Should density overrides for VMAT lung SBRT planning be utilized for small mobile tumors?

I don't think it's a huge issue to change it, but I/we don't usually do that. VMAT plans are very robust to density changes so I bet if you looked between the plans it would be a very subtle difference....like less than 5% if I had to guess. We don't do it that way because it's a bit of a cheat (or, it's treating the physicist/doc vs the patient) but it's also an unnecessary step. Back in the ancient days of lung SBRT Bob Timmerman's study at Indiana University did not use heterogeneity corrections for lung. They didn't feel that the algorithms available at the time were sufficiently accurate and so the whole body was treated as water equivalent. When those algorithms improved the analyzed the dose and found that it was about 10% hotter wit...

The following reference offers a word of caution: Zou et al, Dynamic simulation of motion effects in IMAT lung SBRT., Radiat Oncol, 2014 9:225.

This study calculated the magnitude of dose deviation which can be attributed to the "interplay effect" for both the target and organs at risk in a series of patients treated with 4-fraction VMAT SBRT. While the interplay effect has a negligable impact on the target -- as @Dr. First Last very eloquently stated -- it may have a significantly greater impact on organs at risk, such as the heart or proximal bronchial tree.

Bottom line: When there is a great deal of motion (which our institution defines as ~>1.2 cm), calculated doses to OARs may not be accurate. This may be problematic especially for ce...

The paper referred to by @Dr. First Last is Xiao et ao, IJROBP, 2009, 73(4): 1235-1242

It actually showed that with heterogeneity correction applied, the percent of PTV receiving 60 Gy was reduced, largely due to the physical issue of not having so much depth of high density tissue in the ITV to reach electronic equilibrium. Fig 6 of that paper is here:

Fig. 6. Percentage of volume that received prescription dose of ≥60 Gy (V60) compared against protocol required value of 95%, without and with heterogeneity (Hetero) correction

If the original question from @Dr. First Last concerned whether it is ok to force a PTV density in the surrounding lung tissue so that at least there is the appearance of good dose buildup, I think this practice is...