The greatest fear should be internal contamination. Many of the radioactive isotopes mimic their non-radioactive elements or have a similar biological uptake. (Strontium vs. Calcium), (normal Iodine 127 vs. radioactive Iodine 131).
For dosage calculations it is important to know if it is an alpha particle, beta particle, gamma ray or neutron. Each has different penetrating capabilities and different destructive potential.
If I had a choice between 1 Sievert of gamma radiation given externally or 10 milliSieverts of an internal dose of Iodine 131 (8 day half life) or Polonium 210 (138 day half-life) I would take the external dose.
Polonium is a wicked producer of alpha particles. When ingested or inhaled it is toxic in addition to giving you a constant source of radiation (rate decreasing by 1/2 every 138 days).
I had a thyroid disorder and I opted for the Iodine 131 treatment instead of surgery. It was an internal contaminant and my sweat and urine were radioactive and I emitted enough radiation that I was supposed to stay away from people for several days (there are biological half-lives for elements in the body that are quite different from isotope decay half-lives). The treatment essentially "killed" my thyroid on purpose since it was on it's own version of a nuclear meltdown.
Need to consider external vs. internal, particle types, radiological half-life and biological half-life on any sort of dosage calculation.