There is no easy way to know for us to know what, if any, effect doping had on Lance's cancer. Cancer is a disease caused by the uncontrolled growth of a single cell. This growth is unleashed by mutations, i.e. changes in cell DNA, that affect genes controlling cell hyperplasia. Once this cycle gets going, cancer cells grow and mutate without any of the built-in limitations that normally affect cell growth. Chemicals, including hormones, can play one of two possible roles. First, they can act as mutagens, which alter cell DNA thereby leading to changes that produce the cancerous hyperplasia. Generally, these mutations occur in people already susceptible to the particular mutation that develops. Cancer is essentially lying in wait in our DNA.
Second, chemicals, including hormones, can increase cancer cell hyperplasia. Some cancer cells are sensitive to various hormones. In the presence of these hormones, the cancer cells divide and mutate rapidly. The best example of this are certain types of estrogen-senstive breast cancers. For these cancers, anti-estrogen drugs such as Tomaxifen, which block estrogen receptors on cells, are highly effective treatments.
The problem with attributing Lance's cancer to doping is that, in the first instance, no one can know what specifically caused the initial mutation that led to Lance's cancer. Secondly, unless we know whether Lance had cancer cells that were sensitive to the doping agents he had in his body, we will never know that his doping had an effect on his cancer. Cancer cells are all quite different, and, without testing them for hormone sensitivity, one is simply left to speculate. At the time of Lance's cancer, there was no HGH antagonist with which to treat HGH-sensitive cancers, so it seems unlikely that any testing towards this end was ever done.