About 40% of children develop tumours in both eyes. This is known as bilateral retinoblastoma. The cancer does not spread from one eye to the other. When both eyes are affected and / or multiple tumours form, each tumour arises from a unique retinal cell.
All bilateral retinoblastoma is caused by genetic errors on the RB1 gene which may be inherited from a parent or occur during early embryo development.
Average Age at Diagnosis
In developed countries, the average age at diagnosis is 15 months, but in Kenya (typical of developing countries), it is 26 months due to poor awareness.
When both eyes are affected, multiple tumours usually develop in each eye. Children develop an average of three tumours in total, but as many as 14 tumours between both eyes may occur. The risk of new tumour growth diminishes significantly after three years of age.
Re-growth in old tumours remains a risk for several years after the last tumour activity. Risk of recurrence depends on the size of tumours, previous treatment, and unique molecular changes in the RB1 gene that initiated the cancer.
Genetics of Bilateral Retinoblastoma
All children with bilateral tumours have a constitutional RB1 mutation that may be inherited from a parent, or occurred naturally soon after conception. When parents’ genetic risk is not known, the eyes of their other children under 5 years old should be examined regularly to ensure any tumours are diagnosed and treated early.
Genetic testing determines which children are at risk and which do not need further eye exams. 50% of babies with a known risk are born with tumours already formed.
Second Primary Cancers
Children with bilateral retinoblastoma have a life-long risk of second cancers that is increased by exposure to radiation. Therefore they should avoid radiotherapy, CT scans and x-rays wherever possible. They should also receive genetic counseling as adults so they understand their risks and the potential risks to their children.