In WHO-2004, BACs are one of four specific histologic subtypes of lung adenocarcinoma, along with acinar adenocarcinoma, papillary adenocarcinoma, and solid adenocarcinoma with mucin production. However, approximately 80% of adenocarcinomas are found to contain two (or more) of these four subtypes. Multiphasic tumors such as these are classified into a fifth "subtype", termed adenocarcinoma with mixed subtypes.

There are other classification systems that have been proposed for lung cancers, including BACs and other forms of adenocarcinoma. The Noguchi classification system for small adenocarcinomas has received considerable attention, particularly in Japan, but has not been nearly as widely applied and recognized as the WHO system.

Like other forms of lung carcinoma, BAC possesses unique clinical and pathological features, prognosis, and responses to different treatments.

In situ pulmonary adenocarcinoma (AIS), previously called "Bronchioloalveolar carcinoma" (BAC), is a term describing certain variants of lung cancer arising in the distal bronchioles or alveoli that initially exhibit a specific non-invasive growth pattern. BAC is a type of non-small-cell lung cancer (NSCLC). AIS is defined as a small (≤3 cm) solitary tumour with pure alveolar epithelial appearance (lepidic growth), lacking any invasion of the interstitium. If completely resected, the prognosis of surgically treated AIS is 100%.

Adenocarcinomas are highly heterogeneous tumors. Several major histological subtypes are currently recognized by the WHO and IASLC/ATS/ERS

- Non-invasive or minimally invasive adenocarcinoma

- Adenocarcinoma in situ of the lung (Bronchioalveolar carcinoma)

- Minimally invasive adenocarcinoma of the lung

- Invasive adenocarcinoma

- Acinar predominant adenocarcinoma

- Papillary predominant adenocarcinoma

- Micropapillary predominant adenocarcinoma

- Solid predominant adenocarcinoma

- Invasive mucinous adenocarcinoma

In as many as 80% of tumors that are extensively sampled, components of more than one of these subtypes will be recognized. In such cases, resected tumors should be classified by comprehensive histological subtyping. Using increments of 5% to describe the amount of each subtype present, the predominant subtype is used to classify the whole tumor. The predominant subtype is prognostic for survival after complete resection.

Signet ring and clear cell adenocarcinoma are no longer histological subtypes, but rather cytological features that can occur in tumour cells of multiple histological subtypes, most often solid adenocarcinoma.

Some variants are not clearly recognized by the WHO and IASLC/ATS/ERS classification:

- Enteric adenocarcinoma of the lung

- Cribriform adenocarcinoma of the lung

Nearly 40% of lung cancers in the US are adenocarcinoma, which usually originates in peripheral lung tissue. Most cases of adenocarcinoma are associated with smoking; however, among people who have smoked fewer than 100 cigarettes in their lifetimes ("never-smokers"), adenocarcinoma is the most common form of lung cancer. Its incidence has been increasing in many developed Western nations in the past few decades, where it has become the most common major type of lung cancer in smokers (replacing squamous cell lung carcinoma) and in lifelong nonsmokers. According to the Nurses' Health Study, the risk of adenocarcinoma of the lung increases substantially after a long duration of previous tobacco smoking, with a previous smoking duration of 30 to 40 years giving a relative risk of approximately 2.4 compared to never-smokers, and a duration of more than 40 years giving a relative risk of approximately 5.

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, although it may also occur as central lesions. For unknown reasons, it often arises in relation to peripheral lung scars. The current theory is that the scar probably occurred secondary to the tumor, rather than causing the tumor. The adenocarcinoma has an increased incidence in smokers, and is the most common type of lung cancer seen in non-smokers and women. The peripheral location of adenocarcinoma in the lungs may be due to the use of filters in cigarettes which prevent the larger particles from entering the lung. Deeper inhalation of cigarette smoke results in peripheral lesions that are often the case in adenocarcinomas of the lung. Generally, adenocarcinomas grow more slowly and form smaller masses than the other subtypes. However, they tend to form metastases widely at an early stage. Adenocarcinoma is a non-small cell lung carcinoma, and as such, it is not as responsive to radiation therapy as is small cell lung carcinoma, but is rather treated surgically, for example by pneumonectomy or lobectomy.

FA can produce repeated hemoptysis, possibly related to cavitation of the tumor.

Other presenting symptoms described have included: flu-like syndrome with cough and fever,

Synonyms for FA include well differentiated fetal adenocarcinoma, high-grade fetal adenocarcinoma, pulmonary adenocarcinoma

of fetal type, and pulmonary endodermal tumour resembling fetal lung.

Atypical adenomatous hyperplasia is a subtype of pneumocytic hyperplasia in the lung. It can be a precursor lesion of in situ adenocarcinoma of the lung (bronchioloalveolar carcinoma).

In prostate tissue biopsy, it can be confused for adenocarcinoma of the prostate. The needle biopsy rate is less than 1%.

This particular variant of lung cancer is usually asymptomatic and is found after chest x-rays are taken for other reasons. Hemoptysis is seen occasionally and, in some cases, distal obstruction of bronchi by blood clots or mucus plugs produces cough and/or infection. Lesions often enlarge and progress slowly, over many years.

The 1999 World Health Organization classification system defined MCACL as a cystic adenocarcinoma with copious mucin production that, histologically, resembles (the more common) mucus-producing cystadenocarcinomas originating in the ovary, breast and pancreas. The 2004 revision of the WHO classification noted that the tumors tend to be well circumscribed by a partial fibrous tissue capsule with central cystic change and copious mucin pooling. The thin, fibrous wall circumscribing the tumor is highly characteristic of this lesion. It can sometimes occur within a pulmonary bronchocele, and this tumor entity should be kept in mind after identification of a bronchocele with suspicious or non-prototypical imaging characteristics.

Microscopically, the neoplastic epithelial cells tend to grow along the alveolar walls, in a fashion similar to the mucinous variant of bronchioloalveolar carcinoma, a more common form of adenocarcinoma.

Hemoptysis is seen occasionally.

Positron Emission Tomography (PET) scanning can be of assistance in diagnosing MCACL, as these lesions show intense uptake, typically in the wall of the tumor.

CA 19-9 has been reported to be elevated in MCACL.

Differential diagnosis of MCACL includes secondary metastatic cystadenocarcinomatous lesions, particularly from the pancreas or ovary, mucoepidermoid carcinoma, and pulmonary mucinous bronchioloalveolar carcinoma. The mouse monoclonal antibody 1D3, developed to detect a high molecular weight mucin found in a number of cystic malignancies of various organs, may be of use in differentiating primary mucinous cystadenocarcinoma of the lung from metastatic lung tumors due to mucinous cystic lesions of the uterus and pancreas, as well as those primary in the colon and stomach.

Multifocal micronodular pneumocyte hyperplasia (MMPH) is a subtype of pneumocytic hyperplasia (hyperplasia of pneumocytes lining pulmonary alveoli).

Several synonymous terms have been done for this entity: adenomatoid proliferation of alveolar epithelium, papillary alveolar hamartoma, multifocal alveolar hyperplasia, multinodular pneumocyte hyperplasia.

These multifocal lesions are observed in tuberous sclerosis, and can be associated with lymphangioleiomyomatosis and perivascular epithelioid cell tumour (PEComa or clear cell "sugar tumor")).

It can be diagnosed through lung biopsy using thoracoscopy.

About 20% of DIPNECH patients are symptom free at the time they first present. The most common symptoms include:

- Chronic cough

- Shortness of breath or dyspnea when exercising or exerting one’s self

- Wheezing (less frequent)

- Hemoptysis (Infrequent)

Symptoms may be present for many years prior to diagnosis and are often ascribed to other lung conditions. Erroneous initial diagnoses of asthma or chronic obstructive pulmonary disease often are made in patients with DIPNECH.

Several other terms for this lesion have been used in the past medical literature, including mucinous multilocular cyst carcinoma, pseudomyxomatous pulmonary adenocarcinoma, mucinous cystic tumor of low malignant potential, and others.

Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) is a diffuse parenchymal lung disease which often presents with symptoms of cough and shortness of breath. The pathological definition published by the World Health Organization is “a generalized proliferation of scattered single cells, small nodules (neuroendocrine bodies), or linear proliferations of pulmonary neuroendocrine (PNE) cells that may be confined to the bronchial and bronchiolar epithelium.” The true prevalence of this disease is not known. To date, just under 200 cases have been reported in the literature. However, with an increase in recognition of this disease by radiologists and pulmonologists, the number of cases has been increasing. DIPNECH predominantly affects middle-aged women with slowly progressive lung obstruction. DIPNECH is usually discovered in one of two ways: 1) as an unexpected finding following a lung surgery; or 2) by evaluation of a patient in a pulmonary clinic with longstanding, unexplained symptoms.

Clear-cell adenocarcinoma is a type of adenocarcinoma that shows clear cells.

Types include:

- Clear-cell adenocarcinoma of the vagina

- Clear-cell ovarian carcinoma

- Uterine clear-cell carcinoma

- Clear-cell adenocarcinoma of the lung (which is a type of Clear-cell carcinoma of the lung)

See also:

- Clear-cell squamous cell carcinoma of the lung

Patients typically have no symptoms until the third or fourth decade of life. In most cases, the disease is discovered incidentally on routine chest Xray. The most common symptoms include the following:

- dyspnea

- dry cough

- chest pain

- sporadic hemoptysis

- asthenia

- pneumothoraces

Pulmonary alveolar microlithiasis (PAM) is a rare, inherited disorder of lung phosphate balance that is associated with small stone formation in the airspaces of the lung. Mutations in the gene "SLC34A2" result in loss of a key sodium, phosphate co-transporter (called Npt2b), known to be expressed in distal airway epithelial alveolar type II cells, as well as in the mammary gland, and to a lesser extent in intestine, kidney, skin, prostate and testes. As the disease progresses, the lung fields become progressively more dense (white) on the chest xray, and low oxygen level, lung inflammation and fibrosis, elevated pressures in the lung blood vessels, and respiratory failure ensue, usually in middle age. The clinical course of PAM can be highly variable, with some patients remaining asymptomatic for decades, and others progressing more rapidly. There is no effective treatment, and the mechanisms of stone formation, inflammation and scarring are not known.

Urachal cancer can exist for some years without any symptoms. The most frequent initial symptom is haematuria which occurs when the urachal tumour has penetrated the bladder wall, but mucinuria (mucin in the urine), local pain or swelling, recurrent local or urinary tract infections and umbilical discharge can (but is not always) be seen.

Mucinous tubular and spindle cell carcinoma (MTSCC) is a rare subtype of renal cell carcinoma (RCC), that is included in the 2004 WHO classification of RCC. MTSCC is a rare neoplasm and is considered as a low-grade entity. It may be a variant of papillary RCC. This tumor occurs throughout life (age range 17–82 years) and is more frequent in females.

The signs and symptoms of PAP include shortness of breath, a cough, low grade fever, and weight loss.

The clinical course of PAP is unpredictable. Spontaneous remission is recognized, and some patients have stable symptoms. Death may occur due to the progression of PAP or of any underlying associated disease. Individuals with PAP are more vulnerable to lung infections such as bacterial pneumonia, mycobacterium avium-intracellulare infection, or a fungal infection.

Urachal cancer is a very rare type of cancer arising from the urachus or its remnants. The disease might arise from metaplasic glandular epithelium or embryonic epithelial remnants originating from the cloaca region.

It occurs in roughly about one person per 1 million people per year varying on the geographical region. Men are affected slightly more often than women mostly in the 5th decade of life but the disease can occur in also in other age groups.

It can involve the urinary bladder, but is not bladder cancer in the usual sense. Urachal cancer can occur at any site along the urachal tract.

Urachal cancer was mentioned by Hue and Jacquin in 1863 followed by an elaborate work by T. Cullen in 1916 about diseases of the umbilicus, while C. Begg further characterized urachal cancer in the 1930s. Detailed diagnostic and staging schemes were proposed by Sheldon et al in 1984, which remain widely used today.

MTSCC has a gross appearance close to papillary RCC. Microscopically, it has three histologic components: mucin, tumor cells forming tubules, and spindle cells. It is characterized by the proliferation of cuboidal and spindle cells arranged in tubular or sheet-like arrays, typically with a mucinous or myxoid background.

Hepatoid tumor or hepatoid [adeno]carcinoma are terms for a number of uncommon or rare neoplasms in humans, named for a visual resemblance of the cells under the microscope to those of hepatocellular carcinoma, the most common form of liver cancer. They can arise in several parts of the body, and thus form sub-types of diseases such as stomach cancer and pancreatic cancer. The WHO defines "Hepatoid carcinoma" as "An adenocarcinoma with morphologic characteristics similar to hepatocellular carcinoma , arising from an anatomic site other than the liver".

In dogs it may refer to a Perianal gland tumor, based on a similar resemblance to healthy liver cells.

Clear cell papillary renal cell carcinoma, abbreviated CCPRCC and also known as clear cell tubulopapillary renal cell carcinoma, is a rare subtype of renal cell carcinoma (RCC) that has microscopic morphologic features of papillary renal cell carcinoma and clear cell renal cell carcinoma, yet is pathologically distinct based on molecular changes and immunohistochemistry.

Pulmonary edema, connective tissue diseases, asbestosis, lymphangitic carcinomatosis, lymphoma, lymphangioleiomyomatosis, drug-induced lung diseases

- Lymphadenopathy

Sarcoidosis, silicosis, berylliosis, lymphangitic carcinomatosis, lymphoma, lymphocytic interstitial pneumonia

Alveolar disease is visible on chest radiography as small, ill-defined nodules of homogeneous density centered on the acini or bronchioles. The nodules coalesce early in the course of disease, such that the nodules may only be seen as soft fluffy edges in the periphery.

When the nodules are centered on the hilar regions, the chest x-ray may develop what is called the "butterfly," or "batwing" appearance. The nodules may also have a segmental or lobar distribution. Air alveolograms and air bronchograms can also be seen.

These findings appear soon after the onset of symptoms and change rapidly thereafter.

A segmental or lobar pattern may be apparent after aspiration pneumonia, atelectasis, lung contusion, localized pulmonary edema, obstructive pneumonia, pneumonia, pulmonary embolism with infarction, or tuberculosis.

Alveolar lung disease may be divided into acute or chronic. Causes of acute alveolar lung disease include pulmonary edema (cardiogenic or neurogenic), pneumonia (bacterial or viral), pulmonary embolism, systemic lupus erythematosus, bleeding in the lungs (e.g., Goodpasture syndrome), idiopathic pulmonary hemosiderosis, and granulomatosis with polyangiitis.

Chronic alveolar lung disease can be caused by pulmonary alveolar proteinosis, alveolar cell carcinoma, mineral oil pneumonia, sarcoidosis (alveolar form), lymphoma, tuberculosis, metastases, or desquamative interstitial pneumonia.