Image Guidance is Most Critical in the First Two Weeks for Patients Treated by Radiation Therapy for Head and Neck Cancer: Implications for Adaptive Radiotherapy

Abstract

Daily Megavoltage computed tomography (MVCT) images are acquired at our institution as part of the patient setup for radiation therapy. The purpose of this study was to analyze temporal trends in the gross tumor volume (GTV) to better understand tumor remodeling secondary to radiation therapy for head and neck cancer. We used daily MVCT images from six patients with intact tumors involving the base of the tongue, oropharynx, nasopharynx, and/or hypopharynx to determine the changes in GTV and airway volumes throughout treatment. To assess the tissue changes to radiation therapy, the GTV was compared with the initial simulation images and systematically recontoured on weekly IGRT scans. Additionally, the volume of the oropharynx and hypopharynx was measured using several of the bony aspects of the head and neck as boundaries. These were chosen for consistency in contouring these volumes from week to week. In all cases it was necessary that the daily MV-CT images incorporated enough of the airway to make a measurement. Variables measured weekly included the volume of the GTV and the volume of the luminous airway. Graphical software was used to visually depict and analyze trends in volume changes. In all cases, the most pronounced losses in GTV (shrinkage) occurred in the first two weeks of treatment, after which time the response was markedly variable from patient to patient. Similarly, the airway volume in all of the patients increased during the first two weeks, but then could shrink again, stabilize, or continue to expand. The mean initial volumes of the GTV was 52.3 cc (range, 12.7 to 93.4 cc). The mean change in volume for the GTV over the first two weeks was 14.1 cc (range, 1.8 to 30.0 cc). This corresponded to a mean change of 1.0 cc daily over the first two weeks. On average, each patient's GTV shrank by approximately 23% (range of ΔV, +4.4% to -39%) during the first 2 weeks. During this time, the airway opened by 1.6 to 38.1 cc (relative ΔV of 8% to 580%). In contrast, documented changes in volume of both GTV and the airway slowed dramatically in the last four weeks of treatment at -7% and +7%, respectively. Our results confirm that tumor shrinkage during radiation therapy for head and neck cancer is a dynamic process. The greatest remodeling in GTV and adjacent tissues occur within the first two weeks. These results suggest that IGRT is most beneficial during this time. Additionally, adaptive planning may be most useful after the third week of treatment when changes in internal anatomy level off and stabilize.