Incidence, causes, and management of noncancerous lung nodules and masses

By Lynne Eldridge, MD 

 Medically reviewed by Douglas A. Nelson, MD

Benign lung tumors (pulmonary tumors) are relatively common, and are likely to be found much more frequently with the widespread use of CT lung cancer screening. You may find yourself feeling reassured if you are told a tumor is benign, but what could it be? The most common benign lung tumors include hamartomas and adenomas, but there are many other types of tumors as well. Benign tumors are usually asymptomatic, but when they cause symptoms they may include a cough, respiratory infections due to airway obstruction, or coughing up blood. The diagnosis usually includes imaging studies such as a CT scan, but further testing or a lung biopsy may be needed to make the diagnosis and rule out other conditions. Most benign tumors do not require treatment, but surgery to remove a tumor may be needed in some cases.

Illustration by Joshua Seong. © Verywell, 2017. 

Importance

While most benign tumors are harmless, a major concern with benign tumors is differentiating these from malignant (cancerous) tumors. The survival rate for lung cancer is highest when caught and treated in the early stages.

Terminology

The terminology surrounding lung tumors can be confusing, and it's helpful to first define a few terms:

• Lung lesion: a lung "lesion" simply refers to any abnormality in the lungs. This could be a benign or malignant tumor, scar tissue, granulomas related to rheumatoid arthritis, infectious processes, a lung abscess, and more. The term pulmonary coin lesion is often used to describe round nodules.

• Lung nodule: A nodule is described as an abnormal appearing area of tissue that is 3 centimeters (roughly 1 1/2 inches) in diameter or less.

• Lung mass: The term mass is used to describe an abnormal area of tissue that is 3 centimeters in diameter or larger.

Characteristics and Behavior

There are many different types of benign lung tumors. These tumors can behave similarly to malignant tumors in some ways, but there are important distinctions.

• Size: While malignant tumors are more likely to be large (many lung masses, defined as tumors larger than 3 cm, are cancer), some benign tumors may grow to large sizes as well.

• Growth rate: Malignant tumors tend to grow rapidly, with an average doubling time of roughly 4 months. Benign tumors often grow slowly and sometimes even shrink. That said, some benign tumors may grow very rapidly.

• Recurrence: Both benign and malignant tumors may recur when they are removed, though benign tumors always recur at the site where they originated.

• Invasiveness: Benign tumors may push against nearby structures, but do not invade other tissues.

• Threat to health: While cancers can be life-threatening, most benign lung tumors are harmless. That said, some benign lung tumors can be dangerous due to their location, such as if they are present near the large blood vessels in the chest (like the aorta).

• Age of Onset: Most malignant lung tumors occur in older adults (though lung cancer appears to be increasing in young women who have never smoked). In contrast, benign lung tumors may occur at any age.

• Location: While it was once thought that benign tumors were more likely to occur in the periphery of the lungs and lung cancers centrally, both types of tumors may occur at any location within the lungs.

• Spread: Malignant tumors can spread (metastasize) to other regions of the body. Benign tumors, however, do not spread beyond the lungs.

The exact incidence of benign lung tumors is uncertain and may vary by what is classified as a lung tumor. For example, there are conditions other than benign and malignant lung tumors that may appear as a nodule on imaging studies.

Types and Classification

The World Health Organization classifies lung tumors into several categories (that include both benign and malignant tumors). Several types of benign tumors within these categories (of which the most common include hamartomas and adenomas) include:

Mesenchymal Tumors

• Hamartomas: Hamartomas are the most common type of benign lung tumor, and tend to contain a variety of cell types such as fat, cartilage, and more. They are usually found accidentally but may cause airway obstruction leading to pneumonia and bronchiectasis. Some of these tumors are linked to a genetic syndrome called Cowden syndrome. Hamartomas may occur in many other regions of the body as well. They can be difficult to distinguish from lung cancer, and are not uncommonly found when surgery is done for a possible lung cancer.

• Chondroma: A benign tumor of cartilage cells

• Congenital peribronchial myofibroblastic tumor: A benign tumor that may develop in a baby during pregnancy or shortly after birth, and is composed of what's thought to be primeval lung cells

• Inflammatory myofibroblastic tumor: These are tumors of connective tissue cells that are most often benign, though in some cases can be malignant. They are most commonly found in children and young adults. Though benign, they have a tendency to recur after treatment.

• Granular cell tumors: These tumors are very uncommon, and originate in supportive cells of the nervous system known as Schwann cells.

• Fibroma: Fibromas are benign tumors of connective tissue and may be found anywhere in the body. In the lungs they may be found within the large airways (endobronchial), within the lungs, or on the pleura. They are usually discovered accidentally and don't usually require treatment.

• Lipoma: Lipomas, while common in the skin, are rarely found in the lungs. When they occur they may be present near they airways, within the tissue of the lung, or on the membranes that line the lungs (pleura).

Adenomas

• Alveolar adenomas: These are very rare benign tumors of unknown cell origin.

• Mucous gland adenoma: These are very rare tumors of mucus-producing cells in the lungs. They tend to grow centrally near the airways, and therefore frequently cause symptoms related to obstruction of the airways such as pneumonia or a persistent cough.

• Sclerosing pneumocytoma: These tumors are made up of what is thought to be primitive lung cells and are very uncommon. They are much more likely to be found in women, especially Asian women. Of note is that they can closely mimic lung cancer on imaging studies, including showing increased uptake on a PET scan.¹ This can be made more confusing as lung cancer in Asian women is commonly found in never smokers.

• Mucinous cystadenoma: Most commonly found in the ovary, where they make up roughly 20% of tumors and can grow very large, mucinous cystadenomas of the lung are thought to be unrelated, and composed of mucus-producing epithelial cells. They are usually seen as a cystic tumor that is filled with mucus. While benign, it's recently been noted that these tumors can undergo malignant transformation (become cancerous) into mucinous cystadenocarcinoma.²

Salivary Gland Type Tumors

Myoepithelial tumors: These tumors are actually considered to be malignant, and can sometimes spread, but often behave like benign tumors. They are very rare and therefore little is known about their prognosis or best treatments.

Papillomas

• Squamous cell papilloma: Squamous cell papillomas may occur in both adults and children, and are often associated with the human papillomavirus (HPV, most often types 6 and 11). In rare cases, these tumors may undergo malignant transformation and become cancerous.

• Glandular papilloma: The cause of glandular papillomas is unknown, though they are more common in adults.

• Mixed squamous cell and glandular papilloma: Mixed papillomas are rare and the exact cause is unknown.

Other Tumors

• Xanthoma: Xanthomas are fatty tumors that many people are familiar with as they often occur under the skin. They can, however, occur in the lungs as well.

• Amyloid: The condition of amyloidosis refers to a build-up of abnormal proteins and can occur in a number of regions of the body. In the lung, they can cause symptoms and even result in death if they are extensive and interfere with air exchange in the alveoli. Amyloidosis can also be difficult to distinguish from lung cancer at times. Amyloidosis may occur with multiple myeloma or in conjunction with some autoimmune diseases.

• Hemangiomas: Hemangiomas are benign tumors of blood vessels, and can sometimes be found in the lungs. Mutational studies are also finding that a particular mutation (AKT mutation) found in some lung cancers is present some hemangiomas, and may suggest a common pathway in the formation of these tumors.

Symptoms

Most often, benign lung tumors are asymptomatic (have no symptoms) and are found accidentally when a chest X-ray or chest CT is done for another reason. There are, however, exceptions.

Benign tumors in or near the airways (endobronchial tumors) may result in obstruction of the airways. This can lead to a persistent cough, recurrent respiratory infections such as pneumonia, coughing up blood (hemoptysis), collapse of part of a lung (atelectasis), wheezing, or shortness of breath.

Benign tumors do not usually result in symptoms common with lung cancer such as unintentional weight loss or hoarseness.

Causes

The causes of most types of benign lung tumors are unknown. Some risk factors include:

• Genetics: Genetics may play a role in some hamartomas, and these tumors often occur as part of Cowden's disease, a hereditary syndrome. People with this syndrome are also at risk for cancers such as breast cancer, thyroid cancer, and uterine cancer, often in their 30s and 40s.

• Infections: Squamous papillomas of the lung have been linked to HPV infections.

• Smoking: Smoking has been considered a risk factor for squamous cell papillomas, but it's not certain if tobacco truly plays a role in their development.

Diagnosis

Diagnosing a lung tumor begins with taking a careful history, including that of risk factors, and a physical exam.

Imaging Studies

A chest X-ray is frequently the first test ordered and may pick up a suspicious finding. It's important to note that a chest X-ray alone cannot prove conclusively that a tumor is benign or malignant. In fact, up to 25% of lung cancers are missed on chest X-rays. Lung tumors can be seen on a chest X-ray when they reach about 1 cm in diameter

A chest CT is usually done to further clarify something seen on a chest X-ray, or a benign tumor may be discovered only when a CT is performed. Other imaging tests may also be done at times, including MRI, bone scan, or PET scan.

Procedures

If a tumor is near the large airways, it may be seen on bronchoscopy. A biopsy may also be done via the airways during this procedure (endobronchial biopsy).

When the diagnosis is uncertain, a lung biopsy may be needed. This can be done through the chest wall (fine needle aspiration biopsy), during bronchoscopy, or instead as a surgical procedure (open biopsy).

Characteristics of Benign Lung Tumors on Imaging Studies

Compared with malignant (cancerous) tumors, benign lung tumors are more likely if:

• They are small: Tumors less than 3 centimeters (roughly 1.5 inches)

• They have smooth, regular shapes and borders

• The doubling time is either fast or slow (for example, a doubling time of fewer than 10 days or more than 450 days): The average doubling time with cancerous lung tumors is roughly four months

• They have calcifications that are diffuse, speckled, or popcorn-like (eccentric calcifications are more common with cancer)

• No increase in size of lymph nodes (especially mediastinal, supraclavicular) is seen

• There is no evidence of spread (metastases) to other regions of the body: Lung cancer most commonly spreads to the brain, liver, bones, and adrenal glands.

The location of the tumor (whether in the outer regions (periphery) of the lungs or centrally near the large airways) is not very helpful in distinguishing between benign and malignant tumors.

Treatment

The treatment of a benign tumor will depend primarily on whether the tumor is causing symptoms and the particular type of tumor that is present. When a benign tumor is small, the entire tumor may be removed during a biopsy procedure.

When a benign tumor must be removed surgically, there are now minimally invasive procedures that allow for a much faster recovery. The procedure known as video-assisted thoracoscopic surgery involves making a few incision in the chest wall in order to gain access to the lungs. Special instruments are then used to remove an area of the lungs. This method can be used to remove even an entire lobe of the lungs, but is not possible with tumors in all regions of the lungs.

2 Sources

Lim JH, Lee N, Choi DW, et al. Pulmonary sclerosing pneumocytoma mimicking lung cancer: Case report and review of the literature. Thoracic Cancer. 2016. 7(4): 508–511. doi:10.1111/1759-77.12341

Moneke I, Zeisel C, Elze M, et al. Mucinous cystadenocarcinoma arising from mucinous cystadenoma of the lung: case report and review of the literature. Journal of Thoracic Disease. 2018. 10(4): E243–E249. doi:10.21037/jtd.2018.04.19

Additional Reading

Hashimoto H, Tsugeno Y, Sugita K, Kentaro I. Mesenchymal tumors of the lung: diagnostic pathology, molecular pathogenesis, and identified biomarkers. Journal of Thoracic Disease. 2019. 11(Suppl 1):S9–S24. doi:10.21037/jtd.2018.12.04

Kikano GE, Fabien A, Schilz R. Evaluation of the Solitary Pulmonary Nodule. American Family Physician. 2015;92(12):1084-91.

Massion PP, Walker RC. Indeterminate pulmonary nodules: risk for having or for developing lung cancer?. Cancer Prevention Research (Phila). 2014;7(12):1173-8. doi:1158/1940-6207.CAPR-14-0364

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."