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 cm(3); n=1), esophagus (1 cm(3); n=10), trachea (1 cm(3); n=11), heart (1 cm(3); n=9), aorta (1 cm(3); n=16), superior vena cava (1 cm(3); n=12), brachial plexus (0.2 cm(3); n=2), vagus nerve (0.2 cm(3); n=7), sympathetic trunk (0.2 cm(3); n=4), chest wall (30 cm(3); n=12), and proximal bronchial tree (1 cm(3); n=17). Cumulative dose-volume (D cm(3)) toxicity following reirradiation data included esophagitis grade ≥2 (n=3, D1 cm(3) range: 41.0-100.6 Gy), chest wall grade ≥2 (n=4; D30 cm(3) 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 cm(3) range: 207.5-302.2 Gy), brachial plexopathy (n=1; D0.2 cm(3) = 242.5 Gy), and Horner's syndrome (n=1; sympathetic trunk D0.2 cm(3) = 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.