Treatment of Eye Melanomas - Using Proton Therapy

The Proton Treatment Facility at TRIUMF is dedicated to treating a cancerous growth on the back of the eye, called choroidal melanomas. Before proton treatment became available, the most common course of action was removal of the eye. Other possible treatments included surgical removal of the tumour (which has severe limitations), or implanting a radioactive disk on the wall of the eye under the tumour for some days. These alternatives were unsuitable for large tumours, and could damage sensitive parts of the eye, often resulting in loss of vision. After proton therapy, however, patients can retain useful vision. The protons enter the eye at a carefully controlled energy, and come to rest at a precise, predictable distance inside. They deposit their energy of motion (kinetic energy) in a very narrow layer, destroying living cells in that layer. Because the beam of protons is so concentrated and deposits its energy so predictably, we can successfully destroy a tumour while better preserving the other nearby parts of the eye.

 

Image of eye showing tumour location
Protons

 

Proton Therapy Treatment Process

The ophthalmologist, in conjunction with the radiation oncologist and the patient, decides which type of therapy would be the most effective, based on size and location of the tumour and physical attributes of the patient.

If proton therapy is chosen, these would be the steps undertaken:

The ophthalmologist would first perform a small operation to attach tiny tantalum clips to the outer surface of the eye, which defines the boundaries of the tumour. This is performed under general anaesthetic at the Vancouver Hospital and Health Sciences Centre. The patient's eye and tumour is precisely measured using ultrasound.

Next, the patient comes to the Proton Treatment Facility to have the mask and bite block made, and X-ray photographs taken while sitting in the treatment position. These are just regular X-rays, and not the ones used in X-ray therapy. The markers (called clips) that were sewn on during the operation show up in the X-rays.

The X-rays are then digitized and the position of the clips are used to create a computer model of the patient's eye and tumour. With the model, the ophthalmologist and the radiation oncologist, together with the medical physicist, determine the best treatment position.

Following the development of a treatment plan, a custom beam aperture is designed and the energy of the beam is tailored specifically for the treatment of that particular eye. Once this is determined, technicians test the treatment chair to make sure that everything works properly. The patient is now ready for treatment. The treatment consists of four identical treatment doses spread out over four days. The actual treatment lasts approximately 50-80 seconds.

 

Patient receiving proton therapy