If you click on my link (upper right) to today's NRC event report, you'll see two contrasting medical radiation events.
One was an UNDER-treatment using Y-90 Theraspheres. So this was an internally delivered dose, with a pure beta emitter, to the liver. The intent was to deliver 121 Gy, but only about 62 Gy was delivered due to an equipment problem.
The second was an OVER-treatment using an HDR Afterloader. So this was also an internally delivered dose, using Ir-192, which emits gamma's as well as beta's. I'm not sure what organ was being targeted but the prostate is common. The intent was to deliver 340 cGy, but due to an equipment problem and human error, 600 cGy was delivered.
These sorts of things happen frequently on a nation-wide basis. The risk of it happening to an individual is very small, because lots of treatments take place everyday.
It's odd that the majority of people who are "anti-nuclear power" aren't consistent and aren't also "anti-nuclear medicine".
They both involve risks and benefits.
It's obvious why an over-treatment is reportable to the NRC. A patient might incur deterministic injuries depending on the over-treatment, and has increased his/her cancer risk.
It may seem counter-intuitive to report under-treatments. They're reported because the under-treatment may not be medically efficacious, and the dose has to be delivered again. So the first dose was "wasted" and is similar to an over-treatment.
The criteria for reporting are found in 10CFR35.