it seems that pursuing a pure physics phd does allow for more changes down the road. I am really hesitant to apply for medical physics for that reason! i am a bit scared devote wholly to a single discipline. I have applied to wisc, chicago, ucla, and mit medical physics (originally applied to radiological sciences at mit until i realized that major is no longer offered!) I'm not yet sure on research interests. mri sequence designing is fun, but i have never tried dosimetry or therapy calculations. i know that there are folks within berkeley nuc eng that study a lot of numerical methods for dose calculations, seems very interesting!
other programs applied to: stanford applied physics, berkeley bioengineering. I'm scared too!
here is a cut & paste of a FAQ from the wisconsin medical physics
How competitive is admission to the University of WI – Madison Medical Physics program?
Admission to the Medical Physics program has become increasingly competitive in recent years. Statistics for the past 7 years are:
Year Applied Accepted Admitted International
2008 185 41 23 23%
2007 154 43 26 32%
2006 164 37 22 26%
2005 133 34 20 23%
2004 140 30 23 0%
2003 94 25 19 20%
2002 42 25 12 23%