Pulmonary hamartomas are the most common benign neoplasm of the lung. They are often small (< 2 cm), discreet lesions, radiographically characterized as 'coin lesions'. Occasionally, they have a distinctive radiographic appearance due to calcifications and may be characterized as 'popcorn lesions' When the distinctive look on radiograph is not present, these lesions are often sampled to differentiate from primary lung carcinoma, metastasis, and granuloma. Pulmonary hamartomas are often peripheral, less often endobronchial, making these lesions amenable to fine needle aspiration. A pulmonary hamartoma is comprised of a proliferation of cell types normally present in mature, benign lung. There are benign epithelial and mesenchymal components present in a variable ratio. The epithelial cells are bland, uniform, cuboidal or columnar cells. The mesenchymal component can consist of mature fat, smooth muscle, cartilage, fib rous tissue, or even bone. The most common mesenchymal components are fibromyxoid stroma and chondroid material. If a single cell type predominates, it can be difficult to differentiate from inadvertent sampling of an adjacent rib or bronchus. Pulmonary hamartoma is an important differential to keep in mind while examining specimens submitted from lung lesions, as they have the potential to cause a false positive malignant diagnoses on FNA specimens. A notable pitfall is the presence of cytologic atypia, which can occur secondary to reactive atypia, but is not common in pulmonary hamartomas. If an immediate adequacy assessment is performed, keep in mind that cartilage-rich lesions may be hypocellular. If touch imprints of a core are made, they may shed minimal material. The corresponding core needle biopsy (or cell block in the case of FNA) should demonstrate the cartilaginous component if it is adequately sampled.
The differential diagnosis of pulmonary hamartoma varies depending on which elements of the tumor predominate. For example, if there is a predominance of fibromyxoid stroma, one could consider a tumor of minor salivary gland origin. If cartilaginous / chondroid elements predominate, the differential diagnosis might include sampling of normal anatomic structure (rib) or a low grade cartilaginous neoplasm. Correlation with past medical or surgical history is of utmost importance if considering metastasis from a cartilaginous neoplasm. If only the epithelial component is identified the differential diagnosis includes well differentiated adenocarcinoma, especially if reactive atypia is present.