To determine the incidence and risk factors associated with tumor seeding after radiofrequency ablation (RFA) of hepatocellular carcinoma (HCC), 781 lesions from 352 patients who had undergone ultrasound (US)-guided RFA at our hospital between April 1999 and December 2005 were examined. Of these patients, 6 presented HCC lesions (6 lesions in total) and tumor seeding (7 seedings in total), which were analyzed. RFA using RITA 500 PA, the Cool-tip RFA System or the RTC 2000 System was performed. RFA sessions were repeated until complete necrosis was confirmed by imaging. Subsequently, follow-up was performed every 3-4 months by means of computed tomography (CT) and US scans. The 6 patients were retrospectively analyzed for patient characteristics, CT and histopathological findings, RFA method and complications, and clinical and imaging progress and outcome. Of the 6 lesions, 2 were in a subcapsular location at S7. Mean tumor diameter was 23.3±9 mm. Tumor biopsies indicated that 1 of the 6 lesions was well-differentiated, 4 were moderately-differentiated, and 1 was undifferentiated. The RITA 500 PA was used in 2 cases, and the Cool-tip RFA System in 4. Seeding was identified 14.6±13 months after RFA. Four of the cases with seeding were located on the abdominal wall, 2 on the thoracic wall and 1 in the Douglas pouch. Four of the patients underwent surgical resection, 1 radiation, and 1 conservative treatment for seeding. Five of the 6 patients died 12.6±9 months after seeding was detected, with the exception having undergone surgical treatment. The seeding risks identified in this study include treatment of subcapsular lesions and patient treatment over multiple sessions. The selection of the proper RFA system to avoid multiple sessions and the use of ablation technique are important for the prevention of seeding. Additionally, long-term follow-up after RFA by extensive imaging of the pelvic cavity and the thoracoabdominal wall is needed.

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