Radiation Margins CT vs MRI guided radiotherapy v proton

If I read that article correctly, they were comparing MRI-guided SBRT (photon) vs CT-guided SBRT (photon); neither were proton.

The MRI-guided is IGRT, which has advantages over CT-guided. That’s probably why there’s less toxicity.

Using a rectal spacer to shield the rectum from salvage radiation post-prostatectomy is rarely done. Here’s one case when it was done successfully: https://pubmed.ncbi.nlm.nih.gov/25339311/

Proton has 2mm margins as well. The value of proton is its Bragg-Peak characteristic. (See attached graphic.) The heavy particle (proton) has little entry-dose, scatter, or exit-dose. When it hits a target at a calculated depth, it releases all of its energy right there; none passes through to (possibly) hit nearby organs and tissues (as happens with photons).

You may go a decade or more before he needs salvage radiation. By then they may have completely new techniques. It’s a little early to start planning. Around 60% of RP patients never need salvage radiation.

The marginal differences between CT and MRI are most useful for primary treatment where they are aiming at the intact gland.
SRT does involve aiming but it is more of a blanketed approach to the prostate bed and nodes. Rectal spacers are not used so that the beams can reach PCa cells near/around the rectum. Hope this helps!
Phil

If I had to do salvage Radiation, I would definitely do an Mri guided Machine either the Mridian or Elekta unity as what the doctor can see he can treat. The margins for Proton will vary.

@brianjarvis
Thank you Brian! It's definitely MR-Linac or Proton for us.

The graphic and the link to that spacer using SRT study are good to have!.

And yet they don't use spacer for SRT, I suppose, as @heavyphil says, it is because they are actually aiming for PC cells in the vicinity / on the rectum (erg) -

The cancer was anterior in the prostate, and there was bladder neck involvement, maybe an individualized treatment might deliver a higher dose to the bladder neck, and avoid / deliver less to the rectum by design? ?

Add to all that - he has titanium hip implants. They cause scatter, unintended dispersion of the radiation, I think.

@jeffmarc

From your lips to God's ears, Jeff. we certainly hope so! We'll keep that thought front and center!

Didn't you have an operation coming up soon - an AUS implant? ?? How did that go / best of luck to you, if it is still on the horizon.

@heavyphil
Hi, thanks for the reply!! I hope they do some aiming for sure! There are so many sensitive (and critical) targets in the pelvic area.

I hope that MR guided imaging, during SRT, can help distinguish between scar tissue, normal tissue and cancer during treatment - and moderate the doses correspondingly "crossing fingers"

Then there's those hip implants...

@klein505 Curious as to how they managed the MRI/CT scan with his hip implants?

While many doctors use spacers routinely for SBRT, some studies suggest that with modern, highly precise SBRT and real-time tracking, spacer placement might not provide a noticeable quality-of-life benefit.

@brianjarvis
Good question - I don't have the MRI scan report. The PSMA/PET scan said "Evaluation is
limited by artifact from bilateral total hip replacements."

he said he could feel "tugging" on the implants during the MRI. 🙁 🙁

@klein505
I will have the AUS put in tomorrow. 6/4. It’s been a long wait.

Thanks for the positive Reinforcement