Is there a concern for using FFF beams in single isocenter multi target lung SBRT?

Fun, if forgotten, factoid: in the early Indiana University experience, Bob Timmerman and Lech Papiez designed milled compensators for the purpose of steepening the dose gradient across the profile of the beam. The basic idea was to emulate the steep gradients of GammaKnife beams for the purpose of what was then commonly called "extracranial stereotactic radioablation", later a few other nicknames, and now of course called SBRT or SABR, take your pick. The explicit intent, in other words, was a steep gradient such as you might now come close to with FFF beams, which were not generally available in those days. The idea was to lower dose to nearby normal lung tissue, for which goal you generally need a pretty hot hotspot inside the tumor itse...

With the help of our physics faculty, here are our thoughts.

I wouldn't say it is wrong to treat these cases with a single isocenter, but at some point, you start to lose many of the advantages of it, particularly when the lesions are widely separated. As a general rule, if the jaw opening is exceeding ~10 cm, I think it's worth questioning whether a single-isocenter technique is still the best approach, and once you're beyond ~15 cm, I would be reluctant to use FFF routinely.

The concern is not really the FFF beam itself. Rather, as target separation increases, there are several factors that need to be considered: increased sensitivity to rotational and setup errors, greater low-dose spillage, and increased MLC modulation requirements. Wit...