If a seemingly perfectly healthy person is exposed to toxins, then they may get cancer. And many seemingly healthy people are actually deficient in key nutrients that are needed for optimum health and that is particularly true if they try to rely on diet alone. In today's mineral depleted soils and foods on our grocers shelves which have had most of the nutrition processed out (even as harmful toxins are processed in), which may be replaced, if at all, with crushed rock minerals and synthetic petrochemical vitamins, it is literally impossible to get optitmum nutrtition from diet alone. It is likewise impossible to have an optimum immune system, our first line of natural defense against cancer and other illness, when one has deficiencies in key nutrients.
It may be, as my friend suggested, that Iodine deficiency could play a key role - but based on my decades of research into cancer I would not bet my life on it. Though I would surely make sure that I had plenty of iodine, as well as it's vital co-factor selenium and do the same with any other deficicencies which can be identified. You might check with CureZone sponsor Utopia-Silver - they have a very reasonable hair test which will identify mineral deficiencies. There is also a product named IntraMax which I think is the best all around nutrtitional supplement on the planet. It contains a whopping 415 differect organic, whole food derived nutrients and the body utilizes every single one. Compare that to the woefully inadequate RDA list touted by mainstream medicine.