Sinonasal Smooth Muscle Cell Tumors
Authors: Huang Hsuan-Ying , Antonescu Cristina R. 
Published: January 2021

Context.—Smooth muscle tumors (SMTs) of the sinonasal tracts are very rare mesenchymal neoplasms, and the literature includes very limited data correlating histologic parameters with clinical outcome. As the behavior of SMTs is site-dependent, defining applicable criteria to distinguish among low-grade leiomyosarcoma, SMT of uncertain malignant potential (SMTUMP), and cellular leiomyoma is sometimes difficult and arbitrary.

Objective.—To correlate the clinicopathologic features of 12 well-differentiated sinonasal SMTs with MIB-1 index and clinical outcomes so as to better classify this group of tumors.

Design.—Twelve cases of sporadic well-differentiated SMTs arising from the sinonasal tract were retrieved from both institutions. High-grade leiomyosarcomas were excluded from this analysis. The histologic parameters assessed included circumscription, mucosal ulceration, cellularity, nuclear atypia, mitotic count, necrosis, and destruction of adjacent bony structures. The histologic classification of these tumors was based on the guidelines for SMTs of deep soft tissues, using greater than 4 mitotic figures (MF)/10 high-power fields (HPF) to separate SMTUMP from leiomyosarcoma and the presence of 1 to 4 MF/10 HPF to distinguish between leiomyoma and SMTUMP. Immunostaining of MIB-1 index was performed in 7 cases with available material.

Results.—This study showed a 1:1.5 male-female ratio and a mean age of 40 years (range, 20–67 years). The most frequent symptoms were nasal stuffiness and/or epistaxis. The tumors involved nasal cavity in 8 cases (67%), paranasal sinus alone in 2 cases (16.5%), and both nasal cavity and paranasal sinuses in 2 cases (16.5%). The tumors ranged in size from 0.3 to 5.5 cm (mean, 2 cm) and were classified as follows: 7 leiomyomas, 2 SMTUMPs, and 3 low-grade leiomyosarcomas. All 7 leiomyomas, 1 SMTUMP, and 1 leiomyosarcoma originated from vessel walls. Bone involvement was seen in both the SMTUMPs (1 of 2) and leiomyosarcomas (2 of 3). Focal infiltrative growth was observed only in the 3 leiomyosarcomas, 1 of which also showed microscopic coagulative tumor necrosis. Mitotic figures ranged from 0 to 10 (mean, 2.3), with absence of mitosis in all 7 leiomyomas, 1 to 4 MF/10 HPF in the 2 SMTUMPs, and more than 4 MF/10 HPF in the 3 low-grade leiomyosarcomas. The MIB-1 index was low (≤5%) in both leiomyoma and SMTUMP groups, while the leiomyosarcomas showed a higher (≥15%) proliferative index. All 12 patients were treated by surgical excision, and only 1 patient with leiomyosarcoma received postoperative radiation. In all 12 tumors, there was neither local recurrence nor distant metastasis after an average of 93 months of follow-up (range, 4–221 months).

Conclusion.—Well-differentiated sinonasal SMTs are rare and occur in adults with a slight female predilection. The lesions confined within the nasal cavity were more common than those involving a single paranasal sinus or both nasal cavity and paranasal sinuses. Despite variations in location, clinical manifestation, histologic features, and MIB-1 index, these tumors appear to follow a favorable course. Complete surgical excision appears to provide adequate treatment for these patients.