Purpose: 
To quantify dose heterogeneity due to the interplay effect when pencil beam scanning protons (PBS) are used with normal-breathing versus high frequency percussive ventilation (HFPV).

Methods:
HFPV provides frequent mini-bursts of air (100-400/min) into the patient’s trachea using a controlled ratio of oxygen inspiration to passive exhalation. We recorded chest wall motion curves using the Anzai laser motion-monitoring system for five (N=5) randomly selected volunteers during both normal-breathing and HFPV. Each chest wall curve was programmed to drive a dynamic CIRS phantom holding high-resolution Gafchromic film.  A PBS treatment plan with a known pattern of matchlines was delivered to each film, with and without 4-cm range shifter. We investigated dose heterogeneity, by evaluating film flatness, under normal-breathing, static-film, and HFPV motion conditions.

Results:
The mean peak-to-peak motion amplitude of normal and HFPV breathing curves were 18.94 mm and 2.11 mm, respectively. Median dose heterogeneity with range shifter was: 24.60% (95% CI: 13.11-39.81%) for normal-breathing, 4.90% (95% CI: 3.27-8.25%) for static-film, and 3.8% (95% CI: 2.89-4.67%) for HFPV. Median dose heterogeneity without range shifter was: 31.30% (95% CI: 25.13-37.75%) for normal-breathing, 7.90% (95% CI: 6.37-8.79%) for static-film, and 5.30% (95% CI: 4.17-6.23%) for HFPV. No difference was noticed from the range shifter, but dose heterogeneity was significantly worse during normal breathing than HFPV.  

Conclusion:
HFPV may address the motion concerns that exist for patients undergoing PBS radiotherapy, by immobilizing the target and decreasing absolute dose heterogeneity by 20-25% when compared to normal breathing.  Dose heterogeneity during HFPV was similar to the motionless static-film scenario.

05 - Evaluating dose heterogeneity when using high frequency percussive ventilation as a motion mitigation technique for pencil beam scanning radiotherapy