International Agency for Research on Cancer (IARC) responds to scientific article claiming that environmental and lifestyle
factors account for less than one third of cancers. The International Agency for Research on Cancer
(IARC), the World
Health Organization’s specialized cancer agency, strongly disagrees with the conclusion of a scientific
report
1
on the causes of human cancer published in the journal
Science
on 2 January 2015 by Dr Cristian
Tomasetti and Dr Bert Vogelstein.
The study, which has received widespread media coverage, compares the number of lifetime stem cell
divisions across a wide range of tissues with lifetime cancer risk and suggests
that random mutations (or
“bad luck”) are “the major contributors to cancer overall, often more important than either hereditary or
external environmental factors.”
For many cancers, the authors argue for a greater focus on the early detection of the disease rather than
on prevention of its occurrence. If misinterpreted, this position could have serious negative consequences
from both cancer research and public health perspectives.
IARC experts point to a serious contradiction with the extensive body of
epidemiological evidence as well
as a number of methodological limitations and biases in the analysis presented in the report.
“We already knew that for an individual to develop a certain cancer there is an element of chance, yet this
has little to say ab
out the level of cancer risk in a population,” explains IARC Director Dr Christopher Wild.
“Concluding that ‘bad luck’ is the major cause of cancer would be misleading and may detract from efforts
to identify the causes of the disease and effectively preve
nt it.”
The past five decades of international epidemiological research have shown that most cancers
that are
frequent in one population are relatively rare in another and that these patterns vary over time
. For
example, oesophageal cancer is common among men in East Africa but rare in West Africa. Colorectal
cancer, once rare in Japan, increased 4
-
fold in incidence in just two decades.
These observations are
characteristic of many common cancers and
are
consistent with a major contribution of environment
al and
lifestyle exposures, as opposed to genetic variation or chance (“bad luck”).
Furthermore, IARC experts identify several limitations in the report itself. These include the emphasis on
very rare cancers (e.g. osteosarcoma, medulloblastoma) that together make only a small contribution to
the total cancer burden.
The report also excludes, because of the lack of data, common cancers for which
incidence differs substantially between populations and over time.
The latter category includes some of
the most
frequent cancers worldwide, for example those of the stomach, cervix, and breast, each known
to be associated with infections or lifestyle and environmental factors.
Moreover
, the study focuses
exclusively on the United States population as a measure of lifetime risk. The comparison of different
populations would have yielded different results.
Although it has long been clear that the number of cell divisions increases the risk of mutation and,
therefore, of cancer, a majority of the most common cancers occurring worldwide are strongly related to
environmental and lifestyle exposures.
In principle, therefore, these cancers are preventable; based on
current knowledge, nearly half of all cancer cases worldwide can be prevented.
This is supported in
Tomasetti C, Vogelstein B (2015). Variation in cancer risk among tissues can be explained by the number of stem
cell divisions.
Science
. 347(6217):78
–
81.
http://dx.doi.org/10.1126/sc
ience.1260825
2
Stewart BW, Wild CP, editors (2014).
World Cancer Report 2014
. Lyon, France: International Agency for Research
on Cancer.