Doping

It does not mention whether cancer rates are any different among the cyclists - I would expect them to be at least slightly lower, although the substance abuse might well outweigh any beneficial effects of the exercise.

Nutrition and physiology are way too complex, and largely still unknown in details to be discussed at any length and certitude in an internet forum, but with that proviso in mind I shall add a few remarks.

You should actually expect the various forms of cancer to be on an increase among pro cyclists, or among any other pro sportists with similar level of substance abuse, and with that level of unphysiological physical overload. Physical overload and continuous mental distress in fact compromise the immune system, which is responsible for early detection and destruction of cancer cells (cancel is thought to be a natural phenomena where some cells "go mad" and start to multiply for no reason; that process become an illness when the immune system lose control over the natural occurrence of cancer. That is one of the hypotheses).

The other important point is that, from what I read published, pros' regimes are largely based on pseudo science: evidently deep down they are regarded as disposable, and prescribed anything that works in the short term. For instance the energy (glucose/fructose) drinks used in the sport are extremely harmful as not only they do not provide any benefit (several hours - four or more - are necessary for the ingested drinks to get to the muscle tissue in a form ready to use; therefore they do not fulfil the advertised role of "replenishing" the energy levels), but long term they wreak havoc in the insulin system; and they provide, figuratively speaking, an avalanche of energy that the body find very difficult to use in the most efficient way. The detailed step by step of the cancer process is still unknown, but one think is certain: cancer cells to multiply need a lot of energy (cutting the energy supply to cancer cells was - or perhaps still is - one of the lines of research for treatments). With this circumstantial information in mind it is not entirely baseless to speculate that energy dense supplements and medications/supplements aimed to influence body's hormonal balance and push it into overdrive, on and off repeatedly, may have an indirect canceronenic effect.

Sorry for the (relatively) long post.