A Curious "Mickey Mouse" Sign: Physiological FAPI Uptake in Uterus Didelphys on 68 Ga-FAPI PET/CT.

A 42-year-old woman with invasive lobular carcinoma of the left breast underwent staging with 68 Ga-FAPI PET/CT, which demonstrated intense uptake in the primary breast lesion and ipsilateral nodal metastases. Incidentally, symmetric intense pelvic tracer uptake was identified, corresponding on fused PET/CT to 2 distinct uterine horns consistent with uterus didelphys. The characteristic bilateral rounded appearance on PET images produced a striking "Mickey Mouse" sign, reflecting physiological FAPI accumulation within duplicated uterine stromal tissue. Subsequent ultrasonography confirmed uterus didelphys with normal endometrial and myometrial morphology. Recognition of physiological FAPI uptake patterns in congenital Müllerian anomalies is essential to avoid misinterpretation as pelvic metastatic disease.