Answer: Calcifying pseudotumor

Histology:

Discussion: Calcifying pseudotumor of the pleura (CFPT) is a rare but distinct entity that is histologically similar to calcifying fibrous pseudotumors of soft tissue elsewhere in the body. They appear to be more common in women and most cases occur in the 3rd-4th decade if life. Imaging studies usually show a pleural based mass that are well demarcated and contain calcifications and can be as large as 12 cm. Histologically, these are well demarcated but not encapsulated areas of hyalinized dense fibrosis that are typically paucicelluer and contain interspersed bland spindle cells. Focal chronic inflammation can be present. The calcifications can be of psammomatous and dystrophic types. The spindle cells are usually vimentin positive and may also be positive for actin, desmin and CD34. ALK immunostaining is typically negative. The differential diagnosis includes hyalinized pleural plaques, tumefactive pleural fibrosis, calcified or hyalinized pleural granulomas and amyloidosis. These are benign lesions that are localized and show no tendency towards progression.