Quantification of gross tumour volume changes between simulation and first day of radiotherapy for patients with locally advanced malignancies of the lung and head/neck

Introduction: To quantify changes in gross tumour volume (GTV) between simulation and initiation of radiotherapy in patients with locally advanced malignancies of the lung and head/neck. Methods: Initial cone beam computed tomography (CT) scans from 12 patients with lung cancer and 12 with head/neck cancer (head and neck squamous cell carcinoma (HNSCC)) treated with intensity-modulated radiotherapy with image guidance were rigidly registered to the simulation CT scans. The GTV was demarcated on both scans. The relationship between percent GTV change and variables including time interval between simulation and start, tumour (T) stage, and absolute weight change was assessed. Results: For lung cancer patients, the GTV increased a median of 35.06% (range, -16.63% to 229.97%) over a median interval of 13 days (range, 7-43), while for HNSCC patients, the median GTV increase was 16.04% (range, -8.03% to 47.41%) over 13 days (range, 7-40). These observed changes are statistically significant. The magnitude of this change was inversely associated with the size of the tumour on the simulation scan for lung cancer patients (P < 0.05). However, the observed changes in GTV did not correlate with the duration of the interval for either disease site. Similarly, T stage, absolute weight change and histologic type (the latter for lung cancer cases) did not correlate with degree of GTV change (P > 0.1). Conclusion: While the observed changes in GTV were moderate from the time of simulation to start of radiotherapy, these findings underscore the importance of image guidance for target localisation and verification, particularly for smaller tumours. Minimising the delay between simulation and treatment initiation may also be beneficial.