What is an advantage of using a radioactive seed to treat a cancerous tumor?

One big advantage - probably THE biggest - is time. A standard course of radiation therapy can take weeks, with several trips to the center each week. And each treatment takes time as well. That's a lot of gas, a lot of travel, a lot of time commitment. Whereas with "seeds," it's one day, in and out. The travel may be farther, but you only have to do it once, and it's done. There are drawbacks to "seeds." But time isn't one of them.