SU‐GG‐J‐34: Aperture Design of Two‐Dimensional MLC Motion for Dose‐Rate‐Regulated Tracking

Purpose: Real‐time adaptive tumor tracking called Dose‐Rate‐Regulated Tracking (DRRT) is based on a preprogrammed MLC sequence and real‐time dose‐rate modulation. We have developed an algorithm for designing the MLC sequence reflecting 2‐D MLC motion. MLC apertures were designed from 4D‐CT images, and the algorithm was tested in the 3‐D phantom. Method and Materials: The 2‐D MLC motion and the corresponding tumor shape are derived from ten‐breathing bins of 4D‐CT. The closed MLC leaves that are not participating in the beam aperture are programmed to remain in motion to minimize leaf‐end leakage. This feathering motion is intended to spread the leakage dose to the normal tissue beneath the closed MLC leaves. We performed phantom studies with a 3‐D Phantom (Washington Univ.) for two cases: (1) a circular moving target (2) a patient's lung tumor in the lower lobe. The results of these two measurements were compared with those for the case of a static beam aperture and a static phantom (static‐static case). The accuracy of the 2‐D MLC sequence was determined by γ analysis with those for the static‐static case. The effectiveness of the feathering motion was quantified with the leakage dose normalized to the maximum dose measured by the film. Results and Conclusion: For both of the above cases, the γ analysis showed that 95% of the pixels are less than 1 for 3 % and 3 mm criteria. The feathering motion reduced the leaf‐end‐leakage dose from ∼15% to ∼7%. A careful design of the MLC‐leaf sequence done in advance can not only facilitate real‐time tumor tracking, but also reduce the dose to healthy tissue, and thus can lead to improved results using DRRT.