Cholesterol granuloma forming the solid components within an endometrioma showing intense FDG uptake, mimicking malignancy: A case report

A mural solid component (SC) within an endometrioma (ovarian endometriotic cyst), demonstrating internal vascularity or increased fluorodeoxyglucose (FDG) uptake on positron emission tomography (PET), is considered a characteristic finding suggestive of malignant transformation. We report a rare case of an endometrioma containing a solid component with intense FDG uptake that was pathologically diagnosed as a cholesterol granuloma. A 68-year-old woman was incidentally found to have bilateral ovarian cysts on transvaginal ultrasonography. MRI revealed the left ovary contained a cyst measuring 46 × 36 × 30 mm, showing high signal intensity (SI) on T1-weighted images (T1WIs), which was not suppressed on fat-suppressed T1WIs, and low SI on T2-weighted images (T2WIs) suggesting an endometrioma. Inside this cyst, SC was noted, showing low SI on both T1WIs and T2WIs and partial high signal on diffusion-weighted images. FDG-PET/CT demonstrated intense FDG uptake in the SC (standardized uptake value max = 13.0), with no evidence of distant metastasis. Based on these findings, a malignant tumor arising from left ovarian endometrioma was suspected. However, postoperative histopathological examination revealed that the SC within the background of the endometrioma consisted of granulomatous tissue with hemosiderin deposition surrounding cholesterol crystals, leading to a diagnosis of an endometrioma with a cholesterol granuloma. Characteristic SIs of SC on MRI may be caused by hemosiderin deposition inside it and the resulting susceptibility artifacts. These findings may aid in distinguishing cholesterol granuloma from malignant tumors, although further case accumulation is needed.