Shaneystar.......why was your husband's anal cancer missed? reason, because they we not looking at the anus.....he was having a colonoscopy and they were looking at the colon.
An inexperienced surgeon can easily slip the scope passed the anus without giving if a second glance.......it shouldn't happen. but it does and in other parts of the body.
Another reason may have been that at the time of colonoscopy. it wasn't visible by the naked eye and it grew rapidly in six months.