They are not associated with asbestos exposure. A few are located in the fissures. Some arise in fissures and a few grow into, or are entirely within, lung parenchyma. Large tumors may adhere secondarily to contiguous surfaces, making their site of origin uncertain.
Tumors are usually firm, rounded or lobulated, and sharply circumscribed from lung parenchyma.
The patterns seen in these tumors often resemble those of other tumors. In solid, cellular areas there is little or no collagen. In sclerosing areas, collagen varies from scant to marked. As collagen increases and then predominates, cells become compressed. Other areas may have a myxoid stroma .