Internal radiation, by contrast, emanates from radioactive elements that enter the body by inhalation, ingestion, or skin absorption. Hazardous radioactive elements being released in the sea and air around Fukushima accumulate at each step of various food chains (for example, into algae, crustaceans, small fish, bigger fish, then humans; or soil, grass, cow's meat and milk, then humans). Entering the body, these elements - called internal emitters - migrate to specific organs such as the thyroid, liver, bone, and brain, continuously irradiating small volumes of cells with high doses of alpha, beta and/or gamma radiation, and over many years often induce cancer.
Source: theage.com.au