Effect of dose rate on local control and complications in the reirradiation of head and neck tumors with interstitial iridium-192

Interstitial Iridium-192 was used as the sole treatment for 23 previously irradiated patients with recurrent or second primary malignancy in the head and neck region. Doses used in initial external beam treatment ranged from 50.0 to 72.0 Gy, in daily fractions of 180 to 200 cGy. Retreatment with interstitial implant using the Quimby system delivered total doses between 38.5 and 60 Gy, with hourly dose rates of 30 to 66 cGy. Local control in the implanted volume was seen in 21 of 23 patients at 5 to 34 months post-implant (median 10 months). Dose rate (hourly dose delivered at 0.5 cm beyond the periphery of the implanted tumor volume) did not affect local control, but did have a significant impact on severe complications (soft tissue necrosis and fistulae; p = 0.26). No effect on either local control or complication rate was found for volume of implant, initial external beam radiation dose, time to recurrence, or anatomic site of treatment. These findings suggest that interstitial irradiation is effective treatment for head and neck cancers in previously irradiated sites. Dose rates as low as 32 cGy/hr provided adequate local control, with few long-term complications to date; doses above 42 cGy/hr may be associated with severe and even fatal complications.