LOW-GRADE MYOFIBROBLASTIC SARCOMA - ANALYSIS OF 18 CASES IN THE SPECTRUM OF MYOFIBROBLASTIC TUMORS

The clinicopathologic, immunohistochemical, and ultrastructural featur es of a seemingly distinctive low-grade spindle cell sarcoma showing m yofibroblastic differentiation have been analyzed in a series of 18 pa tients. The age range of the patients (7 women and 11 men) was 19-72 y ears (median: 42 years). A painless, enlarging mass was the most commo n clinical presentation. Five tumors arose in the oral cavity (includi ng four lesions in the tongue), four in the lower extremities and thre e in the upper extremities, four cases in the abdominal/pelvic cavity, and two on the trunk. Eight soft-tissue cases involved skeletal muscl e, three cases were located in perifascial tissues, and two arose in s ubcutaneous tissue. Tumor size ranged from 1.4 to 17 cm (median: 4 cm) ; in six cases (of which four were abdominal/pelvic) the lesion was la rger than 5 cm. All patients were treated surgically, and four receive d additional adjunctive therapy. Histologically, most cases were cellu lar lesions showing a diffusely infiltrative pattern, and were compose d of spindle-shaped tumor cells arranged mainly in fascicles. Tumor ce lls had poorly defined, palely eosinophilic cytoplasm and fusiform nuc lei, which were either tapering and wavy or plumper and vesicular with indentations and small inconspicuous nucleoli. Tumor cells were set i n a collagenous matrix often with prominent hyalinization. Mild nuclea r atypia was noted in 16 cases; in the other 2 cases, and in the metas tases of one other lesion, a greater degree of nuclear atypia was seen . In all but one case, the mitotic rate ranged from 1 to 6 mitoses in 10 HPFs (mean: 2/10 HPFs); in a single case, there were more than 20 m itoses in 10 HPFs. Immunohistochemically, all cases stained positively for at least one myogenic marker; 12 cases were positive for desmin. 11 for or-smooth muscle actin, and 6 for muscle actin (HHF35). Seven n eoplasms were desmin positive/alpha-smooth-muscle actin negative, and five cases were desmin negative/alpha-smooth-muscle actin positive emp hasizing the variable immunophenotype of myofibroblastic lesions. In a ddition, 7 of 10 tumors stained at least focally positive for fibronec tin. Ultrastructural examination in five cases showed characteristic f eatures of myofibroblasts. Follow-up in 11 patients (median: 29 months ) revealed local recurrence in 2 cases, and multiple distant soft-tiss ue, intraosseous, and pulmonary metastases in one other patient. Low-g rade myofibroblastic sarcoma seems to represent a distinct entity in t he spectrum of low-grade myofibroblastic neoplasms and is distinguisha ble from fibromatosis, myofibromatosis, solitary fibrous tumor, fibros arcoma, and leiomyosarcoma.