as far as i understand, the study merely links the levels of omega-3 acids in blood with the risk of cancer, ie. no causal relationship seems to have been proven. it may be a similar thing as with cholesterol and heart diseases. also, the cancer patients may have simply consumed more supplement as a way to improve their condition.
the full paper is available but requires membership for access: _http://jnci.oxfordjournals.org/content/early/2013/07/09/jnci.djt174.abstract