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.