specifically for med rad you need at least a CA of 5.0. you must complete all 10 courses at the end of the winter term or else you may be put on the wait list if you decide to take a course in summer school.
check out the undergraduate calendar
http://registrar.mcmaster.ca /calen...12/pg1606.html which provides you with more details.
Radiation Therapy is generally the most competitive because it has the least seats (approx 20) followed by ultrasound which has around 40 people and then radiography with 60. It changes each year though. If your year has a lot of people that want to go into radiography then that specialization will be the most competitive.