Primary fallopian tube carcinoma presenting as an isolated sigmoid colon metastasis: a case report

Background: Primary fallopian tube carcinoma (PFTC) is a rare gynecologic malignancy, and isolated colon metastasis as the initial manifestation of PFTC is exceedingly rare. To our knowledge, the present report describes the first case of PFTC presenting as an isolated sigmoid colon metastasis in the setting of completely normal uterine and adnexal imaging and negative serum tumor markers. Case Description: A 52-year-old postmenopausal woman who presented with 3 months of recurrent abdominal pain and diarrhea. Imaging revealed a solitary tumor in the sigmoid colon, while the uterus and bilateral adnexa appeared normal, and serum tumor markers were negative. After tumor resection, histopathological and immunohistochemical analysis confirmed a high-grade serous carcinoma (HGSC) of gynecologic origin. Subsequent laparoscopic hysterectomy and bilateral salpingo-oophorectomy identified a 1 mm lesion of HGSC at the fimbrial end of the fallopian tube. The patient received six cycles of adjuvant chemotherapy with carboplatin 500 mg combined with paclitaxel 80 mg, once every 3 weeks, for a total of six cycles, and achieved complete remission at the 1-year follow-up. Conclusions: This case underscores the crucial role of immunohistochemistry and thorough pathological examination in determining the primary origin of malignancies with atypical presentations. It emphasizes the need for precise diagnosis to inform treatment strategies and enhance prognostic outcomes.