Multimodal ultrasound diagnosis of a huge retroperitoneal leiomyoma originating from the levator ani muscle: a case report.

Background: Primary retroperitoneal leiomyomas are exceptionally rare, accounting for approximately 1.2% of all retroperitoneal tumors. Those originating specifically from the levator ani muscle are even rarer. The contrast-enhanced ultrasound (CEUS) features of such tumors are poorly documented in the literature, often leading to diagnostic challenges and misdiagnosis. This case report aims to describe the comprehensive multimodal ultrasound findings of a large retroperitoneal leiomyoma arising from the levator ani and highlight the diagnostic value of integrating advanced ultrasound techniques. Case Description: A 31-year-old asymptomatic woman with a history of uterine fibroids was found to have a large pelvic mass during a routine health check-up. A comprehensive multimodal ultrasound examination was performed, utilizing transrectal double-plane probes, perineal pelvic floor probes, and high-frequency linear probes. Two-dimensional ultrasound revealed a well-defined, solid mass measuring 111 mm × 62 mm × 63 mm, originating from the left levator ani muscle with predominantly low echogenicity. Color Doppler flow imaging (CDFI) showed sparse internal vascularity. High-frame-rate transrectal CEUS demonstrated that the mass began enhancing at 18 seconds, peaked at 31 seconds with lower intensity compared to the adjacent levator ani muscle, and showed early washout, characterized as an "early enhancement, early washout, low enhancement" pattern. Strain elastography and shear wave elastography both indicated that the lesion tissue was softer than the ipsilateral levator ani muscle, with an elastic ratio of 1.16 and shear wave values of 15.73 kPa (mass) Conclusions: To our knowledge, this is the first reported case utilizing a combination of high-frame-rate CEUS and elastography for the preoperative evaluation of a retroperitoneal leiomyoma originating from the levator ani. This multimodal ultrasound approach, encompassing detailed anatomical mapping with specialized probes, dynamic contrast enhancement kinetics, and tissue stiffness assessment, proved crucial in accurately determining the tumor's origin and characterizing its nature. It can significantly improve diagnostic accuracy for complex pelvic masses, potentially reducing misdiagnosis and guiding optimal clinical management, including the decision to forgo biopsy in select cases with compelling benign imaging features planned for definitive surgery.