I would be shocked if that were true about a Medicare limit.
The five treatment approach is a pioneering approach using theory/and treatment methods known as Stereotactic Body Radio Therapy (SBRT). It can be delivered by a wide range of radiation systems, but it is most closely associated with CyberKnife. In fact, studies are going on using some other systems with fewer than the traditional 40 or so external beam treatments but more than the 5 associated with CyberKnife SBRT.
There's an encouraging post about a recent, key research report that documents quite successful follow-up that averages (median) about five years. There was some good sounding theory predicting that the five treatment approach would work, but no one really knew for sure that we could have good five year outcomes until this study.
It's amazing how urban myths get spread around the Internet!