With Rad Onc being as competitive as it is (240/244 on step 1/2 according to charting outcomes 2011), its definitely smart to apply to a backup specialty, especially when it is a backup that you do seem to have genuine interest in. People tend to make a big deal about rad onc and anesthesiology being so different, or any other specialty and anesthesiology being so different. But the bottom line is that people can be happy in multiple specialties even if they specialties are so so different. With the match processes becoming more and more competitive every year, the main goal should be to match. While it would be more noble to shoot for the stars and just apply to rad onc, you should be realistic and realize its better to match in your second choice specialty than not match at all.
Anesthesia is becoming more and more competitive each year. But each year, there are unfilled positions in the match. I know people with stellar step one scores and great CVs who do anesthesia but I also know people who matched that failed step one. Anesthesia is a great backup especially when applying to highly competitive specialties because it is a fantastic hands-on intellectually-stimulating field. Your best bet is your home program. Try to get at least 1-2 letters now for anesthesia even if it means shadowing on your time off.
best of luck
