Clinical Presentation:
A 34-year-old woman, presented with several months history of right-sided chest pain, that occurred at the same time as her menses every month. She had no cough, sputum production, fever, or weight loss. The physical examination was notable for right-sided pleural effusion. Laboratory studies were notable for a hemoglobin level of 10.7 g per deciliter, and normal inflammatory markers. Findings on X-ray and computed tomography of the chest included large right-sided free-floating effusion with normal lung parenchyma (Figure 1A). The pleural fluid was dark-brown (Chocolate effusion) (Figure 1B), with red blood cells of 23,375 cells per microliter, and fluid to serum hematocrit ratio of 0,63. She underwent medical thoracoscopy which showed a brown-pigmented implant (Figure 2). The pleural biopsies revealed groups of endometrial stroma like elements that were positive for CD10, estrogen, and progesterone (Figure 3 ). The periodic chest pain, with the dark-brown appearance of the pleural fluid resulting from invading of the pleural surfaces with endometrial cells that become active during a menstrual period. The patient was referred to a thoracic surgeon and gynecologist for definitive management.

Acknowledgments

We acknowledge the Qatar National Library for funding the open access publication of this article. We also acknowledge the Medical Research Center at Hamad Medical Corporation for their support.

CONFLICT OF INTEREST

The authors report no conflicts of interest in this work.

AUTHORS’ CONTRIBUTIONS

Dr Mousa Hussein: served as corresponding author and involved in manuscript review and submission. Dr Mutaz : served as co‐author and involved in manuscript writing, review, and finalization. Dr. Shanima and Dr Abbas: involved in manuscript review.