Stromal-only endometriosis catamenial pneumothorax: A diagnostic algorithm with potential pitfalls.

Catamenial pneumothorax (CP) is a rare form of recurrent spontaneous pneumothorax occurring synchronously with menstruation and represents the most common manifestation of thoracic endometriosis. CP predominantly involves the right hemithorax as a result of the transdiaphragmatic migration of endometrial tissue through congenital or acquired diaphragmatic defects, a phenomenon thought to be facilitated by the clockwise circulation of peritoneal fluid and by the absence of an effective anatomical barrier on the right side to limit transdiaphragmatic spread. Diagnosis may be particularly challenging when endometrial implants are composed exclusively of stromal cells without identifiable glands, are focal in distribution, and minute in size. We herein describe two challenging cases of recurrent right-sided pneumothorax in women of reproductive age, in whom paucifocal endometrial deposits were consistent with stromal-only endometriosis in the absence of endometrial glands. Based on these cases, we propose a practical diagnostic flowchart addressing the major pitfalls in the recognition of stromal-only thoracic endometriosis and highlighting the critical importance of close clinicopathologic correlation, particularly in gland-deficient lesions.