Capsular collagen staining of follicular thyroid neoplasms by picrosirius red: role in differential diagnosis

A key criterion in the diagnosis of thyroid follicular carcinoma is capsula r invasion, but invasion cannot always be demonstrated histologically. Sinc e invasion is likely to evoke reactions in the capsular collagen, we examin ed the effects of invasion on capsular collagen with the picrosirius orange -red (PSR) staining technique for collagen. Under polarized light, the colo r of PSR-stained collagen varies as a function of the structural and bioche mical properties of the collagen fibers. Capsules of widely invasive carcin omas (n = 10), minimally invasive carcinomas (n = 10), and adenomas (n = 28 ) were stained with the PSR method. Carcinomas were assessed along the thic kened capsule for sites of definite invasion, minimal invasion, and no evid ence of invasion. In adenomas, sites of thickened capsules (similar to carc inomas) were compared to sites of thin capsules. All foci were evaluated fo r the color and color intensity of collagen fibers. We found a significantl y higher frequency of yellow-green collagen fibers than of orange-red fiber s at sites of invasion, whereas orange-red fibers significantly predominate d at non-invaded sites. In a minority of cases both colors occurred but the non-dominant color was of lesser intensity in all but 1 case. There were n o significant differences in staining between minimally and widely invasive carcinomas. Thick capsules of adenomas consistently stained with an intens e orange-red color, although weakly stained yellow-green fibers were also o bserved in some of these cases. We conclude that PSR staining can provide d iagnostically useful information in capsular samples of carcinomas, when bo th color and color intensity of PSR staining are evaluated at the same site . Specifically, intense yellow-green birefringence of collagen in a thicken ed capsule is additional evidence for capsular invasion.