Tolerance of the Brachial Plexus to High-Dose Re-Irradiation

Abstract Purpose The re-irradiation of recurrent tumors of the head and neck provides a unique opportunity to study the tolerance of the brachial plexus to high doses of radiation exceeding historically accepted limits. Methods and Materials Data from 43 patients who were confirmed to have received overlapping dose to the brachial plexus after review of radiation treatment plans from the initial and re-irradiation courses were used to model the tolerance of this normal tissue structure. A standardized instrument for symptoms of neuropathy believed to be related to brachial plexus injury was utilized to screen for toxicity. Cumulative dose was calculated by fusing the initial dose distributions onto the re-irradiation plan, hereby creating a composite plan via deformable image registration. The median elapsed time from the initial course of radiation therapy to re-irradiation was 24 months (range, 3 to 144 months). Results The dominant complaint among patients with symptoms was ipsilateral pain (54%), numbness/tingling (31%), motor weakness and/or difficulty with manual dexterity (15%). The cumulative maximum dose (Dmax) received by the brachial plexus ranged from 60.5 Gy to 150.1 Gy (median, 95.0 Gy). The cumulative mean (Dmean) dose ranged from 20.2 Gy to 111.5 Gy (median 63.8 Gy). The 1-year freedom from brachial plexus-related neuropathy was 67% and 86% for subjects with a cumulative Dmax greater and less than 95.0 Gy, respectively (p=0.05). The 1-year complication-free rate was 66% and 87%, for those re-irradiated within and after 2 years from the initial course, respectively (p=0.06). Conclusion The development of brachial plexus-related symptoms was less than expected due to repair kinetics and to the relatively short survival of the subject population. Time-dose factors were demonstrated to be predictive of complications.