Microcystic lymphatic malformation localized within the diaphragm

Localization of a microcystic lymphatic malformation within the diaphragm is an extremely rare vascular anomaly of the lymphatic system. We report the case of a 3-year-old boy who had a localized intrathoracic mass that was detected after birth. As the feeding artery to the mass was detected on computed tomographic images before surgery, we suspected the mass to be an extralobar pulmonary sequestration. Thoracoscopic findings indicated that the mass was localized within the left diaphragm and had an indiscrete margin with the diaphragm. An excisional biopsy was performed to avoid resection of the diaphragm itself. The mass was diagnosed as a microcystic lymphatic malformation based on pathological findings. When an accurate diagnosis of the mass within the diaphragm cannot be made preoperatively, a biopsy under thoracoscopic surgery may be an effective method to diagnose microcystic lymphatic malformations.