Ectopic Pelvic Kidney Mimicking Sacral Metastasis on Post-Therapy Iodine-131 Scan of a Thyroid Cancer Patient

A 25-year-old woman had total thyroidectomy and iodine-131 ablation therapy for papillary thyroid carcinoma. Whole body imaging on the 7^th day of therapeutic activity demonstrated radioiodine uptake in the remnant tissue and intense heterogeneous uptake at the sacral region prominently in the posterior image. Initial interpretation was suspicious for sacral metastasis. Technetium-99m-methylene diphosphonate bone scan demonstrated normal bone uptake and the absence of left kidney. On blood-pool phase of bone scan, the absence of left renal activity and an extra area of uptake in the sacral region suggestive of pelvic kidney were noticed. Magnetic resonance imaging scan confirmed the ectopic pelvic kidney overlying the sacrum.