Bilateral, multifocal and unilateral types of retinoblastoma

A person with bilateral (both eyes affected) or multifocal retinoblastoma (more than one tumour in an eye) has the heritable form: the altered gene is present in all of their cells.

The situation for people with unilateral retinoblastoma (one eye affected) is more complicated. In 80-85% of unilateral cases most cells in the body will not have the altered gene, and there is a no increased chance of retinoblastoma occurring again or in their families. However, around 15-20% will have the heritable form.

Mosaic retinoblastoma

Mosaicism is the term used when someone has a mixture of cells – some of the cells have a gene change and some do not. Where a child has a mosaic RB1 mutation, the Rb gene is altered in some of their cells (the amount will vary from child to child). If the other normal copy of the Rb gene in a retinal cell becomes damaged, a retinoblastoma can develop. Read more about mosaicism and how it relates to retinoblastoma.

Chromosome 13q deletion

About 5% of people with retinoblastoma (Rb) have lost the part of chromosome 13 that contains the Rb gene. They have what is called chromosome 13q deletion.

From a retinoblastoma point of view they have heritable retinoblastoma. In cases of chromosome 13q deletion other genes may also be lost or damaged and this may cause some physical or developmental delays or disabilities. The amount of genetic information lost varies from person to person.

N-Myc gene amplification

It is thought that around 1-2% of non-heritable forms of retinoblastoma may have a ‘non-Rb gene’ cause. In these cases it is not damage to the RB1 gene which causes retinoblastoma to develop. It is linked instead to the over-expression of the N-Myc gene, a cancer-causing gene which can cause cancers when extra copies are present.

N-Myc gene testing is carried out on tumour tissue, not blood, and therefore is only possible post-enucleation.