Iridium-192 interstitial brachytherapy in carcinoma of the tongue. The importance of various tumor and physical parameters

A detailed analysis of host-tumor factors and interstitial physical factors influencing the disease-free control in carcinoma of the tongue was carried out. Twenty-eight cases of carcinoma of the tongue T1-3, NO-1, MO were treated radically with combined external irradiation and 192Iridium interstitial brachytherapy (one patient received brachytherapy only). The teletherapy dose ranged from 44 Gy to 56 Gy (average 48 Gy), the brachytherapy dose ranged from 16 Gy to 55 Gy (average 22 Gy). The interstitial practice involved loop technique in all the cases. Dose distribution analysis to assess factors influencing local control included indices of dose rate, source activity, inter-planar distance, and discontinuity in the prescribed isodose in other planes when compared to mid-plane. Two-year actuarial disease-free survival (DFS) was 46% with primary treatment and 63% when salvage treatment was also included. DFS was significantly poorer when the interplanar distance at mid-plane exceeded 10 mm (p<0.05). Similarly, tumour control was poorer (p<0.008) when there was discontinuity in the prescribed isodose in 1 or 2 planes (1 cm superior and inferior to mid-plane). Interplanar distance of around 10 mm, prescribed isodose continuity in all three planes and limiting the dose maximum within a factor of 2 will optimize the results of interstitial implants. (orig.)