If Retinoblastoma Requires Mutation in Two Copies of the RB

Short Answer

If retinoblastoma requires mutation in two copies of the RB gene, and familial cases inherit one such mutation, doesn't it seem unlikely that a second mutation would occur in a retinal cell at this specific gene? Yet 90 percent of those individuals who inherit one mutant RB gene develop a second mutation and retinoblastoma. Why does this occur with such a high frequency?