Variation of dose distribution of stereotactic radiotherapy for small-volume lung tumors under different respiratory conditions

PurposeTo clarify the effects of respiratory condition on dose calculation for stereotactic radiotherapy of small lung tumors.Methods and materialsComputed tomography (CT) data were obtained for nine tumors (diameter, 2.1–3.6 cm; mean, 2.7 cm) during the stable state, deep expiration, and deep inspiration breath-hold states. Rotational Irradiation with 3 non-coplanar arcs (Rotational Irradiation) and static irradiation with 18 non-coplanar ports (Static Irradiation) using 6-MV photons were evaluated using Fast Fourier Transform (FFT) convolution and Multigrid (MG) superposition algorithms. Dose-volume histograms (DVHs), mean path-length (PL) and mean effective path-length (EPL) were calculated.ResultsAlthough the PL was larger for the inspiration state than for the stable state and the expiration state, the EPL was 0.4–0.5 cm smaller in the inspiration state than in the expiration state (p = 0.01 for Rotational Irradiation; p = 0.03 for Static Irradiation). The isocenter dose obtained by the FFT convolution algorithm was 7–12% higher than that obtained with the MG superposition algorithm. A leftward shift of the DVH obtained by MG superposition was noted for the inspiration state compared with the expiration state.ConclusionsThe choice of the proper algorithm is important to accounting for changes in respiration state. Differences in isocenter dose were not large among the respiratory states analyzed. EPL was a little shorter for inspiration than for expiration, although there were larger and reverse trends in path length. A leftward shift of the DVH obtained for the inspiration state when MG superposition was used.