Trial Information

Scientific Protocol for the Study of Leukemia and Other Hematologic Diseases Among Clean-up Workers in Ukraine Following the Chernobyl Accident

Leukemia holds a special place in the study of radiation-related cancer because bone marrow
is one of the tissues most sensitive to the carcinogenic effect of ionizing radiation.
Radiogenic leukemia has the shortest latent period among radiation-induced cancers, and its
appearance suggests that solid tumors may follow. These same characteristics also contribute
to its considerable significance in radiation protection. There are, nevertheless, important
gaps in existing knowledge of radiation-induced leukemia, gaps that derive from
characteristics of the study of the atomic bombing of Hiroshima and Nagasaki, and from
studies of the effects of medical irradiation and studies of nuclear workers; these are the
studies that have provided most of the information to date. The gaps include the presumed
reduction in risk resulting from dose-fractionation and low dose-rate, as well as the time-
response function in the first five years after exposure. The study described herein is
intended to fill those gaps.

The Chornobyl accident exposed hundreds of thousands of people to radiation, notably those
involved in its cleanup operations. The second phase of a case-control study of ionizing
radiation and leukemia is being conducted in a cohort of approximately 100,000 Ukrainian
"liquidators" involved in cleanup work following the accident at the Chornobyl Nuclear Power
Plant in northern Ukraine which occurred on April 26, 1986. The cohort is restricted to
liquidators who first worked around the power plant between 1986 and 1990 and were
residents, when first registered in the Chornobyl State Registry, in Kyiv City or in one of
five oblasts (major civil divisions) that comprise the study area. This cohort, consisting
of males of working age, received mean bone marrow doses of approximately 80-100 milli-gray
(mGy) at low to moderate dose rates, with those sent earlier receiving the highest doses.
The primary objective of this study is to investigate leukemia risk as a function of such
radiation; it constitutes probably the largest epidemiologic study conducted to date among
working-age males, a group of particular concern in establishing occupational radiation
safety standards.

The primary scientific objectives of the proposed study are to evaluate whether there is a
doserelated increase in risk of leukemia among these liquidators, and how it compares with
the observed risk per unit dose seen in the atomic bomb survivors, exposed to essentially
instantaneous radiation. A secondary objective is to identify any modifiers of risk,
including time since exposure, age at exposure, etc.

The field work for the second phase of the study, with an extended case ascertainment for
2001 through 2006 has been complated. As a result of the second phase, a total of75
confirmed cases of leukemia and 12 cases of multiple myeloma have been ascertained. We are
currently in the process of database construction, dosimetric calculation for data analyses
for leukemia, and manuscript preparation.

The small sub-study on uncertainties associated with human factors is to be started upon
SSIRB approval.