Well the most common road to a PhD in medical physics begins with a bachelor's degree in physics. It's not too uncommon for people who come from other STEM fields to enter medical physics as graduate students, provided they have the necessary background. Most often that means engineering students.
For someone who's doing a bachelor's degree in radiation therapy... one of the biggest hurdles that you'll encounter is that you're probably not gaining the skill sets in physics and mathematics that you need to get through a medical physics PhD. Most RT programs don't go into enough detail that the students come out comfortable with the mathematics or programming skills needed to understand medical imaging (image reconstruction, designing image filters, MRI pulse sequences, etc.), radiation transport problems (numerical methods for solving the Boltzmann equation, Monte Carlo simulations of radiation interactions, convolution-superposition), fluence optimization techniques, medical statistics, and machine learning. And remember some (though not all) medical physics programs require their students to pass the same qualification exams as the physics students.
That said, not all graduate medical physics programs are created equal. Some don't go into as much depth as others and may be more flexible on the background of the students they'll take in.
And, I know people from hybrid physics-RT programs who've gone on to do master's degrees in medical physics. So it is *possible.*
Something else you might want to consider though, if you end up working as a dosimetrist, is the financial angle. Sure, by median salary
medical physicists tend to make more than dosimetrists. But consider the opportunity cost. A medical physics PhD is going to take at least four years, if not longer. During that time, you probably won't be earning much money. Whereas you'll have a comfortable living as a dosimetrist over that same period. That's not to dissuade you, but it is something to think about.
