Ask a lot of questions to the specialists (H&N surgeons, surg oncs, med-oncs) about their area of expertise. Go to all the tumor boards you can, even the ones off your service if you have time. If an opportunity to give a talk arises, take it - you learn a lot, and you'll gain respect amongst the other specialists. Steph is right - we aren't 'radiation therapists'. We don't do what a specialist wants - we do what's right for the patient.
Dosimetrists are crucial to a junior resident. When contouring, ask a million questions about fields, wedges, collimator angles, etc. Ask why they do things the way they do. Most dosimetrists like to teach the residents, and they can break it down very simply for you. I'd say talk to the physicists, but they just seem to confuse me further right now. I'm sure as I learn more, I will learn from them, too.
RTs are great and the key to delivery of treatment. At the beginning, when you are new and asked to approve things, be wary and don't be rushed. They just want to get things rolling, so if you aren't sure, don't just say 'ok' to appease the tech. Take another film if you need to, call an attending if you need to, and don't get rushed. But, when looking at films, ask them for pointers on landmarks and quick ways to match up things. They do this a million times a week, so they have good shortcuts.
As far as studying, I don't know what to say but read lots. Some UpToDate, some textbook, some journal articles, just a little bit every day and more before conference or when giving a talk. Nothing really clicks at the beginning. It's too overwhelming, and it doesn't seem to help. But, each time you go over something, some of it sticks. It's like "Karate Kid", when Daniel-san is asked to clean the floors and paint and doesn't get why and gets all mad (like me reading Khan). Then finally, when sen-sai spars with him, he realizes why - he was building baseline skills and reflexes (like when I drop a tight four-field plan for a rectal ca).
-S