Value of electron microscopy and immunohistochemistry in the diagnosis of soft tissue tumors.

Light microscopy alone is often insufficient to type a soft tissue tumor. In this study 142 consecutive soft tissue tumors (benign or malignant), for which both electron microscopy (EM) and immunohistochemistry (IHC) had been performed, were reviewed to assess the contribution of these two modalities to making the final diagnosis. A tentative diagnosis (58 cases) or differential diagnosis (84 cases) was made following light microscopical examination. Ultrastructural and immunohistochemical features were then reviewed. Overall, EM more often contributed to diagnosing the tumor type than IHC (80 vs. 65%), with a statistically significant difference (p = .001). This was most apparent in high-grade sarcomas (grade III); the contribution of the two modalities was similar in benign and low-grade malignant tumors. IHC more often than EM (33% vs 22%) did not provide information to aid in typing of the tumor. However in 47% of the cases in which one of the two modalities was noncontributory, the other technique was helpful in reaching a diagnosis. These findings suggest that both IHC and EM are necessary to properly evaluate soft tissue tumors.