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

A woman in her 30s presented to a referring hospital with abnormal genital bleeding, menorrhagia, and dysmenorrhea. She had no relevant medical or surgical history and no history of infertility treatment. Physical examination was unremarkable. Transabdominal ultrasonography at the referring institution suggested a left adnexal mass, prompting referral to our hospital for further evaluation. Transvaginal ultrasonography showed mild enlargement of the left ovary and a cystic lesion in the left adnexal region. The mass demonstrated good mobility and appeared adjacent to, but not clearly continuous with, the left ovary. No significant ascites was present. Pelvic contrast-enhanced computed tomography (CT) performed before MRI revealed a multilocular cystic lesion with an adjacent discrete fat-containing nodule in the left adnexal region ( Fig. 1 ). Subsequent MRI demonstrated a multilocular cystic mass measuring approximately 66 × 37 mm in the left lower abdomen, located inferior to the left ovary ( Fig. 2 ). The lesion contained a well-defined intralesional fat component measuring approximately 20 mm ( Fig. 2 , arrow), which showed high signal intensity on T1-weighted images and signal suppression on fat-saturated sequences. The cystic components exhibited high signal intensity on T2-weighted images. Importantly, the left ovary was visualized separately from the mass and appeared morphologically intact ( Fig. 2 D, white arrowhead). Fig. 1 CT of the pelvis. (A) Noncontrast-enhanced axial CT. (B) Contrast-enhanced axial CT. Noncontrast-enhanced CT (A) and contrast-enhanced axial CT (B) images reveal a 20-mm fat-containing nodule (arrow) in the left pelvic cavity. Medial to the fat-containing nodule, a lobulated low-attenuation mass (arrowhead) is observed. However, the extent and margins of the lesion are poorly defined on CT, and evaluation of the nonfatty component is limited. Fig 1 – dummy alt text Fig. 2 Pelvic MRI. (A) Axial T2-weighted image. (B) Axial T1-weighted image. (C) Axial fat-suppressed T1-weighted image. (D) Coronal T2-weighted image. The axial T2-weighted image (A) reveals a well-defined round lesion (arrow) and an adjacent multilocular cystic component with high signal intensity (arrowhead). The axial T1-weighted image (B) illustrates marked high signal intensity within the round lesion (arrow), consistent with fat. On the axial fat-suppressed T1-weighted image (C), this signal is suppressed, confirming the presence of intralesional fat. The coronal T2-weighted image (D) clearly shows that the normal left ovary (white arrowhead) is separately identifiable from the fat-containing lesion (arrow) and the multilocular cystic component (black arrowhead). The presence of a preserved ipsilateral ovary separate from the lesion is a key imaging finding suggesting fallopian tube origin rather than ovarian origin. Fig 2 – dummy alt text CT of the pelvis. (A) Noncontrast-enhanced axial CT. (B) Contrast-enhanced axial CT. Noncontrast-enhanced CT (A) and contrast-enhanced axial CT (B) images reveal a 20-mm fat-containing nodule (arrow) in the left pelvic cavity. Medial to the fat-containing nodule, a lobulated low-attenuation mass (arrowhead) is observed. However, the extent and margins of the lesion are poorly defined on CT, and evaluation of the nonfatty component is limited. Pelvic MRI. (A) Axial T2-weighted image. (B) Axial T1-weighted image. (C) Axial fat-suppressed T1-weighted image. (D) Coronal T2-weighted image. The axial T2-weighted image (A) reveals a well-defined round lesion (arrow) and an adjacent multilocular cystic component with high signal intensity (arrowhead). The axial T1-weighted image (B) illustrates marked high signal intensity within the round lesion (arrow), consistent with fat. On the axial fat-suppressed T1-weighted image (C), this signal is suppressed, confirming the presence of intralesional fat. The coronal T2-weighted image (D) clearly shows that the normal left ovary (white arrowhead) is separately identifiable from the fat-containing lesion (arrow) and the multilocular cystic component (black arrowhead). The presence of a preserved ipsilateral ovary separate from the lesion is a key imaging finding suggesting fallopian tube origin rather than ovarian origin. Although the lesion was initially interpreted as a left ovarian mature cystic teratoma due to the presence of macroscopic fat, this interpretation did not adequately consider the spatial relationship between the mass and the left ovary, making definitive preoperative diagnosis challenging. No enlarged lymph nodes or other pelvic abnormalities were identified. Serum tumor markers, including CA125 and CA19-9, were within normal limits. Based on imaging findings, a left ovarian mature cystic teratoma was suspected, and laparoscopic surgery was performed. Intraoperatively, the left ovary was confirmed to be morphologically normal ( Fig. 3 ). A dilated segment of the left fallopian tube containing a mass was identified, clearly separate from the ovary, confirming a tubal origin. A left salpingectomy was performed. Fig. 3 Intraoperative laparoscopic findings. The left ovary (black arrowhead) appears morphologically intact. A dilated segment of the left fallopian tube is identified and is continuous with the mass (arrow), confirming its tubal origin and clear separation from the normal ovary. Multilocular cystic components are indicated by white arrowheads. Fig 3 – dummy alt text Intraoperative laparoscopic findings. The left ovary (black arrowhead) appears morphologically intact. A dilated segment of the left fallopian tube is identified and is continuous with the mass (arrow), confirming its tubal origin and clear separation from the normal ovary. Multilocular cystic components are indicated by white arrowheads. Gross examination of the resected specimen showed a dilated fallopian tube containing a round mass ( Fig. 4 , arrow) and a papillary lesion ( Fig. 4 , arrowhead). Histopathological analysis demonstrated mature tissues derived from multiple germ layers, including squamous epithelium with skin appendages and adipose tissue, consistent with a mature cystic teratoma ( Fig. 5 A). Furthermore, a separate area composed of benign serous epithelial proliferation with fibrous stroma was identified, consistent with serous adenofibroma arising within the same fallopian tube ( Fig. 5 B). No immature components or malignant features were observed. The postoperative course was uneventful, and the patient recovered without complications. Fig. 4 Gross findings. Gross specimen demonstrating a dilated fallopian tube containing a round mass (arrow) and a papillary lesion (arrowhead). Fig 4 – dummy alt text Fig. 5 Histopathological findings. Histopathological analysis reveals mature squamous epithelium with skin appendages and fatty component, consistent with a mature cystic teratoma (hematoxylin and eosin stain) (A). A teratoma (*) is identified beneath the fallopian tube epithelium. Fig. 5 (B) Shows features consistent with serous adenofibroma (hematoxylin and eosin staining). These findings confirm the coexistence of 2 benign tumors within the same fallopian tube, corresponding to the imaging components. Fig 5 – dummy alt text Gross findings. Gross specimen demonstrating a dilated fallopian tube containing a round mass (arrow) and a papillary lesion (arrowhead). Histopathological findings. Histopathological analysis reveals mature squamous epithelium with skin appendages and fatty component, consistent with a mature cystic teratoma (hematoxylin and eosin stain) (A). A teratoma (*) is identified beneath the fallopian tube epithelium. Fig. 5 (B) Shows features consistent with serous adenofibroma (hematoxylin and eosin staining). These findings confirm the coexistence of 2 benign tumors within the same fallopian tube, corresponding to the imaging components.

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Primary fallopian tube mature teratoma is a rare but important differential diagnosis for fat-containing adnexal masses. The presence of macroscopic fat alone should not be used to assume ovarian origin. Careful identification of a morphologically preserved ovary separate from the lesion may provide a key clue for accurate diagnosis. Recognition of this imaging feature can help prevent misdiagnosis and support appropriate surgical management.

Primary mature cystic teratoma of the fallopian tube is an extremely rare condition, with fewer than 100 cases reported in the English-language literature to date [ [4] , [5] , [6] , [7] , [8] , [9] , [10] , [11] , [12] ]. In contrast, mature cystic teratoma is one of the most common benign ovarian tumors, accounting for approximately 16%-20% of all ovarian neoplasms [ 1 , 2 ]. Because of this striking difference in prevalence, along with the close anatomic relationship between the ovary and fallopian tube, fat-containing adnexal masses are almost always assumed to be ovarian in origin on preoperative imaging. The main educational value of the present case is that a preserved ipsilateral ovary may serve as an important imaging clue suggesting tubal rather than ovarian origin, even when the lesion contains macroscopic fat. In the present case, the lesion was initially interpreted as an ovarian mature cystic teratoma despite multimodal imaging evaluation. In our case, several factors likely contributed to this misinterpretation. First, the presence of macroscopic fat on CT and MRI is a classic feature of mature cystic teratoma and can introduce a strong interpretative bias toward ovarian origin. Second, the fallopian tube is frequently not clearly delineated on cross-sectional imaging, limiting accurate identification of the site of origin. Third, the close spatial proximity between the ovary and fallopian tube can obscure their anatomical separation. Furthermore, insufficient emphasis on confirming ovarian continuity may result in an assumption of ovarian origin when typical imaging features are present. Although MRI is highly sensitive for detecting fat-containing lesions, it is not reliable for determining their exact site of origin. The imaging appearance of tubal teratomas is essentially indistinguishable from that of ovarian mature cystic teratoma (dermoid cyst) [ [4] , [5] , [6] , [7] , 10 , 11 ]. Therefore, accurate differentiation depends primarily on careful anatomical assessment rather than signal characteristics alone. Detailed evaluation of the relationship between the mass and the ovary is essential. In practice, identification of the ovary on MRI may be facilitated by recognition of ovarian follicles, characteristic ovarian stromal morphology, and confirmation of anatomical separation between the lesion and the ovary. Notably, prior reports have indicated that visualization of a morphologically preserved ovary clearly separate from the lesion is an important clue suggesting a tubal origin [ 4 , 11 , 12 ]. In this case, both CT and MRI demonstrated a normal-appearing ipsilateral ovary distinct from the mass. This finding was subtle but diagnostically significant and could have suggested a nonovarian origin preoperatively. From a radiologic perspective, careful assessment should not focus on the presence of fat but should also determine whether the lesion replaces, distorts, or is clearly separate from the ovary. Other fat-containing pelvic lesions, including lipoleiomyoma, pelvic lipoma, and extraovarian teratoma, may also mimic ovarian mature cystic teratoma. These entities should be considered when the ovary can be identified separately from the lesion. Another unique feature of this case was the coexistence of mature cystic teratoma and a serous adenofibroma within the same fallopian tube. Although the clinical significance of this coexistence remains unclear, it may contribute to altered internal morphology and further complicate radiologic interpretation. Clinically, tubal teratomas are often asymptomatic but may present with nonspecific symptoms such as pelvic pain or menstrual irregularities [ [8] , [9] , [10] , [11] , [12] , 18 , 19 ]. Serum tumor markers are typically within normal limits, as observed in this case. Although CA125 and CA19-9 were within normal limits, AFP, β-hCG, LDH, and SCC antigen were not measured because imaging findings strongly suggested a benign mature cystic teratoma. This represents a limitation of the present report. Complete surgical excision is curative, and the prognosis is excellent. Compared with ovarian mature cystic teratomas, fallopian tube mature cystic teratomas are exceedingly rare and remain less well characterized. While malignant transformation has been reported in ovarian mature cystic teratomas, its incidence in fallopian tube teratomas remains unknown because of the limited number of reported cases. In conclusion, this case highlights an important diagnostic pitfall in the evaluation of fat-containing adnexal masses. In routine radiologic practice, particularly when assessing such lesions, recognition of a preserved ipsilateral ovary separate from the mass is a key imaging clue suggesting fallopian tube origin. When evaluating a fat-containing adnexal lesion, radiologists should first determine whether the ipsilateral ovary is preserved or replaced before assigning ovarian origin.

Mature cystic teratoma is the most common benign germ cell tumor of the ovary, representing approximately 16%-20% of all ovarian neoplasms and typically occurring in women of reproductive age [ [1] , [2] , [3] ]. In contrast, primary mature teratoma of the fallopian tube is extremely rare, with only a limited number of cases reported in the literature [ [4] , [5] , [6] , [7] , [8] , [9] , [10] , [11] , [12] , [13] ]. Because of the close anatomic relationship between the ovary and fallopian tube, such lesions are frequently assumed to be ovarian in origin. Magnetic resonance imaging (MRI) plays a key role in evaluating adnexal masses. The presence of intralesional fat, shown as high signal intensity on T1-weighted images with signal suppression on fat-saturated sequences, is strongly suggestive of a mature cystic teratoma [ [1] , [2] , [3] ]. However, imaging features that reliably differentiate ovarian from tubal origin remain poorly defined. Serous adenofibroma of the fallopian tube is also a rare benign epithelial tumor [ [13] , [14] , [15] , [16] , [17] ]. Reports of synchronous benign germ cell and epithelial tumors arising within the same fallopian tube are exceptionally uncommon. We present a rare case of primary fallopian tube mature cystic teratoma coexisting with serous adenofibroma, initially misdiagnosed as an ovarian mature cystic teratoma, and emphasize the key imaging feature of a preserved ipsilateral ovary, which may suggest tubal origin. The key educational message of this report is that a preserved ipsilateral ovary separate from a fat-containing adnexal mass may provide an important clue to fallopian tube origin, even when imaging findings otherwise strongly suggest an ovarian mature cystic teratoma. Accurate determination of tumor origin is clinically important for appropriate surgical planning and preservation of ovarian function.