Prostatic amyloidosis: pathological features of an underdiagnosed condition.

Amyloidosis is a rare disease that can affect genitourinary organs but the involvement of the prostate has been documented in a limited number of cases. We have reviewed morphologic and immunohistochemical features of prostate biopsies or surgical specimens in which an initial diagnosis of amyloidosis was made. Prostatic amyloidosis was diagnosed in 25 patients, 21 of them were needle biopsies (1.16% of these ones). Amyloid was observed inside vessel walls (25 cases) and the stroma (3). No significant differences in the number of affected biopsy samples between patients with and without cardiac amyloidosis were found. In prostatectomies, amyloid was visualized in all the regions of the prostate, being more abundant in the periphery and the posterolateral tissue. Three patients with abundant amyloid in the prostatectomy did not have cardiac amyloidosis. Immunohistochemically prostatic amyloid was positive for transthyretin and P amyloid (24 cases). A amyloid, kappa, and lambda chains were negative. The genetic analysis revealed transthyretin wild-type amyloidosis. Immunohistochemistry was not conclusive in one case of immunoglobulin light chain amyloidosis. In conclusion, prostate amyloidosis is an infrequent finding characterized by the deposition of amyloid inside small vessel walls, and less often in the stroma. It occurs mainly in the periphery of the gland. Amyloid deposits are often subtle and overlooked but relevant as this may be the first sample in which systemic amyloidosis is diagnosed. Immunohistochemistry can be used to subtype amyloid, although there are limitations when confronted with immunoglobulin light chain amyloidosis. Most cases have corresponded to wild-type transthyretin amyloidosis.