Intra- and inter-fractional variations of tumors with fiducial markers measured using respiratory-correlated computed tomography images for respiratory gated lung stereotactic body radiation therapy.

Purpose: This study evaluated the intra- and inter-fractional variation of tumors with fiducial markers (FMs), relative to the tumor-FM distance, to establish how close an FM should be inserted for respiratory-gated stereotactic body radiation therapy (RG-SBRT).

Methods: Forty-five lung tumors treated with RG-SBRT were enrolled. End-expiratory computed tomography (CT) (CT) and four-dimensional-CT (4D-CT) scans were obtained for planning. End-expiratory CT (CT) scanning was performed before each fraction. The FMs were divided into two groups based on the median tumor-FM distance in the CT (D). For the intra-fractional variation, the correlations between the corresponding tumor and FM intra-fractional motions, defined as the centroid coordinates of those in each 0-90% phase, with the 50% phase of 4D-CT as the origin, were calculated in the left-right, anterior-posterior, and superior-inferior directions. Furthermore, the maximum difference in the tumor-FM distance in each phase of 4D-CT scan, based on those in the 50% phase of 4D-CT scan (D), was obtained. Inter-fractional variation was defined as the maximum distance between the tumors in CT and CT, when the CT scans were fused based on each FM or vertebra.

Results: The median D was 26.1 mm. While FM intra-fractional motions were significantly and strongly correlated with the tumor intra-fractional motions in only anterior-posterior and superior-inferior directions for the D > 26 mm group, they were significantly and strongly correlated in all directions for the D ≤ 26 mm group. In all directions, D values of the D ≤ 26 mm group were lower than those of the D > 26 mm group. The inter-fractional variations based on the D ≤ 26 mm were smaller than those on the D > 26 mm and on the vertebra in all directions.

Conclusions: Regarding intra- and inter-fractional variation, FMs for D ≤ 26 mm can increase the accuracy for RG-SBRT.