Tumor volume changes on serial imaging with megavoltage CT for non-small cell lung cancer during conformal radiotherapy

Abstract

17041 Background: Adaptive radiotherapy allows treatment plan modification based on data obtained during treatment. Assessing volume changes during treatment is now possible with intra-treatment imaging capabilities on radiotherapy devices. This study assesses non-small cell lung cancer (NSCLC) volume changes during treatment with conformal intensity-modulated radiotherapy by evaluating serial megavoltage CT (MVCT) scans. Methods: MVCTs were retrospectively reviewed for 25 patients treated with the TomoTherapy Hi-Art system at the University of Wisconsin. Twenty-one patients received definitive radiotherapy, 4 with extracranial stereotactic radioablation (60 Gy in 5 fractions) and 17 on a dose-per-fraction escalation protocol (57–80.5 Gy in 25 fractions). Four patients were treated palliatively (22–30 Gy in 8–10 fractions). Gross tumor volumes (GTVs) were contoured on serial MVCTs at weekly intervals, by individuals other than the treating physician, to minimize bias. Each patient had 3–25 scans including one at the beginning, mid-way, and one at the end of treatment. Initial GTVs ranged from 1.4 - 565.5 cm3 (mean = 70 cm3). Results: At completion of treatment, no patient demonstrated a complete response (CR). Partial response (PR) defined as a >65% decrease in tumor volume occurred in 3 (12%) and marginal response (MR) defined as a 35 - 65% reduction in tumor volume was noted in 5 (20%). The remaining 17 patients (68%) showed stable disease (SD) defined as <35% reduction or <40% increase in size. The minimum “scorable threshold” for volume discrepancy between scans to account for inter-scan assessment variability was set at >25% volume change; 10 patients (40%) had >25% tumor regression. None of the patients treated ablatively or palliatively showed tumor regression during treatment. Conclusions: Although gross tumor regression during treatment may be objectively measured using MVCTs, substantial volumetric decrease occurs only in a minority. The clinical significance of this regression is questionable, as there is no way to document histologic tumor clearance and therefore field reductions during radiotherapy cannot be recommended. [Table: see text]