Time trends in target volumes for stage I non-small-cell lung cancer after stereotactic radiotherapy

Purpose: To identify potential time trends in target volumes and tumor mobility after stereotactic radiotherapy (SRT) for Stage I non-small-cell lung cancer. Patients and Methods: Repeat planning computed tomography (CT) scans were performed for 40 tumors during fractionated SRT delivered in either three (n = 21), five (n = 14), or eight fractions (n = 5). The planning CT scans used to define internal target volumes (ITVs) consisted of either six multislice CT scans or a single four-dimensional CT scan. All repeat CT scans were coregistered with the initial (D₀) scan to determine volumetric or spatial changes in target volume, and tumor mobility vectors were determined from each scan. Results: A significant decrease in target volumes (ITVs and gross tumor volumes) relative to baseline values was observed starting at the fourth week of SRT (p = 0.015). No trends in tumor mobility were detected during SRT. Significant positional shifts in the ITV, of more than 5 mm, were seen in 26-43% of patients at different times during SRT. Conclusion: Significant changes in target volumes can occur during SRT for Stage I non-small-cell lung cancer. A failure to account for such changes e.g., by repeat CT planning or verification using on-board volumetric imaging can lead to inadequate target coverage.