An accurate diagnosis between leiomyoma and leiomyosarcoma is essential for patient management. IMP3 is a member of the insulin-like growth factor (IGF-II) mRNA binding protein (IMP) family that consist of IMP1, IMP2, and IMP3. IMP3 is an oncofetal protein associated with aggressive and advanced tumors and is specifically expressed in malignant tumors but not found in benign tissues. The aim of this study was to determine the expression and diagnostic value of IMP3 in leiomyoma and leiomyosarcoma. A total of 216 cases (resection, n = 183; biopsy, n = 33) consisting of 82 leiomyosarcomas (uterine, n = 15; soft tissue, n = 67), 62 leiomyomas (uterine, n = 50; soft tissue, n = 12), and 72 uterine-variant leiomyomas (atypical, n = 19 [14%]; cellular, n = 21 [16%]; mitotically active, n = 12 [9%]; myxoid, n = 11 [8%]; vascular, n = 3 [2%]; epithelioid, n = 1 [1%]; benign metastasizing, n = 1 [1%]; and smooth muscle tumors of uncertain malignant potential, n = 4) were examined by immunohistochemistry for IMP3 expression. IMP3 showed strong cytoplasmic staining in 43 (52%) of 82 leiomyosarcomas, regardless of histologic grades. There was no difference in IMP3 expression between uterine and soft tissue leiomyosarcomas. In contrast to malignant tumors, IMP3 expression was not found in any of the typical leiomyomas (0/62 cases). All uterine-variant leiomyomas were negative, except for 3 cases (atypical variant, n = 2; cellular variant, n = 1) for IMP3 staining. In summary, we are the first to describe IMP3 expression in smooth muscle tumors. Our findings indicate that the expression of IMP3 in both uterine and soft tissue leiomyosarcomas can be used as a positive biomarker to increase the level of confidence in establishing a definitive diagnosis of a malignant smooth muscle tumor.
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