Toxicology of Industrial Compounds

360 CARCINOGENESIS AT LOW DOSE
years. This is equivalent to an increase from 40000 to 40001 as a
cumulative incidence 0–64, but it has a completely different meaning. It can
now be excluded that the additional individual would have lived tumour
free for 80 years in the absence of the exogenous carcinogen.
Final remarks
With the ideas presented, fear of cancer from low dose or from rare
exposures can possibly be reduced. This does not mean that small cancer
risks should be tolerated. Carcinogens in the environment, for instance,
affect all of us; the tumour-free life span is reduced in the entire
population.
The model should be valid for tissues with a high spontaneous tumour
incidence and an exponentially steep age dependence indicative of a
multistage requirement of 5–6 steps. For cells that can be transformed in 2
or 3 steps or that are specifically sensitive in certain phases of the
development (in utero or during childhood), the model has to be
reconsidered. This might be necessary for tumours with incidence peaks at
a young age (leukaemia, tumours of the lymphatic tissues, brain, testis).
Nevertheless, the latter tumour types are rare in comparison with cancer of
the old age so that the concept should hold for the majority of the tumours
in humans.
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