New - Going from a Firehose to a Trickle in 3 Days Flat?

If it is that big, how is it that they didn't feel it on a DRE?

SBRT doses don't increase with risk - it already has a very high BED. What they increase is the margin.

Alan Katz originally started with 36.5Gy/5fx and then switched to 35 Gy/5 fx. He found there was no difference in oncological effectiveness, but that 35 Gy had a better side effect profile. Since then, he has stuck with 35 Gy - never higher, even for his high risk patients. The BED is higher than conventional IMRT (80 Gy/40 fx) and meets the threshold that some believe is optimal. No one has found a correlation between risk level and radioresistance at these kinds of BEDs.

Chris King started with 36.5 Gy (if memory serves) when he used CyberKnife. When he switched to RapidArc, he upped the dose to 40 Gy and found no difference in toxicity. He uses the same dose for favorable and unfavorable risk patients, but expands the margin in his high risk protocol.

Fuller and Gottschalk increase the BED using more extreme hypofractionation (38 Gy/4 fx).

Zelefsky is doing a dose-finding trial, incrementally raising doses to see if toxicity is acceptable. Timmerman did a disastrous dose-finding trial. Loblaw also did a trial, which you can read about here:

/pcnrv.blogspot.com/2017/05/sbrt-dose-escalation.html

The dose/response curve is s-shaped. They want to be at the top of the steep part of the curve where an incremental dose increase has very little increase in effectiveness, but only increases toxicity. Getting an extra 1% or 2% in bRFS (that is, going from 97% to 99%) does not seem warranted if it leads to fistulas.

It's more complicated than this. These are just the prescription doses. The doses received by a higher percent of the prostate may be set differently. So, for example, if Katz sets his 35 Gy to cover at least 100% of the planned target volume (including margin), he may be delivering an equivalent dose to another RO who sets 40 Gy to cover 90% of the PTV. Hot spots and cold spots are looked at in the planning as well.

Your link doesn't work. Sorry, never heard of it. If it has to do with dose homogeneity, it's a controversial subject. Some, like Fuller and Gottschalk, believe that SBRT ought to mimic HDR brachy as closely as possible, which means dose heterogeneity (lots of hot spots) is important. Others, like my RO, argue that dose homogeneity gives better toxicity outcomes. He reasons that if you believe (w/o evidence) that higher dose is required, you ought to be delivering that dose everywhere and not just in random places. For the same reason, he doesn't believe in SIBs to DILs -(look it up )