Fat-containing adnexal mass with a preserved ovary: Primary fallopian tube mature teratoma mimicking an ovarian lesion

Primary mature cystic teratoma of the fallopian tube is an exceptionally rare condition with fewer than 100 cases reported in the English-language literature and is often misdiagnosed as an ovarian mature cystic teratoma due to its close anatomic proximity to the ovary. We report a case of a woman in her 30s who presented with abnormal genital bleeding, in whom imaging revealed a fat-containing left adnexal mass. Contrast-enhanced computed tomography and magnetic resonance imaging showed a multilocular cystic lesion with a distinct round fat-containing component located inferior to a morphologically preserved left ovary. Although the mass was adjacent to the ovary, the ipsilateral ovary was clearly visualized as separate and appeared structurally normal. Despite these findings, we initially interpreted the lesion as an ovarian mature cystic teratoma because of its characteristic imaging features, reflecting the high prevalence of ovarian mature cystic teratoma among fat-containing adnexal lesions. Laparoscopic exploration confirmed that the tumor originated from the fallopian tube, and a salpingectomy was performed. Histopathological analysis demonstrated a mature cystic teratoma coexisting with a serous adenofibroma within the same fallopian tube. This case underscores a key diagnostic pitfall: the presence of intralesional fat in an adnexal mass does not necessarily indicate ovarian origin. Careful evaluation of the spatial relationship between the lesion and the ovary is essential for accurate preoperative diagnosis and improved diagnostic precision.