasic syndromes).
Anal sac AC is well known for causing malignant abnormal amount of calcium in the blood(hypercalcemia); however, other malignancies, especially lymphoma, may also cause this potentially life threatening paraneoplastic syndrome. (See section on Paraneoplastic)
Nasal AC may extend pass the ethmoid plate to the brain. Primary central nervous system (CNS) brain tumors are very pleomorphic (existing in two or more distinct forms). Only choroid plexus tumors of the brainstem, which are poorly responsive to treatment, are classified as carcinomas. AC may appear in the ciliary body in the eye and occasionally in the glands of the eyelids.
Aortic body [heart base tumors (chemodectoma)] and bronchogenic (passage of the lung) AC originate in the chest cavity. Bronchogenic AC is often found as a round solitary mass in the posterior part of the chest. If it is located toward the end (distal) part of the lung lobe without local lymph node spread (metastases), affected dogs may have good survival times following surgical removal of the lobe (lobectomy). Cats rarely have primary lung cancer, but when they do it is generally SCC, which may metastasize to the digits and resist treatment. (See section on Chest Cavity Tumors).  
Mesotheliomas originate from the epithelial lining of the pleura, pericardium and peritoneum.  They originate in the chest and abdominal cavity causing malignant effusions. In humans, mesotheliomas are associated with asbestos and are rare in dogs and cats.  Most mesotheliomas are AC’s despite their misnomer. Pulmonary lymphomatoid granuloma (PLG) is a. rare condition that may confuse the doctor.
It is categorized as a pre-cancer. It occurs in middle-aged dogs and appears as massive pulmonary involvement with huge nodules and infiltrates resembling end stage AC or sarcoma. It is important to distinguish this disease from AC as most cases of PLG respond nicely to treatment and have a favorable prognosis despite the enormity of the lesions (wound/injury).
Some malignant tumors have cells that are so undifferentiated or “dedifferentiated” that the pathologist can only report them as anaplastic carcinomas with no idea as to the cell type of origin. This was the case with Alfie, Dr. Villalobos’ 11-1?2 year old Australian Shepherd. Ultrasound guided FNA cytology of the mass revealed two populations of bizarre cells that looked like they could be AC or sarcoma. The diagnosis on histopathology was undifferentiated carcinoma with no definite gland of origin. We know that Alfie’s AC was hepatic in origin from visual inspection at exploratory laparotomy (incision through abdominal wall).
The biological behavior for most AC’s is aggressive with a persistent tendency for metastasis. Local recurrence and lymph node invasion is common followed by widespread metastasis. Most AC’s expand to a detectable size by outward growth (like an onion) and by direct extension into local tissue. The cells use matrix metalloproteinase (metal in the catalytic mechanism) to dissolve neighboring cell walls for local invasion. AC cells gain entry by diapedesis into local lymphatic and capillary vessels, and then disseminate throughout the body. Metastatic AC generally appears as nodular cell clones in the lungs, liver (abdomen), brain, eyes, and may further metastasize to bone and the dermis.
On rare occasions, AC may metastasize to the digits (fingers/toes) in cats. German Shepherds may develop a rare form of renal AC, classified as cystadenocarcinoma, which causes the bizarre formation of multiple benign fibrous nodules in the skin.
AC’s are commonly visualized as fluffy or miliary infiltrates or nodules in chest x-rays of pets in advanced stage disease. Some pets develop pulmonary effusion and cough up blood. Nasal cancer patients exhibit chronic unilateral nasal discharge and/or a bloody nose. Some dogs and cats develop occult (secretive) disease and the only warning signs that the pet’s owner detect are coughing, gagging, exercise intolerance or dyspnea due to pulmonary compromise from the metastatic disease process.

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