On 1 March 1954, a thermonuclear test explosion, called ‘Castle Bravo’ test, was performed by U.S. authority at Bikini atoll in North Pacific Orcean. The explosion resulted in large amount of radioactive fallout. In contrast to the predicted wind direction, the fallout spread out to the northeast direction and resulted in moderate to high level of radiation exposures to large number of people including the native Marshallese (some 240 Marshallese living on Rongelap-, Sifo-, Ailinginae- and Utrik-atolls) and 28 American military men on Rongerik-atoll.
     At the time of explosion, Japanese tuna-hunting vessel, ‘Fukuryu-maru No. 5’ (sometimes also called ‘No. 5 Lucky Dragon’), was fishing at about 160 km northeast of the hypocenter, where 23 fishermen were suffered from the fallout for about 4.5 hrs. Without knowing the radioactivity of the fallout, although they expressed nausea and vomiting, they returned to the home port, Yaizu, on 14 March 1954, where the fallout was found to be radioactive, in which more than 50% of radioactivity was that from radioactive rare earth elements.
     They were hospitalized in two hospitals in Tokyo by 28 March 1954 (seven in the University of Tokyo Hospital, and 16 in the First National Hospital of Tokyo). One of the fishermen (KuAi) died 23 September 1954. All the others were discharged from the hospitals in May 1955. Since then, annual medical follow-up examinations, including chromosome aberration examinations, have been performed. The follow-up studies have been performed by Dr. Toshiyuki Kumatori and Dr. Takaaki Ishihara, National Institute of Radiological Science, Chiba.

Dose reconstruction:
     Dose to the fishermen for 14 days have been estimated by (1) direct measurement of the fallout adhered to the body, (2) fallout in the vessel and the behavior of the fishermen therein, and (3) internal exposures by incorporated radionuclides as estimated from the radioactivity in urine and thyroid, and supplementarily by the radioactivity data in organs on the fatal case (KuAi).

Chromosome aberration analysis 

     It should be noted that the lymphocyte culture technique became available in 1960 (Moorhead et al., Exp. Cell Res., 20:613, 1960). In the early stages of human radiation cytogenetics, in particular before 1969 (e.g., 1969 report of UNSCEAR), the lymphocyte culture method for quantitative analysis of radiation-induced chromosome aberrations has not been established; for instance the chromosome aberration analysis has been made by 72-h culture, which eliminates significant number of unstable aberration by cell division in culture. In the Bikini cases, standardized conventional 2-day cultures have been applied for the blood samples taken 13-28 year after exposure.
     Nevertheless, in the history of human radiation cytogenetics, the Bikini cases marked a unique landmark in that (1) large number of persons were exposed to relatively high dose of beta-rays in short time, externally as well as internally; (2) radiation exposure to healthy male adults with similar age; (3) long-term follow-up of radiation symptoms, including hematological pictures and sperm count; (4) comparison between chromosome aberrations and clinical symptoms; and (5) first attempt to the use of chromosome aberration analysis for dose reconstruction. 

References 

     Ishihara, T. and Kumatori, T. (1967): Chromosome studies on Japanese exposed to radiation resulting from nuclear explosion. In: Evans, H. J., Court Brown, W. M. and Mclean, A. S. eds., ‘Human Radiation Cytogenetics’. North-Holland Publ. Co., Amsterdam. pp.144-166.
      Ishihara, T. and Kumatori, T. (1983): Cytogenetic follow-up studies in Japanese fishermen exposed to fallout radiation. In: Ishihara, T. and Sasaki, M. S., eds., ‘Radiation-induced chromosome damage in man”. Alan R. Liss, Inc., New York. pp.475-490.
    Ishihara, T. and Kumatori, T. (1969): Cytogenetic studies in fishermen exposed to fallout radiation in 1954. Jpn. J. Genet., 44 (suppl 1):242-251. 
     Kumatori, T., Ishihara, T., Hirashima, K., Sugiyama, H., Ishi, S. abd Miyoshi, K. (1980): Follow-up studies over 25 year period on the Japanese fishermen exposed to radioactive fallout in 1954. In: Hübner, K. F. and Fry, S. A., eds. ‘The medical basis of radiation accident preparedness.’ Elsevier North Holland, Inc., Amsterdam. pp. 33-54.
     Kumatori, T., Ishihara, T., Ueda, T. and Miyoshi, K. (1965): Medical survey of Japanese exposed to fall-out radiation in 1954: a report after 10 years. Chiba National Institute of Radiological Sciences, Chiba. pp. 1-14.

Chromosome aberrations in peripheral blood lymphocytes and bone marrow cells 

Scenario 

     In addition to the Fukuryu-maru No. 5 vessel, other tuna hunting vessels were working in the vicinity (420-1200 km) of the test site and exposed to the fallout of varying degrees. In this study, lymphocyte chromosome aberrations were examined in 17 crew members from 8 fishing boats and 2 from one cargo ship. They include fishing boats Junko-maru No. 8, Kaifuku-maru No. 5, Kosei-maru No. 2, Myoga-maru No. 5, Konpira-maru No. 1, Housei-maru No. 10, Dai-maru No. 7 and Myojin-maru No. 2, and a cargo ship Yaiko-maru. They are reportedly contaminated with fallout radiation at a level of almost 4000 cpm at the time of their return to harbor in Japan (Muroto, Kochi Prefecture).
     Blood sampling was made for those 19 persons 60 years after the Bravo nuclear test explosion. Chromosome aberrations were studied by G-banding method. The frequencies of chromosome aberrations, such as dicentrics and rings, were significantly higher for the fishermen as compared with the age-matched controls.