@climateguy
This may help explain things:
So why would a radiation oncologist recommend IMRT (photons) over protons?
Because treating the seminal vesicles requires wider, deeper radiation fields.
And in that situation:
IMRT often provides better coverage and fewer risks than protons.
Specifically:
Seminal vesicles are long, thin structures that extend upward and outward.
They are harder to target precisely.
IMRT handles complex shapes better.
It can “wrap” dose around anatomy more smoothly.
Proton beams can be more sensitive to motion and tissue changes (like bowel gas or bladder filling).
For deep or irregular targets like seminal vesicles, this can reduce accuracy.
Several major cancer centers prefer IMRT for prostate cancer with extracapsular extension or seminal vesicle invasion (SVI) for these.
I summary: For prostate cancer that has spread into the seminal vesicles, IMRT is often the safer, more reliable option to cover all cancer areas fully while protecting organs as much as possible.