Our bodies are made up of tiny building blocks called cells, sometimes when new cells are being made by the body one is made incorrectly. Usually the body knows that one is wrong and gets rid of it, but sometimes that one cell keeps growing, copies its self and divides to make more incorrect cells. This group of faulty cells is called a cancer or tumour. A tumour which develops in the back of the eye (the retina) is called a retinoblastoma (Rb). For some people who had Rb a cancer just grows by chance or by accident because the body didn't get rid of the wrong cell.
Some people can develop a retinoblastoma (Rb), not by chance but because someone in their family has Rb. If a family member has Rb, the chances of another family member getting Rb may be higher. This is because some people have a type of retinoblastoma which is in their genes, it is known as genetic or heritable retinoblastoma.
Genes are the instructions in the cell and they tell the cells how to grow. People with genetic/heritable retinoblastoma have a mistake in the instructions in their cells. This mistake can be in all or many cells in the body. Genetic/heritable retinoblastoma can be passed from generation to generation. It is possible for more than one person in a family to have this type of Rb. If someone has the genetic type of Rb they have more chance of developing retinoblastoma tumours and also in some cases other cancers too. Not everyone who has genetic/heritable retinoblastoma will get retinoblastoma themselves. Special genetics blood tests have to be designed to find this out for each person. Also screening under anaesthetic and follow up clinics will check each person who has genetic/heritable Rb for their risk of developing cancer.