Answer: Metastatic endometrial stromal sarcoma

Histology: none provided

Discussion: This patient had a TAH-BSO for a low grade endometrial stromal sarcoma 22 years prior to developing a 2.2 cm right upper lobe mass. Further workup also detected a pelvic mass clinically most consistent with a local recurrence. Microscopically, the tumor is composed of bland oval cells that are arranged concentrically around small thick walled arteries with scattered mitotic figures. Immunostains performed on the lung mass showed strong staining for estrogen receptor (ER) and weak staining for CD10 in support of the diagnosis. Endometrial stromal sarcomas are typically slowly progressing tumors with repeated local recurrences and occasional metastases. They can be grouped as low grade and high grade tumors based on their mitotic count (< 10 versus >10 mitosis per 10 HPF). Lung is a common site for metastasis and endometrial stromal sarcomas can mimic primary lung tumors (solitary fibrous tumor, spindle cell carcinoid and other neoplasms). Immunostains can be helpful in confirming the diagnosis since endometrial stromal sarcomas are typically positive for ER and CD10, and negative for TTF-1.