CONTENTS:
3.8 Benign Lung Tumors
3.8.1 Hamartoma
3.8.2 Solitary Fibrous Tumor of the Lung
3.8.3 Clear Cell Tumor (‘Sugar Tumor’)
3.8.4 Benign Epithelial Lung Tumors
i. Papillomas (Squamous, Glandular, Mixed)
ii. Sclerosing Hemangioma (‘Pneumocytoma’)
iii. Alveolar Adenoma
iv. Type II Pneumocyte Papilloma
v. Salivary Gland–Type Tumors
vi. Mucinous Cystadenoma
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3.8 Benign Lung Tumors
Benign tumors or ‘benign neoplasms’ are uncommon, but when they occur they are usually of epithelial and mesenchymal origin. Benign lung tumors are usually small (< 3 cm) and may be found in the bronchi (endobronchial) or the lung parenchyma.
Individuals with benign endobronchial lung tumors can have symptoms such as cough, hemoptysis, and post-obstructive pneumonia, but can be asymptomatic and only detected on CT screening as solitary pulmonary nodules.
In 1970, Arrigoni and colleagues reviewed the 10-year surgical lung pathology experience at the Mayo Clinic and analyzed all 130 cases of benign lung tumors. The majority of the benign lung tumors were asymptomatic, and endobronchial location was found in 6 %. This study showed the following benign tumor diagnoses:
- lung hamartoma (76 %),
- benign fibrous mesothelioma/solitary fibrous tumor (12.3 %), inflammatory pseudotumor (5.4 %),
- leiomyoma (1.5 %),
- lipoma (1.5 %),
- and single cases of hemangioma, adenoma of the mucous glands, and ‘mixed’ benign tumor.
The World Health Organization (WHO) classification of lung tumors, from 1999/2004, will be followed in giving tumor descriptions (Travis et al., 2004).
3.8.1 Hamartoma
A hamartoma is a disorganized collection of epithelial and other tissue elements that form a focal nodule. When occurring in the lung, most hamartomas will contain nodules of cartilage and fat. These are benign lung tumors and have no malignant potential.
Hamartomas cause approximately 10 % of benign lung nodules and occur in middle age, grow very slowly. As well as cartilage (with scattered calcification) they can contain fat, muscle, and fibroblasts.
On screening CT, a well-described appearance of a lung hamartoma is a SPN with ‘popcorn‘ calcification (< 10 %). A high-resolution CT (HRCT) of the lung nodule may show focal areas of fat. The presence of only one type of mesenchymal tissue raises the possibility of lipoma, leiomyoma, or chondroma.
3.8.2 Solitary Fibrous Tumor of the Lung
Solitary fibrous tumor (SFT) is a rare, benign, mesenchymal tumor that can occur at any soft tissue site. In the lung, SFT is usually found attached to the visceral pleura.
This benign tumor has a variety of names that include, ‘pleural fibroma,’ ‘localized fibrous mesothelioma,’ and ‘localized fibrous tumor.’ They were also known as ‘hemangiopericytoma’ but this term has been removed from the latest WHO Classification of Lung Tumors (Travis WD et al., 2004).
Solitary fibrous tumors present as a solitary nodule and can be any size; some have been reported to be as large as 30 cm in diameter. Whatever their size, they are usually asymptomatic and detected by routine imaging. They are a recognized cause of solitary pulmonary nodule (SPN) in CT lung screening.
3.8.3 Clear Cell Tumor (‘Sugar Tumor’)
The histology shows uniform proliferations of clear and granular cells with few mitoses. The ‘clearing’ is due to glycogen. Recent studies have shown that these benign tumors may be derived from perivascular epithelioid cells (PECOMAs).
3.8.4 Benign Epithelial Lung Tumors
Because the commonest forms of lung cancer are squamous cell carcinomaand adenocarcinoma (non small cell carcinomas), both of which are epithelial cell cancers, any lung tumor consisting of epithelial cells warrants careful examination.
i. Papillomas (Squamous, Glandular, Mixed)
When papillomas completely obstruct the airway, post-obstructive pneumonia can result. Solitary papillomas have a central location and male predominance and are usually squamous papillomas, but they can also be glandular papillomas, or of mixed type. Benign lung papillomas occur in the lower respiratory tract and can be solitary or multiple; multifocality can be associated with multiple papillomas of the upper respiratory tract, including the trachea.
The squamous type is associated with human papillomavirus (HPV) infection, while the glandular type is not. There is an association between squamous carcinoma with squamous papillomas. HPV types 16, 18, 31, 33, and 35 may represent high-risk HPV types in these lesions, as they are in other sites.
The histology of exophytic papilloma shows an epithelial cell layer covering central fibrovascular core fronds that protrude into the lumen of the airway. The differential diagnosis includes benign fibroepithelial polyp.
Because of their uncertain malignant potential as well as their potential for recurrence, conservative complete excision of lung papillomas is advised.
ii. Sclerosing Hemangioma (‘Pneumocytoma’)
Sclerosing hemangioma, or ‘pneumocytoma,’ is a benign tumor of epithelial, type II pneumocytes. This benign tumor is mainly found in women and has an average age at presentation in the fifth decade. It is usually an asymptomatic solitary nodule, although multiple lesions have been described in less than 5% of cases.
Sclerosing hemangiomas have an average size of 3 cm and are rarely larger than 5 cm but are occasionally cystic. They are gray to yellow and may contain small areas of hemorrhage. They are benign.
The histology shows two components; cuboidal epithelium (resembling type II pneumocytes) and round to oval stromal cells. Four patterns have been described: solid, papillary, hemorrhagic, and sclerotic. These tumors may be hormonally responsive (to estrogens).
iii. Alveolar Adenoma
Alveolar adenomas are benign and may present as peripheral lung nodules. They occur in adults. Macroscopically, they are soft and lobulated and contain multiple cysts.
They are an incidental finding on imaging where their multicystic appearance may be appreciated. It is still debatable whether their origin is mesenchymal with epithelial entrapment or whether these are true epithelial tumors.
The histology shows that the cysts are lined by a cuboidal and sometimes ‘hobnailed’ epithelial cells., but without papillae. The differential diagnosis of alveolar adenoma includes sclerosing hemangioma (pneumocytoma) and mesenchymal tumors including hamartomas and lymphangiomas.
iv. Type II Pneumocyte Papilloma
This is a rare benign lung tumor composed of papillae lined by type II pneumocytes. The tumor is solitary, peripheral and usually small (1.5 cm). The histology shows uniform cuboidal cells with foamy cytoplasm consistent with type II pneumocytes.
v. Salivary Gland–Type Tumors
Malignant salivary gland–type lung tumors are very rare, but benign salivary gland–type lung tumors are more common.
Mucous gland adenoma and pleomorphic adenoma are recognized in the lung as centrally located benign tumors. The histology of the mucous gland adenoma shows mucin-filled glands lined by mucous secreting cells, but without squamous epithelial cells.
Pleomorphic adenomas are very rare, central, benign lung tumors with a bi-phasic growth pattern. Circumscription, smaller size (<3.0 cm), and low mitotic activity are features associated with a more benign course.
vi. Mucinous Cystadenoma
Mucinous cystadenoma of the lung is a rare benign epithelial tumor. It usually presents as a peripheral, non-symptomatic cystic nodule, Histologically, benign features include, an absence of cell atypia, low cellularity, absence of necrosis and solid areas, absence of invasion, and a low mitotic count.
References:
Arrigoni MG, Woolner LB, Bernatz PE, et al. (1970). Benign tumors of the lung: a 10-year surgical experience. J Thorac Cardiovasc Surg 60, 589–599. (Retrieved 5th Feb 2015): http://www.ncbi.nlm.nih.gov/pubmed/?term=Arrigoni+MG+1970
Travis WD, Kavita G, Franklin WA, Wistuba II. et al. Bronchioloalveolar Carcinoma and Lung Adenocarcinoma: The Clinical Importance and Research Relevance of the 2004 World Health Organization Pathologic Criteria (2006) Journal of Thoracic Oncology: Nov:Volume 1-Issue 9. pp S13-S19. (Retrieved 15th April 2015): http://journals.lww.com/jto/Abstract/2006/11001/Bronchioloalveolar_Carcinoma_and_Lung.4.aspx
Patient Information:
American Cancer Society What is salivary gland cancer? (Retrieved 15th April 2015): http://www.cancer.org/cancer/salivaryglandcancer/detailedguide/salivary-gland-cancer-what-is-salivary-gland-cancer
Web MD Lung Disease & Respiratory Health Center: Benign Lung Tumors and Nodules. (Retrieved 13th April 2015): http://www.webmd.com/lung/benign-lung-tumors-and-nodules
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