Complex sentence: In the United States, radiation oncologists undergo four years of residency (in addition to an internship), which is more oncology training than any other specialty and is currently the most competitive specialty for medical students applying for residency.
Split into simpler sentences: In the United States, radiation oncologists undergo four years of residency (in addition to an internship), which is more oncology training than any other specialty. It is presently the most competitive specialty for medical students applying for residency.