The following study took a look at the evidence (or lack of it) supporting the use of SBRT rather than other forms of radiation therapies:
http://www.annals.org/content/early/2011/05/02/0003-4819-154-11-201106070-00343.full
Apparently, we will have to wait for the results of 2 large ongoing trials scheduled to end in 2013 to know whether it is a better option. The article is nevertheless interesting as it explains the differences with the other types of radiation therapy.