CT-guided iodine-125 brachytherapy for sacral metastasis with neural canal invasion: a case report of clinical outcome and dosimetry.

Sacral metastasis with neural canal invasion may cause severe pain and progressive neurological dysfunction, while local treatment is challenging because of the proximity of the sacral nerve roots, lumbosacral plexus, rectum, and bladder. We report a 65-year-old man with lung adenocarcinoma and a progressive sacral metastasis causing radiating pain, sensory impairment, lower-extremity weakness, and difficulty walking despite systemic treatment. CT-guided permanent iodine-125 seed implantation was performed with a prescription dose of 140 Gy. A total of 84 seeds, each with an activity of 0.5 mCi, were implanted. An exploratory postoperative CT-based analysis of the anatomically estimated neural region yielded a calculated mean dose of 316 Gy and a maximum voxel dose of 907 Gy; however, interpretation of these values was limited by the highly heterogeneous dose distribution, contouring uncertainty, and the use of a point or voxel-based maximum dose. At 2 weeks after implantation, the Visual Analogue Scale score decreased to 3, and at 1 month it decreased to 1. Lower-extremity muscle strength improved to grade 4, and the patient regained independent ambulation. During the available 12-month follow-up, imaging showed a marked metabolic response and no radiographic evidence of local progression. No new or worsening clinically apparent neurological deficit attributable to the procedure was identified. This case suggests that CT-guided iodine-125 seed implantation may be a feasible local treatment option for selected patients with sacral metastasis involving neural structures. However, a single case with 12 months of follow-up cannot establish long-term neural tolerance or support modification of existing neural dose constraints.