Dosimetric Factors and Toxicity in Highly Conformal Thoracic Reirradiation

Purpose We determined cumulative dose to critical structures, rates of toxicity, and outcomes following thoracic reirradiation. Methods and Materials We retrospectively reviewed our institutional database for patients treated between 2008 and 2014, who received thoracic reirradiation with overlap of 25% prescribed isodose lines. Patients received courses of hyperfractionated (n=5), hypofractionated (n=5), conventionally fractionated (n=21), or stereotactic ablative radiation therapy (n=51). Doses to critical structures were converted to biologically effective dose, expressed as 2 Gy per fraction equivalent dose (EQD2; α/β = 2 for spinal cord; α/β = 3 for other critical structures). Results We identified 82 courses (44 for retreatment) in 38 patients reirradiated at a median 16 months (range: 1-71 months) following initial RT. Median follow-up was 17 months (range: 3-57 months). Twelve- and 24-month overall survival rates were 79.6% and 57.3%, respectively. Eighteen patients received reirradiation for locoregionally recurrent non-small cell lung cancer with 12-month rates of local failure and regional recurrence and distant metastases rates of 13.5%, 8.1%, and 15.6%, respectively. Critical structures receiving ≥75 Gy EQD2 included spinal cord (1 cm3 ; n=1), esophagus (1 cm3 ; n=10), trachea (1 cm3 ; n=11), heart (1 cm3 ; n=9), aorta (1 cm3 ; n=16), superior vena cava (1 cm3 ; n=12), brachial plexus (0.2 cm3 ; n=2), vagus nerve (0.2 cm3 ; n=7), sympathetic trunk (0.2 cm3 ; n=4), chest wall (30 cm3 ; n=12), and proximal bronchial tree (1 cm3 ; n=17). Cumulative dose-volume (D cm3 ) toxicity following reirradiation data included esophagitis grade ≥2 (n=3, D1 cm3 range: 41.0-100.6 Gy), chest wall grade ≥2 (n=4; D30 cm3 range: 35.0-117.2 Gy), lung grade 2 (n=7; V20combined-lung range: 4.7%-21.7%), vocal cord paralysis (n=2; vagus nerve D0.2 cm3 range: 207.5-302.2 Gy), brachial plexopathy (n=1; D0.2 cm3 = 242.5 Gy), and Horner's syndrome (n=1; sympathetic trunk D0.2 cm3 = 130.8 Gy). No grade ≥4 toxicity was observed. Conclusions Overlapping courses of reirradiation can be safely delivered with acceptable toxicity. Some toxicities occurred acutely at doses considered safe for a single course of therapy (esophagus). We observed rib fracture, brachial plexopathy, and Horner's syndrome for patients receiving high cumulative doses to corresponding critical structures.