Gastrointestinal stromal tumor (GIST) has only recently been distinguished histologically and immunochemically from morphologically similar neoplasms of the abdomen.
The authors reviewed 15 cytologic cases of GIST (14 fine-needle aspiration [FNA] specimens and 1 peritoneal fluid specimen) and compared them with 23 cases of leiomyosarcoma (LMS) arising in the abdomen or pelvis (all FNAs). Immunochemistry (IC) was performed on both the cytologic and subsequent tissue specimens if sufficient specimen was available.
Cytologic samples of GISTs typically showed irregularly outlined clusters of uniform spindle cells that were spread easily without crush artifact. The cells had wispy cytoplasm with long, delicate, filamentous extensions (13 cases; 87%). A prominent vascular pattern was common (9 cases; 60%); pleomorphism (1 case; 7%) was uncommon. The LMSs showed three-dimensional, tightly cohesive, sharply marginated syncytia of spindle cells, often with nuclear crush artifact. The cytoplasm/stroma had a distinct wiry, refractile appearance (21 cases; 91%); delicate filamentous cytoplasmic extensions (5 cases; 22%) and prominent vessels (3 cases; 13%) were less common. LMSs more commonly exhibited pleomorphism (14 cases; 61%). Epithelioid cytomorphology, mitoses, and necrosis occasionally were observed in both tumor types. IC for c-kit (on cytologic material) was positive in 10 of 10 cases of GIST (usually diffuse and strong) and 2 of 19 cases of LMS (focal). CD34 positivity favored GIST (4 of 9 cases) over LMS (1 of 19 cases). Smooth muscle actin was positive in 20 of 20 LMSs (strong and diffuse) and 6 of 10 GISTs (usually focal). Desmin was positive in 12 of 20 LMSs and was only focally positive in 1 of 11 GISTs. Correlation of IC results was excellent between cytologic and tissue specimens.
Delicate cytoplasmic processes; a prominent vascular pattern; a lack of nuclear pleomorphism; and a c-kit-positive, desmin-negative immunoprofile are characteristic features of GIST and help distinguish these tumors from LMS in cytologic specimens.