Myxoid pattern in smooth muscle tumor of uncertain malignant potential (STUMP) is a significant prognostic parameter with difficult diagnostic dilemmas

Background: Uterine smooth muscle tumors of uncertain malignant potential (STUMPs) represent a heterogeneous group of tumors with varying histological and biological characteristics. Tumors featuring a dominant myxoid stroma are uncommon in the uterus. This case emphasizes the importance of recognizing the myxoid pattern in smooth muscle tumors, especially within the STUMP category. Case: In this study, we present a 39-year-old woman with a large transmural intrauterine nodule. The findings indicated smooth muscle tumor tissue with a fascicular pattern, without significant ischemic necrosis or cytological pleomorphism. The mitotic count was 1/10 High Power Field (HPF). The tumor exhibited more than 50% myxoid histomorphology, indicative of a rare and specific type of smooth muscle differentiation. Immunohistochemical analysis showed positive reactions for smooth muscle differentiation, retained progesterone receptor expression, and no aberrant tumor protein p53 (p53) or cyclin-dependent kinase inhibitor 2A (p16) expressions. The marker of proliferation Ki-67 was low. Considering all the characteristics, this myxoid smooth muscle tumor was diagnosed as a STUMP myxoid neoplasm. The patient underwent a hysterectomy with bilateral salpingectomy and bilateral ovarian conservation. The patient is currently under follow-up and has not experienced any complications. Conclusions: This case illustrates an extremely rare instance of STUMP, which continues to pose challenges in both diagnosis and treatment. Myxoid patterns are exceptionally uncommon and often hard to detect, making them surprising in smooth muscle tumors. Recognizing this distinct differentiation is crucial, as accurate diagnostic criteria must be applied during the histopathological evaluation of these tumors. The presence of a myxoid pattern suggests a worse prognosis, highlighting the need for more vigilant follow-up in these patients.