Purpose: A novel bi-polar (BP) gated motion management strategy for lung SBRT is proposed. The feasibility is evaluated geometrically and dosimetrically.
Methods: In BP strategy, patient holds the breath at the end of inhale and exhale under audio/video coaching with dose only been delivered during breath hold. 10 previous lung SBRT patients with 4DCT were selected, each having tumor motion larger than 1cm. BP was compared with other strategies: free breathing (FB), gating at end of inhale (GI), exhale (GE), and real-time tracking (RT). Lung volume and tumor position were used to determine the end-exhale and end-inhale phases. The GTV was contoured at each 10-phases of 4DCT to determine the ITV for each strategy. PTV is formed by 1mm expansion of ITV. IMRT/VMAT plans were generated to give 60Gy to >95% of PTV and lung dose was compared. Breathing patterns’ stability and repeatability were investigated on 8 volunteers and recorded by RPM and analyzed.
Results: The breath-hold of the BP strategy is found to be comfortable over 3 minutes, with repeatability in periodicity at 1.00±0.03 and duty cycle at 65%. Using GI as reference, the ITV for FB, GE, RT, and BP are 1.83±0.50, 1.00±0.25, 0.63±0.14, 0.59±0.26, respectively. RT is larger than BP due to the limited number of phases in 4DCT that produce larger GTV volume in fast-moving phases. In IMRT plans from 3 patients, the lung V5Gy normalized to GI are 1.33, 1.17, 1.02 and 0.95, respectively, with similar trends in VMAT plans.
Conclusion: The bi-polar strategy is feasible for lung SBRT. The breathing pattern can sustain a long treatment time with a high duty cycle. Compared to other strategies, it has the smaller ITV, therefore a smaller treated volume and lower normal tissue toxicities. It is less complex to implement than real time tracking.