[1] Hokuto Hosp, Dept Otolaryngol Head & Neck Surg, Inadacho Kisen 7-5, Obihiro, Hokkaido 0800833, Japan
[2] Hokuto Hosp, Dept Biol & Genet, Lab Canc Med Sci, Obihiro, Hokkaido 0800833, Japan
[3] Hokuto Hosp, Dept Neurosurg, Obihiro, Hokkaido 0800833, Japan
[4] Asahikawa Med Univ, Dept Otolaryngol Head & Neck Surg, Asahikawa, Hokkaido, Japan
[5] Hokkaido Univ, Grad Sch Med, Dept Pathol, Sapporo, Hokkaido, Japan