Cells are tiny building blocks that form the tissues and organs of the body. Healthy cells develop, divide to make new cells, and die in a naturally well-balanced manner. This is how the body grows and heals.
The body regulates the number of normal cells being produced, so it does not have more than required at any time.
Cancer (malignant) cells are damaged copies of normal cells. They behave and reproduce in an unpredictable way that is not controlled by the body’s defences alone. Cancer cells clump together to form a tumour. They can invade surrounding tissue and travel in the blood or lymphatic system to other parts of the body. Cancer that has spread to another area of the body is called “metastasis”.
Retinoblastoma is malignant eye cancer of infancy and early childhood. Tumours may affect one eye (unilateral) or both eyes (bilateral), and can escape the eye (extraocular). Rarely, a separate tumour may grow in the brain (trilateral).
The word “retinoblastoma” contains three parts.
retino – of the retina.
blast – an immature cell.
oma – a swelling or tumour.
Retinoblastoma arises from immature cells of the retina.
The retina is the light-sensitive layer of cells forming the inner surface of the eye, that converts light into nerve signals for transmission to the brain.
A retinoblastoma tumour is a cluster of tightly packed cancer cells. Sometimes these cells form a rosette shaped pattern. As the tumour grows, the cells also die and often become calcified.
Inside the eye, retinoblastoma is very soft and can easily break apart, and float into the vitreous gel (vitreous seeds) or into the space under the retina (subretinal seeds).
Causes of Retinoblastoma
Retinoblastoma is caused by errors on the RB1 gene (a tumour suppressor gene) or MYCN gene (an oncogene). These errors most often occur spontaneously in the affected child. About 45% of children have a constitutional RB1 mutation that can be passed from one generation to the next, but only 10% of children inherit an RB1 mutation from a parent.
Nearly all retinoblastoma tumours have both copies of the RB1 gene damaged or missing. About 1.4% of children with unilateral cancer and no family history have two normal copies of RB1, but develop retinoblastoma due to too many copies of MYCN.
Sometimes, parents or siblings of a child with retinoblastoma have a retinoma, a benign precursor of retinoblastoma that does not grow. Such a relative will carry the same constitutional RB1 mutation as the child with retinoblastoma, but never developed retinoblastoma.
All types of retinoblastoma are highly malignant, but some grow more aggressively than others, depending on the unique molecular changes that have occurred in the cancer cells. Tumours seem to grow faster in infants, perhaps because the whole baby is growing rapidly. MYCN tumours are especially aggressive.
Cancer cells that survive chemotherapy or other treatments do so because they have acquired biological changes that make them resistant to that therapy. Each recurrence of a cancer is more likely to have become resistant to therapy. This is the nature of cancer progression.