Microscopic description and IHC:

The biopsies show well circumscribed nodules composed of fascicles of bland smooth muscles admixed with entrapped glandular spaces lined by cuboidal cells with cilia. No significant mitotic figures or atypia are seen.

The smooth muscle components are positive for desmin, h-caldesmon, ER, and PR. The entrapped glands are positive for TTF1 and negative for PAX8 and WT-1.

Discussion:

Benign metastasizing leiomyoma is a rare and controversial entity. It is thought as benign spread of uterine leiomyoma. Lung is the most common site and multiple small solitary nodules are typical. Patient usually have no symptoms although rare complications such as pulmonary symptoms or respiratory obstruction due to large size have been reported. Prognosis is excellent and the nodules appear to be hormonally dependent and may regress during pregnancy or after oophorectomy. Proliferative activities tend to be low and many of them may overexpress P53 and have deletion of the long arm of chromosomes 19 and 22.

Alternatively, many have thought of them as metastasis of a well differentiated leiomyosarcoma of low malignant potential (STUMP).

The admixed glands most likely represent respiratory epithelium entrapped within the nodules.

References:

Lee, Hee Jin, Jene Choi, and Kyu-Rae Kim. "Pulmonary benign metastasizing leiomyoma associated with intravenous leiomyomatosis of the uterus: clinical behavior and genomic changes supporting a transportation theory."International Journal of Gynecologic Pathology 27.3 (2008): 340-345.

Sapmaz, Feride, et al. "Benign metastasizing leiomyoma." Lung 186.4 (2008): 271-273.

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