IMRT

actual dose coverage of smaller target is more susceptible to tumor motion in lung SBRT.

formity and increases dose gradient, which indirectly causes CTV underdose as shown in Fig. 6(a) . In this study our pre- scription is to cover 95% PTV. In case of a reduced PTV cov- erage, motion will easily lead CTV underdose. To raise the prescribed volume coverage, e.g., 99%, will possibly allevi- ate the problem, however with a cost of increased organ dose. Same theory applies to increasing margin size. 6634 Zhao et al. : Dosimetric effect of tumor motion in gated SBRT

DMLC Movement simulation IV. DISCUSSION The target motion data used in this study were obtained from Cyberknife treatment where no coaching was adminis- 6633

6633 Zhao et al. : Dosimetric effect of tumor motion in gated SBRT

F IG . 6. Dose distribution map with and without tumor motion for Patients #3 and #6. Solid curves are delineations of PTV (outer-circle) and CTV (inner-circle). A margin of 5 mm was applied on CTV to obtain PTV. The dose distribution is color-shaded for 100% isodose and above.

Movement tracks from Cyberknife

Medical Physics, Vol. 39, No. 11, November 2012

• Target motion converted into leaf motion using PDF from movement tracks • Segment per segment modified • Recalculation

Zhao et al, Medical Physics, Vol. 39, No. 11,