Is Post-Induction GTV Lymph Node Delineation Sufficient in Nasopharyngeal Cancer?

Abstract

<h3>Purpose/Objective(s)</h3> In patients with nasopharyngeal carcinoma (NPC), induction chemotherapy (IC) is an appealing option in case of bulky lymphadenopathy. In our institution, we prefer IC followed by chemoradiotherapy in these patients. When delineating the lymphatic target volumes, we define the gross tumor volume (GTV) lymph node according to the post-IC magnetic resonance (MR) images. In this study, we investigated the sufficiency of GTV delineation according to post-IC MR images, especially in lymph nodes with a high risk of clinical extranodal extension (ENE) at the time of diagnosis. <h3>Materials/Methods</h3> From June 2011 to December 2020, medical records of 413 patients with NPC treated in our department were retrospectively analyzed. In 58 patients, 81 lymph nodes with ENE and/or ≥3 cm diameter (according to data on increased incidence of ENE) in lymphatic stations of the neck, including retropharyngeal stations, irradiated with concurrent cisplatin with a definitive intent after two or three cycles of IC were included in this study. GTV was contoured according to the post-IC volume of the lymph node. Clinical target volume (CTV) was created with a 5-mm margin to GTV, and planning target volume (PTV) was created with a 3-mm margin to CTV. The prescription schedule was 70 Gy to the lymph node and 60 Gy to the involved nodal station, in 33 fractions. A new GTV covering the pre-IC nodal volume was retrospectively contoured on the planning computed tomography of each patient, then a new CTV and PTV were created with the same margins used in the previous CTV and PTV. The difference between pre-IC and post-IC target volumes were compared with the Wilcoxon test. Local control (LC), overall survival (OS), and disease-free survival (DFS) were calculated using the Kaplan-Meier method. <h3>Results</h3> The median lesion size at diagnosis and after IC were 4.4 cm (interquartile range (IQR): 4-5.2 cm) and 2.5 cm (IQR: 2-3.1 cm, p<0.001). The median pre- and post- IC GTV, CTV, PTV were 19.2 cc and 5.2 cc (p<0.001), 50.1 cc and 21.3 cc (p<0.001), 57 cc and 25.6 cc (p<0.001), respectively. The median volume reduction was 72.5% for GTV, 53.4% for CTV, and 53.1% for PTV. With a median follow-up of 43 months (range: 3-134 months), 2- and 5-year LC, OS, and DFS were 98.7% and 97.2%, 92.5% and 85.1%, and 88.9% and 81.3%, respectively. Two (%2.4) of 81 lymph nodes recurred, and both were within the 95% isodose line. <h3>Conclusion</h3> IC yields a significant reduction in nodal target volumes, and post-IC nodal volume-based intensity-modulated radiation therapy provides excellent LC in patients with NPC even in lymph nodes with high-risk features.