Leiomyosarcoma. A 56-year-old woman presented with abdominal pain and a 30-lb weight loss; a retroperitoneal mass was found. By electron microscopy, all features diagnostic for a metastatic leiomyosarcoma were present. These include caveolae, basal lamina, attachment plaques (all best seen at higher magnification in B), and bundles of microfilaments with dense bodies enclosed (closed arrowheads in A). Additional features of these spindle-shaped tumor cells of smooth muscle cell origin include large electron-dense masses of contracted actin filaments (open arrowheads in A), blunt, club-shaped surface projections (seen at higher magnification in B) and electron-lucent cytoplasmic pools representing admixed glycogen and lipid (P). Note that strands of rough endoplasmic reticulum are not dilated and are inconspicuous in these leiomyosarcoma cells displaying primarily a contractile phenotype. The light microscopic immunoperoxidase profile was positive for vimentin, muscle-specific actin and desmin, and negative for S-100, neurofilament, keratin, and α-1-antitrypsin for this tumor. (A) Original magnification × 8,000; (B) Original magnification × 24,000. (Courtesy of Ann M. Dvorak, MD, Department of Pathology, Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA 02215.)