there is absolutely a risk of increasing the rate of growth of cancerous cells through igf-i. this seems to depend on the type of cancer. many studies have indicated a big part of why nolvadex is so good at stopping all kinds of breast cancer, even ones that have mutated past the need for estrogen, is that it also suppresses igf-i levels severely. people with acromegaly(excessive GH production) are at a much greater risk for cancers like colon cancer. at the same time, there is no demonstrated link between serum igf-i, igfbp, or other GH-related levels and cancer risk or mortality. so, it may be there aren't many cancers reliant on serum igf-i for their growth. this would be consistent with the idea that autocrine production of igf-i triggered by gh is much more important than hepatic igf-i for most of these organs.
needless to say, this is a subject of intense controversy in the medical community right now as GH is prescribed to aging patients. there is no consensus and more epidemiological and clinical data is needed.
if you're young i wouldn't worry too much about it, but this is one of the greatest risks of igf-i use.