Concerning cancer mortality, the risk is known to increase as the exposure dose rate increases after short-term exposure exceeds 100-200 mSv.
Following the accident at the nuclear power station, a "Basic Survey" targeting approximately 2,060,000 people in Fukushima Prefecture was implemented to estimate external exposure doses over a period of the first four months after the accident. As a result, it was found that the exposed dose was "less than 2 mSv" among 93.8% of respondents (approximately 466,000 people excluding persons engaged in radiation work).
UNSCEAR has reported as follows: "No discernible radiation-related increases in rates of leukaemia or breast cancer (two of the most radiogenic cancer types), nor in other types of solid cancer besides possibly thyroid cancer, had been expected."; also, "A large excess of thyroid cancer due to radiation exposure, such as occurred after the Chernobyl accident, could be discounted, because the estimated thyroidal doses due to the Fukushima Daiichi Nuclear Power Station accident were substantially lower than those sustained around Chernobyl., it is suffice to assume that there will be no large excessive occurrence of thyroid cancer like that experienced in the aftermath of the Chernobyl nuclear power station accident."
It is thus deemed unlikely that cancer patients will increase due to radiation exposure arising from the nuclear power station accident.
Ministry of the Environment Government of Japan: BOOKLET to Provide Basic Information Regarding Health Effects of Radiation
Fukushima Minpo&Nagasaki University: Radiation Q&A
UNSCEAR: Fukushima 2017 White Paper
Fukushima Prefecture Fukushima Reconstruction Station "Health of residents of the prefecture"