Room: Exhibit Hall
Purpose: The purpose of this study is to investigate the effect of range shifter and air gap on in-air spot size (Ïƒ) for ProteusPLUS pencil beam scanning (PBS) proton therapy.
Methods: A range shifter (WET=7.5 cm) was placed at the end of a dedicated nozzle snout holder (DNSH). An air gap (AG) is defined as the distance between the distal end of the range shifter and isocenter. All in-air spot measurements in this study were done using Lynx device placed at the isocenter for gantry angle 270Â°. For each AG (11,15,20,25 & 30 cm), a single spot was delivered for energies from 105 MeV to 225 MeV in an increment of 5 MeV. The energy herein means the proton beam energy at the end of nozzle exit. The difference in Ïƒ (Î”Ïƒ) was calculated by comparing the results of AG=11cm with that of other air gaps as mentioned above.
Results: For a given air gap, the Ïƒ increased with decrease in energy. Similarly, for a given energy, the Ïƒ increased with increase in air gap. The Ïƒ values were 3.6â€•7.9 mm for AG=11cm, 4.0â€•9.3mm for AG=15cm, 4.5â€•11.1mm for AG=20cm, 5.0â€•13.0mm for AG=25cm, and 5.5â€•15.0mm for AG=30cm. Additionally, the increase in Ïƒ with an air gap was more pronounced at lower energies. The Î”Ïƒ between AG=11cm and AG=15cm was 0.9mm for Eâ‰¤125MeV. Comparisons with larger air gaps showed: AG=11cm vs. AG=20cm: Î”Ïƒ>1.9mm for Eâ‰¤125MeV); AG=11cm vs. AG=25cm: Î”Ïƒ>3.1mm for Eâ‰¤125MeV, (iv)AG=11cm vs. AG=30cm: Î”Ïƒ>4.4mm for Eâ‰¤125MeV.
Conclusion: The in-air spot sizes of ProteusPLUS PBS proton beams were measured by varying air gap between range shifter and isocenter. From this study, it is recommended to keep the air gap as small as possible while treating shallow targets (E<125MeV) in order to minimize the increase in in-air spot sizes.