Dosimetric Analysis of Radiation-Induced Brachial Plexopathy after Stereotactic Body Radiotherapy: The Contouring of Brachial Plexus Matters

Abstract

<h3>Purpose/Objective(s)</h3> Radiation-induced brachial plexopathy (RIBP) is quality of life impairing, with uncertain dosimetric limits for stereotactic body radiation therapy (SBRT) in lung cancer. This study aimed to examine its relationship with the dosimetric factors of brachial plexus (BP) and determine the effect of BP definition. <h3>Materials/Methods</h3> The primary study population started from an institutional database of 6291 patients. Lesions with PTV ≤2 cm from the BP treated with SBRT from Jan 2016 to Feb 2021 were eligible. All patients had simulation CT scans on 1.25-mm-thickness. BP was delineated according to the atlas of RTOG1106. The primary endpoint was radiation-induced brachial plexopathy (RIBP), with symptoms graded by LENT-SOMA scales. The primary dosimetric variable of our interest was the maximum BP dose (BP-Dmax). The best constraint for BP was determined using logistic regression and the Youden index. <h3>Results</h3> At a median follow-up of 30. 5 months (range 10.9 - 71.0), 126 patients (128 lesions) were eligible for this study. Nine out of 126 patients (7.1%) experienced grade 1-3 (G1-3) RIBP (4 grade 1, 4 grade 2, and 1 grade 3). For patients with RIBP-related symptoms, the median maximum BP dose (BP-Dmax) reached 45.7Gy (range 36.4 - 61.5) with 4-6 fractions (61.5Gy/4f, 42.23-53.11Gy/5f, 41.80-58.13Gy/6f). The best-estimate of fitting variables for logistic regression was BEDmax (α/β=3, LQ model) of BP (BP-BEDmax), and generated a best cut-off BEDmax of 159.44 Gy, with AUC of 0.859 (95% CI: 0.774-0.943). The 2-year Kaplan-Meier risk of RIBP was 33.5% vs 2.0% (p < 0.001) for BP-BEDmax more or less than 159.44 Gy, respectively. Using subclavian vein (SV) for the BP surrogate, the maximum dose was significantly lower than that of the BP defined according to RTOG1106. This underestimation was significantly greater for lesions with PTV ≤ 1 cm from the BP (median dose difference 9.42Gy, range -13.22-43.20). <h3>Conclusion</h3> This study demonstrated that the BP-BEDmax threshold of 150 Gy (less than 36Gy/4F, 40Gy/5F, 43Gy/6F) and improperness of using vascular bundle as a surrogate BP for dosimetric assessment as this severely underestimated the maximum point dose received by anatomic BP