A Case of Ossified Tissue Misdiagnosed as Amyloid Deposits by Congo Red Stain and Birefringence.

Pulmonary amyloidosis is diagnosed by identifying amyloid deposits using Congo red stain (CR) and birefringence under polarized light. However, collagen fibers can also produce similar staining results, complicating diagnosis. We report a case of a 55-year-old male patient with lung opacities, initially suspected to have amyloidosis based on CR positivity and green birefringence. Thoracoscopic biopsy revealed pulmonary ossification (PO) rather than amyloidosis. The parallel alignment of lamellar bone fibers likely caused false-positive CR findings and birefringence. This case underscores the need for careful differentiation between bone tissue and amyloid deposits. Misdiagnosis can lead to unnecessary treatments, especially when the actual condition requires only conservative management, as in PO. Awareness of PO as a mimic of amyloidosis is crucial, especially in cases lacking other systemic symptoms or calcification.