Prognostic factors of local outcome for T1, T2 carcinomas of oral tongue treated by iridium 192 implantation.

ABSTRACT

The results of Iridium 192 implantation for 121 node negative T1 or T2 squamous carcinomas of mobile tongue were reviewed to look for predictors of local control and necrosis. Age, sex, total dose, dose rate, linear activity, and intersource spacing were examined. Minimum follow-up was 2 years but no patient with local recurrence or necrosis was excluded. There were 57 T1N0 tumors, 45 T2aN0 (2.1-3.0 cm), and 19 T2bN0 (3.1-4.0 cm). Local failures occurred in 14% of T1, 11% of T2a, and 26% of T2b. Univariate analysis showed that local control increased with increasing dose (55-60 Gy 73%; 65-75 Gy 92%, p = 0.005), whereas multivariate analysis revealed both sex and total dose to be significant. Radiation necrosis occurred in 17% of T1, 29% of T2a, and 47% of T2b (p = 0.034). Half were limited to soft tissue and the majority healed with conservative management. Univariate analysis showed that necrosis increased with increasing dose (55-60 Gy 16%; 65-75 Gy 33%, p = 0.037), as well as increasing dose rate, linear activity, and intersource spacing. With multivariate analysis only stage, dose rate, and spacing remained predictive of necrosis. Total dose was not adjusted for dose rate or tumor volume. This analysis suggests that within the therapeutic range of low dose rate brachytherapy, correction of total dose according to dose rate is unnecessary. We recommend 65 Gy. Lower dose rate (0.4-0.5 Gy/hr) and closer intersource spacing (12-14 mm) should be aimed for to minimize necrosis.