You always want to know what the MTD is in any dose escalation...

You always want to know what the MTD is in any dose escalation study. The issue you want to avoid is not being able to explain to the FDA (or other regulators) why you stopped raising the dose if the side effect profile was manageable and higher doses are expected to work better. These days you need to justify scientifically to the regulators why you chose the dose level you did for Phase 2 - the RP2D (recommended phase 2 dose).

Having said this moving to multiple doses and then assessing the toxicity profile is the way to go. If at the end of 4 cycles at the 12GBq dose level the side effects are still below the tolerability limit then you can add on more patients at a higher dose until you get to MTD. CU6 have already shown a willingness to change the trial design as it is running based on results so I can't see why they would not add in another cohort at a higher dose level if 4 cycles is tolerated easily.

My one fear about the upcoming Phase 3 trial is Cu67SAR-bisPMSA being forced to be trialed in patients who have failed current Lu177 or Ac225 PMSA therapies. CU6 have been treated very well by the FDA to date so I am hopeful that they don't have to go down this path, but until we see the Phase 3 trial design it is a risk.