Dose-rate effect on local control and complications in irradiation of mobile tongue cancer with interstitial iridium-192

This is a retrospective analysis of 50 patients with squamous cell carcinoma of the oral tongue who were treated with interstitial iridium-192 brachytherapy alone between 1984 and 1995. Local control and incidence of late complications were analyzed with respect to dose and dose rate. We analyzed tumors of up to 30 mm in long diameter, which were treated with single plane implants. Local control was 46/50. Local control rates were comparable with age, sex, tumor size, macroscopic size and dose rate. Local control was relatively poor (13/15) in dose <63 Gy. Ulcer incidence was 11/50. Ulcers occurred in 3/28 at a dose of less than 60 cGy/hr compared with 8/22 at a dose of 60 cGy/hr or greater (p=0.0490 by Log-rank test). The significant factor of mucosal ulcer formation was dose rate by multivariate analysis. Ulcers occurred in 7/18 in case of 65 Gy or greater and 60 cGy/hr or greater. To maximize local control and minimize ulcers, we recommend a dose of 63-70 Gy at a dose rate of less than 70 cGy/hr. (author)