When we employ proton beams to treat thoracic malignancies, we often encounter a situation where the proton beam has to pass a tissue and then an intermediate air and then reach the tumor.
(1) In such cases, how to treat the in-between air gap for dose computation?
(2) Will there be any dose deposition in the air portion?
(3) Apart from intensity degradation, should we also account for proton energy degradation?
(4) What would be the impact on the SOBP curve in such scenarios?
Your TPS should take the different densities of the different targets that the proton beam encounters. And yes, air gaps should also be considered as they will affect the energy, scattering and SOBP characteristics. Precise details of the impact will depends on the proton beam energy, size of the air gap and relative position of all the other targets along the beam path.
With regards to your specific questions:
1) air gaps will have to be treated as any other targets
2) Yes there will be dose deposited to air too. The precise extent depends again on energy, volume, relative position etc
3) Yes, the proton beam will suffer energy degradation going through the air gap. Whenever that is relevant or significant depends on the beam parameter, target(s) size and your end point.
4) Position and size of the SOBP are likely to be affected by introduction of an air gap along the proton beam
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