Toward Improved Classification of Vulvar Smooth Muscle Tumors: A Clinicopathologic Study of 35 Cases.

Vulvar smooth muscle tumors have been evaluated using site-specific (1979 and 1996 Vulvar Criteria) or Uterine Criteria. We compared the performance of 3 classification systems for predicting clinical behavior in the largest series of 35 such tumors to date. Follow-up was available for 30 patients (median: 63 mo, 1 to 346). Most presented as a painless mass in premenopausal patients and showed spindled or myxoid morphology. Diagnoses were concordant across all 3 classification systems in most cases (80%, 28/35), including 22 leiomyomas and 6 leiomyosarcomas. Of the 7 discordant tumors, 3 with ≥5 cm, moderate to marked cytologic atypia, 5 mitoses per 10 high-power fields (HPF), and no tumor cell necrosis or infiltrative borders were diagnosed as smooth muscle tumors of uncertain malignant potential (STUMP) by the Uterine Criteria but as leiomyosarcoma/low-grade leiomyosarcoma by the site-specific criteria. Two tumors (≥5 cm, 8 to 9 mitoses per 10 HPF, no cytologic atypia or tumor cell necrosis, and unevaluable borders) were classified as mitotically active leiomyoma by the Uterine Criteria and the 1996 Vulvar Criteria but as low-grade leiomyosarcoma by the 1979 Vulvar Criteria. Two tumors with tumor cell necrosis, moderate atypia, 9 and 66 mitoses per 10 HPF, but <5 cm and well-circumscribed borders were categorized as leiomyosarcoma by the Uterine Criteria but as leiomyoma by both site-specific criteria. Our findings support the validity of the Uterine Criteria relative to the site-specific criteria for vulvar smooth muscle tumors, although the applicability of these criteria to the STUMP category warrants further study.