SBRT TrueBeam dose

I don't have an answer to this one - maybe Tall Allen will.

I had the "usual" 36.25 Gy CyberKnife treatment, and had some temporary issues afterward. All settled down within a couple of months, though, and now I'm just on the quarterly test and monitor plan.

dafight-

I had 40 Gy (using Truebeam with RapidArc). 40 Gy was also the dose used in the recent large multi-institutional study that had 5-yr cure rates of 97%. Dosimetry is a very complex subject that goes a lot further than just the "prescribed dose." The prescribed dose is what they are delivering to the prostate plus some margin around it (called the planned target volume), but the margin (which is important because of toxicity) can be dealt with in many ways. For example, Alan Katz uses a prescribed dose of just 35 Gy, but the dose received by 95% of the prostate is higher (36.25 Gy - just like what Paxton got) and the dose in the margin is much lower. He sort of feathers the edges. He adopts this strategy to mitigate toxicity and it works very well.

45 Gy seems to be safe as used in the following dose escalation study. But everything depends on the dose delivered to the organs at risk - the rectum, bladder, urethra and penile bulb. These dose constraints must be very strictly observed, and it may not always be possible to satisfy them all simultaneously when delivering higher prescribed doses. (BTW- this is one time where extra belly fat may be a benefit.)

UTSW did an experiment, in my opinion, unethical, where they raised the dose as high as 50 Gy. The results were horrific. Rectal injury was so bad that several men had to have diverting colostomies and some developed fistulas. 98.6% were cancer-free at 5 years vs 97% at 40 Gy in the aforementioned study - so not appreciably different and certainly not worth the extra side effects incurred. Here's an analysis of their experiment and a similar experiment at Sunnybrook:

/pcnrv.blogspot.com/2016/08/safety-limits-of-sbrt-dose-escalation.html

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

My RO, who was the first one to ever use CyberKnife for prostate cancer, now uses Truebeam with RapidArc, so he is in a unique position to compare them. They both do an excellent job. He tells me that his Truebeam system is more conformal so that margins are easier to control tightly, and it also delivers a more homogeneous plan. But he emphasizes that with enough care, the plans are very similar. The most obvious feature is that it's much faster. It can take close to an hour for each CK session vs about 5 minutes for each of my sessions. I suspect that higher doses would take CK even longer, so that may be a reason why doses have been lower on that platform.

BUSMAN - I've seen the UTSW SpaceOAR presentation at ASTRO. (there are some peculiarities with the study that I won't go into here.) Rectal injury is normally not a problem with SBRT as it is normally used -- late term grade 2 rectal SEs are typically under 5%, and grade 3 is almost unknown. However, considering their dose-escalation study (above), if I were being treated at UTSW, I would want a spacer.

A lot of the dosimetry will depend on your personal anatomy from the CT/MRI. My margins were 2-3 mm except for the part abutting the rectum, where it was zero mm. (My RO uses wider margins for high risk patients.) The plan is the most important in terms of side effects. If they set and adhere to tight rectal dose constraints, you should be fine. Ask to see the "dose/volume histogram" and ask them whether it meets or exceeds the constraints they've set. It never hurts to let them know that you're concerned about such things.

Mine was done every other day with a couple of weekends in between - FMWFM. Everyone does it differently, but I wanted the extra time for recovery in between. You're right that it doesn't affect the dose.

I thought UTSW had a Vero platform, but I don't know if they use it for prostate SBRT.

I had 27 grays in 3 fractions to femur met via SBRT with trubeam. If your RO is experienced he knows how many grays per fraction is safe depending on the area being radiated.
Bob

I don't either, but my SEs were mild anyway. Visceral fat adds a natural spacer between all the organs, so that the radiation dose to nearby organs-at-risk is diminished. We saw that a hydrogel spacer had a fairly small effect on rectal SEs, so it's probably not that big a deal with modern image-guidance systems. It may be more important in men who are getting whole pelvic radiation. I remember when Nomar Lupron (RIP), who had quite a bit of belly fat, was having whole pelvic radiation. He showed me his dose/volume histogram and I was amazed at how rapidly dose fell off as it reached the bowels and rectum. He had absolutely no SEs.

dafight,

I had SBRT via the Varian Edge at the Cleveland Clinic. For my particular treatment, they employed "simultaneous integrated boost" for a maximum dose of 45 Gy. The theory was to be able to drive the heterogeneity of the dose into each of the nasty prostate nodules (as seen on my MRI), while delivering 36.25 to rest of my gland.
You will have to pick sides in the debate regarding why you need to escalate a dose, but I found merit in the idea that some cancer cells may be radio resistant and where they cluster could increase the odds of resistant cells residing there.
CC freely admits that their 'dose painting' SBRT strategy deliberately plagiarizes HDR treatment. And since HDR was my first choice (but couldn't due to pubic arch issue) this choice seemed reasonable.
My SEs were mild at first and don't exist today. Time will tell if I did the right thing. I wish nothing but the best for you!