Atypical ovarian fibroma and thecoma mimicking malignancy: clue for differentiation from malignant solid tumors

Abstract Purpose To evaluate the characteristic findings of ovarian fibroma and thecoma (FT) with atypical magnetic resonance imaging (MRI) findings that mimic malignancy and to validate the improvement in diagnostic performance in differentiating between FTs and malignant solid tumors. Materials and methods This retrospective study included 114 patients (119 lesions: 34 FT and 85 malignant tumors) with pathologically confirmed solid ovarian tumors who underwent preoperative MRI. Signal intensity on T2-weighted imaging (T2WI), diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) maps, and time–intensity curves from dynamic contrast-enhanced MRI (DCE-MRI) were evaluated. FT lesions showing low signal intensity on both T2WI and DWI were defined as classic, whereas others were considered atypical. The diagnostic performance of combined imaging findings was also assessed. A reader study was also performed by three radiologists. Results Among the FT lesions, 55.9% demonstrated intermediate signal intensity on T2WI, and 32.4% showed diffusion restriction. Atypical FT accounted for 56% of all FT cases. The combination of T2WI and DWI showed high specificity (98.8%) but low sensitivity (44.1%) for diagnosing FT as benign. In contrast, for the diagnosis of benign lesions, the combination of T2WI and DCE-MRI demonstrated superior diagnostic performance, with a sensitivity, specificity, positive predictive value, and negative predictive value of 84.0%, 95.9%, 87.5%, and 94.6%, respectively. Reader performance improved when the combined interpretation of T2WI and DCE-MRI findings was emphasized. Using this approach, the estimated misdiagnosis rate of FT as malignant decreased from approximately 40% to 16%. Conclusion Atypical FT frequently mimics malignant ovarian tumors on MRI. The combination of T2WI and DCE-MRI findings provides the best diagnostic performance and significantly reduces misdiagnosis. However, a subset of FT remains indistinguishable from malignancy, highlighting the need for careful interpretation.