Research Spotlight: Comparing Proton Beam Therapy and Plaque Brachytherapy in Ocular Melanoma

Alexei Trofimov, PhD, a Medical Physicist in the Department of Radiation Oncology at Massachusetts General Hospital and Assistant Professor of Radiation Oncology at Harvard Medical School, is the lead author of a new study in the International Journal of Radiation Oncology Biology Physics, A Systematic Comparison of Dose Distributions Delivered in 125 I Plaque Brachytherapy and Proton Radiation Therapy for Ocular Melanoma.

What was the question you set out to answer with this study?

Proton beam radiation therapy and plaque brachytherapy using radioactive iodine seeds are both established treatments for ocular melanoma (a rare form of cancer that affects the eye), and both options are available to the patients within our joint MGH-MEE Ocular Melanoma Program.

Our group was interested in characterizing how the dose distribution delivered with these two therapeutic radiation modalities varies depending on the size of the tumor and its location with respect to the critical structures of the eye, such as the optic disc, retina, eye lens.

We were particularly interested in identifying specific tumor presentations for which either protons or plaques provided significantly better sparing of critical structures.

What are two or three key takeaways?

We identified over 100 cases of interest: tumors differing in size and location within the eye.

Then radiation treatment plans were created for these cases with the software used for clinical treatments.

Although the tools we used were not exactly new or unique, we are one of the very few hospital systems with expertise in both radiation modalities.

What were your conclusions?

The attractive physical property of proton beams is that they can be stopped at the desired depth and can completely spare healthy tissues posterior and lateral to the irradiated tumor.

One interesting finding of our study was that although protons in a typical treatment could completely spare the center of the eye lens, the dose delivered at the periphery of the lens was more effectively reduced with plaques in most cases.

Similarly, protons were able to completely spare large sections of the retina, but the areas irradiated to 50% of the prescription dose and higher were more tightly conformed to the target in plaque treatments.

Plaque treatments also showed dosimetric advantage in sparing of the optic nerve and fovea in nasally-located targets, especially those of larger circumference and height. Protons did fare better in temporal, superior and inferior locations.

Free access to the article (no sign up, registration or fees are required) is provided until November 13, 2022 at the following link https://authors.elsevier.com/a/1fowu1Hx52CHPc.

What are the Clinical Implications?

Our team identified several important distinctions between proton and plaque dose distributions. Dosimetry however is only one of the factors affecting care decisions. We anticipate that our findings will be useful to providers and patients in making decisions pertaining to individualized treatments.

What’s Next?

One aspect that was not considered in our comparison is the difference in the biological effect of doses delivered by proton beams and gamma-radiation from iodine plaques.

Not only different species of radiation particles are used in these therapies, but also the duration of the treatment, and the rate at which the dose is delivered vary substantially.

Future research will improve our understanding of the biological effects of different types of radiation on the tissues and critical structures of the eye.

About the Massachusetts General Hospital

Massachusetts General Hospital, founded in 1811, is the original and largest teaching hospital of Harvard Medical School. The Mass General Research Institute conducts the largest hospital-based research program in the nation, with annual research operations of more than $1 billion and comprises more than 9,500 researchers working across more than 30 institutes, centers and departments. In July 2022, Mass General was named #8 in the U.S. News & World Report list of "America’s Best Hospitals." MGH is a founding member of the Mass General Brigham healthcare system.