Health Risks of Ionizing Radiation: - Clark University

Leukemia
Leukemia, a general term to describe cancers of the
blood, was among the first effects to be observed
in survivors of the atomic bomb. There is also information
regarding the nature of radiation-induced
leukemia from nuclear workers, people exposed to
radiation for medical reasons, and people exposed to
fallout from Chernobyl and nuclear weapons tests.
This chapter begins by describing the disease and
then covers what we know and don’t know about the
dose-response relationship for leukemia.
A.1 About the Disease
Appendix A
Leukemia is diagnosed in about 30,000 Americans
each year. Variations of the disease are typically
grouped into several types and we will try to keep
the distinctions clear because the different types of
leukemia appear to have distinct patterns of response
to radiation. Leukemias are divided into acute and
chronic types; this used to refer to the duration of the
illness but the newer classification refers to the maturity
of the cells in question--acute leukemias develop
from immature cells and chronic leukemias develop
from more mature cells. Leukemias are further divided
by cell type--malignant lymphoid cells (white
blood cells involved in immune response including
B-cells and T-cells) are classified as lymphoblastic
or lymphocytic leukemias; malignant myeloid cells,
as well as malignant red blood cells, are classified as
myelocytic or myeloid leukemias. This gives us the
following four major types of leukemia:
Acute Lymphoblastic Leukemia (ALL). ALL
originates in immature lymphoid cells in the bone
marrow, blood, or body tissues. This is the most
common malignancy in children, affecting over
3,000 children in the U.S. each year, but it also affects
about half as many adults.
Acute Myeloid Leukemia (AML). AML risk
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increases with age and it is roughly four times more
common in adults than ALL. This type can originate
in immature white (myeloid, monocytic) or red
(erythrocytic) blood cells or immature platelet cells
(megakaryocytes). Over 10,000 cases of AML are
diagnosed each year in the U.S.
Chronic Lymphocytic Leukemia (CLL). CLL is
another common type of leukemia in the U.S. with
about 7,000 new cases diagnosed each year. At the
same time it is the only leukemia subtype for which
the evidence of a radiation association is equivocal.
CLL is almost always comprised of malignant Bcells.
Chronic Myeloid Leukemia (CML). CML is
sometimes called chronic granulocytic leukemia
because one of the distinguishing characteristics is
overproduction of granulocytes, the largest group of
white blood cells. CML incidence peaks in young
adulthood and is diagnosed in over 4,000 Americans
each year.
All of these types have been clearly associated
with radiation except CLL. For this reason many researchers
will study the incidence or mortality of all
leukemias excluding CLL (or non-CLL leukemia).
Unlike most solid cancers, which can take many
years to develop, leukemia has a short latency period
of just a couple of years and generally declining
risk after a peak period. This makes it important to
consider how many years have passed since exposure.
There is also an apparent sensitivity at younger
ages making it important to consider childhood exposures
as a separate category. Since leukemia is
likely to originate in bone marrow researchers have
made estimates of the bone marrow dose when possible.
The dose-response curve for leukemia is often
described as non-linear, concave-up, or linearquadratic,
but dose-response models will typically
include a linear term that plays a more important
role at lower doses. When appropriate and possible
we will mention these linear terms as a guide to the