Large Interstitial Pregnancy Treated With Methotrexate: A Case Report and Literature Review.

Interstitial pregnancy (IP) is an ectopic pregnancy in the segment of the fallopian tube within the muscular wall of the uterus. Due to its rarity, the applicability and safety of systemic methotrexate (MTX) administration for IP remain unclear, especially when the mass is relatively large. We encountered a 21-year-old patient who was diagnosed with an asymptomatic IP of 42 mm × 52 mm and beta-human chorionic gonadotropin (β-hCG) of 7,640 mIU/ml. A single dose of systemic MTX at 50 mg/m2 gradually decreased her β-hCG levels, which became undetectable 161 days after treatment; however, the remnant mass persisted. On further follow-up, the mass disappeared 21 months after MTX administration. Our clinical experience suggests that MTX can be considered for large IPs (>4 cm) if the patient is asymptomatic and hemodynamically stable, and that remnant tissue should be recognized as an unignorable complication of MTX treatment for IPs requiring long-term follow-up.