Clinically, yes, we move past old modalities. However, in residency we are forced to learn how crap was done 20+ years ago so we can have a "historical perspective".
For example, for head and neck radiation (which is done with IMRT near 100% of the time in the current age), we still have to learn how old plans were created on X-ray (2D planning), despite the fact that it was night and day in terms of areas covered. Why did we have to learn that? Because somebody who trained back in the era where the only imaging you could plan on was an X-ray is still practicing as an attending. Because apparently all of the literature we have currently on what needs to and doesn't need to be covered isn't as useful as knowing how somebody did radiation treatment 30 years ago.
Even in 3-field breast plans, we now have contours for lymph node areas, that we can conform our planning fields to and minimize dose to normal tissue, but "it looks different than when we planned it with x-ray alone" is a very common criticism.
The issue is that most areas that are covered with radiation are extrapolations of how things were done back in the 2D era, meaning most of the time it's similar enough, so anytime there's a discrepancy, somebody can say "that's not how we treated it 20 years ago", and everyone who hasn't been in the field for 15+ years has to resist the urge to roll their eyes.
