Purpose: To examine the incidence of mandibular bone complication in patients who underwent radiotherapy for T1 and T2 carcinomas of the oral tongue and to analyze the factors contributing to its occurrence.
Methods and materials: The clinical records of 148 patients with T1 and T2 carcinoma of the oral tongue treated with radiotherapy alone between 1978 and 1989 were examined retrospectively. Interstitial brachytherapy, used as the major treatment modality, was performed using cobalt needles, radium needles, or iridium hairpins. The prescribed dose at the plane 5 mm from the plane of the radioactive sources was 65-70 Gy in interstitial brachytherapy alone, and 50-60 Gy in the combined treatment with external irradiation. An external irradiation dose of 30 Gy was usually used.
Results: Eleven of the patients showed radiation-induced mandibular bone complication. Two (1 T1, 1 T2) had been treated with interstitial brachytherapy alone, and nine (2 T1, 7 T2) with the combination of external irradiation and interstitial brachytherapy. The incidence of radiation complication of bone was significantly higher in the patients with T2 tumors (p = 0.04) and in those who received the combined treatment (p < 0.01). Multivariate analysis revealed that the total dose (p = 0.04) and dose rate of interstitial brachytherapy (p = 0.03) were significant factors contributing to radiation bone complication. A significant difference in the incidence of bone complication was also seen between patients who received a total dose of 90 Gy or more and those who received less than 90 Gy (p < 0.01), as well as between patients who were treated with 0.55 Gy/h or higher and those who were treated with less than 0.55 Gy/h (p = 0.03).
Conclusion: A significant increase in the incidence of bone complication was found at the total dose of 90 Gy or more and at the dose rate of 0.55 Gy/h or higher. In combined treatment with external irradiation and interstitial brachytherapy, the interstitial brachytherapy dose of 60 Gy appears to be the threshold at which mandibular bone complication is induced when the external irradiation dose is 30 Gy.