Dosimetric Impact of Simulated Daily Adaptive Radiotherapy with Significantly Reduced Setup Margins in the Definitive Treatment of Head and Neck Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Even when treating head and neck cancer (HNC) with daily image-guided radiation therapy, setup margins of 3-5 mm are required to ensure adequate coverage of disease due to setup error. These margins lead to additional normal tissue dose that may compromise acute and long-term quality-of-life. The use of daily adaptive radiotherapy (ART) using an online adaptive system significantly reduces the need for a PTV margin, since the targets and organs-at-risk (OAR) are recontoured each fraction. In this study, we hypothesized using daily ART (simulated) with 1 mm margins based on our involved nodal radiotherapy (INRT) paradigm would improve coverage and reduce OAR dose. <h3>Materials/Methods</h3> Ten patients with HNC treated on our institutional phase II INRT trial were selected for this study. Cone-beam CT scans were uploaded to the ETHOS emulator for daily adaptive replanning which was simulated using fractions 5, 10, 15, 20, 25, 30, and 35. Two plans were generated with a 1 mm (reduced) and a 5 mm (conventional) PTV expansion. The dose grid from the delivered, conventional margin plan was transposed onto the CBCT and adapted contours. Separately, a reduced target margin plan optimized on the online adaptive system was created. PTV coverage (70 Gy-gross/primary disease, 66.5 Gy-suspicious nodes, 63 Gy-extended volume surrounding the primary disease) and D1 were calculated for target structures along with the dose to OARs. Comparisons performed via paired two tailed t-test. <h3>Results</h3> Six patients with oropharynx cancer and four patients with larynx cancer were included. Median V100% coverage for 70 Gy (96% vs 95.6%), 66.5 Gy (98.5% vs 76.5%), and 63 Gy (98.9% vs 74.9%) were improved with the adaptive planning (all p-values<0.03). Plan homogeneity was improved in the ART plan (e.g., PTV70 median D1% 72.8 Gy vs 74.8 Gy, all p-values<0.02). The total median dose reduction was 10.8 Gy (interquartile range, IQR 8.0-12.6 Gy) for the ipsilateral submandibular gland (SMG), 12.2 Gy (IQR 7.3-23.5 Gy) for the contralateral SMG, 8.3 Gy (IQR 3.8-12.6 Gy) for the ipsilateral parotid, 8.1 Gy (IQR 4.4-11.0 Gy) for the contralateral parotid, 7.0 Gy (IQR 4.5-9.1 Gy) for larynx, 6.6 Gy (IQR 2.7-8.4 Gy) for oral cavity, and 8.6 Gy (IQR 5.9-12.1 Gy) for constrictors. <h3>Conclusion</h3> Simulated daily ART using a 1 mm PTV margin provided significant improvement over conventional PTV margins in coverage and critical OAR dosimetry. This small proof-of-concept study suggests this approach may lead to improved quality-of-life while preserving or improving tumor coverage. Further evaluation is needed to assess the resources needed to implement daily adaptations. A phase II randomized trial of daily ART is underway to determine if dosimetric improvements lead to similar gains in patient-reported outcomes.