End of Treatment Cone-Beam CT Correlates with Post-Treatment Follow-up Imaging Response in Head and Neck Squamous Cell Carcinoma

Abstract

Image guidance in radiation therapy (RT) is a critical tool for delivering accurate therapy. Over the past decade, the resolution and quality of cone beam computed tomography (CBCT) has increased significantly such that discrete masses and lymph node conglomerates are readily discernable. During treatment of head and neck squamous cell carcinoma (HNSCC), CBCT imaging is commonly obtained daily or weekly. However, to date, this imaging has not been utilized to assess or predict treatment response despite this readily available source of CT data. Here, we assess whether changes in CBCT measurements obtained during the treatment course correlate with responses on 3-month routine follow-up CT imaging. Patients with oropharyngeal primary tumors who received radiation therapy between 2015 and 2018 were included. Measurements were collected of the primary tumor and largest nodal deposit at the following time points: CT simulation, mid-treatment CBCT, end of treatment (EOT CBCT) and routine 3-month post-RT imaging. At each timepoint, anteroposterior (AP), mediolateral (ML) and craniocaudal (CC) measurements were obtained. The AP and ML measurements were then used to create a 2-dimensional (2D) maximum. CBCT measurement data was reported as percent size change from CT simulation. Patients were excluded if they did not complete the prescribed course of RT, underwent surgical resection or did not undergo imaging within 3-5 months of completing RT. CBCT data from 64 patients who met inclusion criterion were analyzed. All patients had stage III-IV disease based on AJCC 7thedition staging. The largest nodal 2D maximum and CC measurements on EOT CBCT showed a statistically significant correlation with complete response on 3-month post-RT imaging (r=0.313, p=0.02 and r=0.318, p=0.02, respectively). Furthermore, patients with a smaller magnitude of change from CT simulation were more likely to have only a partial response or stable disease on post-RT imaging. The change in size of the largest nodal conglomerate measured using EOT CBCT in HNSCC patients with oropharyngeal primary tumors correlates with response on 3-month post-RT follow-up imaging. Further analysis is underway to determine whether responses determined using CBCT imaging data correlate with patient outcomes. CBCT data may have utility as an early predictor of treatment response in oropharyngeal HNSCC.