Evaluation of 4D dose to a moving target with Monte Carlo dose calculation in stereotactic body radiotherapy for lung cancer.

We evaluated the four-dimensional (4D) dose to a moving target by a Monte Carlo dose calculation algorithm in stereotactic body radiation therapy (SBRT) planning based on the isocenter dose prescription. 4D computed tomography scans were performed for 12 consecutive patients who had 14 tumors. The gross tumor volume (GTV) and internal target volume (ITV) were contoured manually, and the planning target volume (PTV) was defined as the ITV with a 5-mm margin. The beam apertures were shaped into the PTV plus a 5-mm leaf margin. The prescription dose was 48 Gy in 4 fractions at the isocenter. The GTV dose was calculated by accumulation of respiratory-phase dose distributions that were mapped to a reference images, whereas the ITV and PTV doses were calculated with the respiration-averaged images. The doses to 99 % (D(99)) of the GTV, ITV, and PTV were 90.2, 89.3, and 82.0 %, respectively. The mean difference between the PTV D(99) and GTV D(99) was -9.1 % (range -13.4 to -4.0 %), and that between the ITV and GTV was -1.1 % (range -5.5 to 1.9 %). The mean homogeneity index (HI) for the GTV, ITV, and PTV was 1.14, 1.15, and 1.26, respectively. Significant differences were observed in the D(99) and HI between the PTV and GTV, whereas no significant difference was seen between the ITV and GTV. When SBRT planning is performed based on the isocenter dose prescription with a 5-mm PTV margin and a 5-mm leaf margin, the ITV dose provides a good approximation of the GTV dose.