What is the difference between sporadic and familial retinoblastoma?

Retinoblastoma develops when both copies of the tumor suppressor Rb gene are damaged. The sporadic form of the disease can affect anyone and is dependent on genetic changes aquired during the lifetime of the affected individual. The familial form of the disease results when affected individuals inherit a defective copy of the Rb gene from one of their parents. In these individuals every cell contains one normal and one defective copy of the gene.