Wait — how could "Japan" confirm the cause of a particular cancer, leukemia in a middle-aged man, whose radiation exposure was well within official limits of safety? Here's how: Japanese government policy is to award compensation to any Tepco/Fukushima worker who developed cancer subsequent to the earthquake-tsunami-reactor flooding of March 2011, if their radiation exposure was over a minimal level. Over 40,000 workers could potentially become "victims" like this one, whose identity has not been revealed.
Such a scenario is far from unlikely and much to be feared by those handing out funds to cover costs of cancer treatments. The current "victim" worked at the plant for several months in 2012-13, long after the disaster, and had a measured dose of just over 15 millisieverts, which is actually lower than a typical radiation load from a CT scan. Some radiation experts believe, with solid evidentiary support, that such low levels of exposure actually confer a health benefit (as opposed to the official policy, the "linear no-threshold" theory, which holds that there is no safe level of radiation exposure, a holdover from A-bomb extrapolations). Given the expected incidence of cancers as the population ages, and knowing that cancer is the leading cause of death in Japan (heart and vascular diseases are #1 in the US and Europe), it is likely that hundreds, or thousands, of current and former Tokyo Power and Electric Co. workers will be seeking similar certification and compensation for "Fukushima-caused cancers" over the ensuing decades.
At the time of the nuclear reactor flooding and radiation leakage, with many heroic workers toiling to stave off an environmental catastrophe, it was predicted that many so exposed would suffer acute or lingering radiation toxicity, especially cancers of various types known to be linked to radiation. Further, thousands of residents of Fukushima Prefecture were evacuated to escape local radiation effects; and although the actual measured radioactivity rapidly diminished to non-threatening levels, many locals remained too fearful to return. The disruption of this internal exile resulted in many adverse health effects, both physical and mental, while the radiation leakage accounted for approximately zero illnesses and deaths. Nevertheless, the reactor flooding, although the result of the historically-unprecedented earthquake and tsunami, was responsible for accelerating the pervasive anti-nuclear-energy coterie's crusade against this clean and safe source, to the degree that Germany decided to close their nuclear industry entirely.
So when reading about this "confirmed" radiation-caused cancer in Japan, let's keep it in perspective: just because the government and the power company have decided to compensate this gentleman's care does not certify his illness' actual cause. As with almost all other cancers (except those related to smoking), the cause of this one remains a mystery.