Proton Beam Versus Plaque

By Paul T. Finger, MD

Proton Beam Radiotherapy

Proton Beam Radiotherapy- Graphic demonstration
Proton Beam Radiotherapy: Graphic demonstration of an anterior beam moving through the eye to the tumor

This form of external beam irradiation typically involves directing a column of radiation through the front of the eye (anterior segment, eye lids and/or orbit) in order to reach the intraocular tumor. Since the radiation typically enters the front of the eye, eyelash loss, eyelid excoriation, corneal neovascularization and ulceration, dry eye, neovascular glaucoma, and cataract are more common after treatment with proton beam radiation therapy (compared to low-energy plaque radiation therapy). Some of these effects may occur within weeks of treatment, others take years to develop.

In order to aim the tube-shaped proton beam, surgical clips are sewn (surgically placed) on to the eye, around the tumor’s base. Then the patient returns for 3 to 5 daily outpatient treatments. During treatment, the eye must not move. Eye movements displace the column of radiation away from the tumor, causing unnecessary radiation of normal ocular structures. Therefore, the eye must be closely monitored for movement while the radiation beam is traveling through the eye.

Eye cancer specialists commonly suggest protons or other external beam treatments when the tumor is touching (juxtapapillary) or surrounding (circumpapillary) the optic nerve. Since beam therapy can include the entire tumor (plus a tumor-free margin), beam therapy is likely to destroy the tumor. However such treatment places the optic nerve within the targeted radiation zone that also results in radiation-related optic nerve damage and loss of vision (in that eye).

VS. Eye-Plaque Radiotherapy

Eye-Plaque Radiotherapy: Graphic demonstration
Eye-Plaque Radiotherapy: Graphic demonstration of plaque radiation as it moves through the eye in treatment of a posterior tumor.

Plaque Therapy typically involves attaching a dish-shaped radiation source beneath the tumor and leaving it there for 5-7 days.

Compared with proton-beam, the front of the eye usually receives much less radiation with plaque radiation therapy, but parts of the back of the eye receive more. This is why anterior “front of the eye” complications (eye lash loss, severe dry eye, neovascular radiation glaucoma) are unusual after low-energy (iodine-125, palladium-103) ophthalmic plaque radiation therapy.

Tumors that touch the optic nerve are more difficult to treat with ophthalmic plaque radiation therapy. Special notched plaques have been used to treat certain tumors that touch the optic nerve. Dr. Finger has recently developed specialized “Finger’s slotted plaques” for treatment of tumors the touch or encircle the optic nerve (see below). Due to the advent of super-sized 24 mm (for extra-large tumors) and Finger’s slotted eye plaques (for tumors around the optic nerve), fewer than 8% of patients require enucleation (removal of the eye) as treatment for choroidal melanoma (at The New York Eye Cancer Center). Clearly, ophthalmic plaque radiation therapy is the most widely available and most commonly used eye and vision-sparing treatment for choroidal melanoma.

An Overview of Plaque Radiation Therapy in Treatment of Choroidal Melanoma (1,2)

*Note* These results were published largely prior to the discovery of intraocular anti-VEGF therapy for radiation retinopathy and optic neuropathy.

Plaque Therapy Chart: Published results after several forms of plaque radiation therapy.
Plaque Therapy Chart: Published results after several forms of plaque radiation therapy.

Summary

When possible, most centers offer radiation as an eye and vision-sparing alternative for patients with intraocular cancer. The two main types of radiation are eye-plaque brachytherapy (iodine-125, ruthenium-106, palladium-103) and external beam (Proton-Beam). Radioactive plaques are more commonly used and available in more centers throughout the world.

Side effects/complications

An Overview of Plaque Radiation Therapy in Treatment of Choroidal Melanoma (1,2)

Related links

References

  1. Finger PT. Radiation Therapy for Choroidal Melanoma. Survey of Ophthalmology (Review Article) 42:215-32, 1997.
  2. Finger PT., Berson A, Ng T, Szechter A. Palladium-103 Plaque Radiotherapy for Choroidal Melanoma: An 11-year study. Int. J Radiation Oncology Biol. Phys. Vol 54, No. 5, pp. 1438-1445, 2002.

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