Proton therapy, or proton beam therapy, is a type of radiation therapy used in the treatment

of cancer. Unlike photon-based forms of external beam radiotherapy, proton therapy enables

an intense dose distribution pattern, depositing radiation in the precise dimensions of a tumor

while eliminating the exit dose and damage to adjacent normal tissue.

Because of its unique dose-deposition characteristics, proton therapy is indicated for treating

tumors near critical organs and brain tissue, and for the treatment of pediatric tumors.

Over the last 40 years, the 5-year survival rate for children diagnosed with cancer has

skyrocketed from 10% to nearly 90%. Yet, because of the radiosensitive nature of developing

tissue, approximately 60% of survivors suffer from late effects, such as growth deficiencies

and secondary cancers. Proton therapy has given pediatric oncologists a promising option in

the complicated matter of treatment planning for child patients.

Children tolerate proton therapy well because it is non-invasive and painless, and it typically

results in fewer side effects. Most importantly, proton therapy may reduce the risk of late

effects, including secondary malignancies, which is of particular concern for pediatric cancer

survivors.

The idea of using protons in the treatment of cancers has been in existence since 1946, with

the first patient being treated with protons in 1954 at the Berkeley Radiation Laboratory. A

limited number of physics laboratories offered proton therapy over the next few decades, as

advancements in imaging, accelerator, and treatment-delivery technology made proton

therapy more viable for routine medical applications. The pediatric population was not

excluded from treatment with protons during this time. In fact, the Mass General Department

of Radiation Oncology has been treating children with fractionated proton radiotherapy since

1974.

The development of pencil beam or spot-scanning technology in proton therapy has

contributed to a more widespread acceptance of proton therapy as a radiation therapy

modality, particularly in the pediatric population.

Pencil beam scanning is the most precise form of proton therapy. Using an electronically

guided scanning system and magnets, pencil beam scanning delivers proton therapy

treatment via a proton beam that is just millimeters wide. With pencil beam scanning, beam

position and depth are able to be controlled, allowing for highly precise deposition of

radiation to be delivered in all three dimensions of the tumor. The technology’s precision

reduces neutron contamination generated by proton scatter and scatter produced from

beam shaping devices required with non-scanned proton beams, thereby reducing the risk

of secondary malignancies.

Not coincidentally, the development of pencil beam scanning technology corresponds with

the rise of pediatric-specific proton therapy treatment programs.

Sparing normal tissue and improving quality of life is important for all patients. But because

of the long natural life expectancy of pediatric patients — and the particularly radiosensitive

nature of developing tissue — pediatric oncologists place heightened emphasis on late

effects.

Childhood cancer survivors often overcome one enormous battle only to encounter another,

months, years, or even decades after treatment has ended. Cosmetic, hormonal,

neurocognitive, reproductive, and other physical impairments are prevalent among survivors.

This is particularly true for pediatric patients whose disease sites occur near the brain stem,

spinal cord, and other sensitive organs.

Of utmost concern are radiation-induced secondary cancers. The relationship between

radiation therapy and secondary cancers has been clarified. For example, the Childhood

Cancer Survivor Study has assessed more than 14,000 childhood cancer survivors to

determine how different treatment plans have affected their long-term health. This data

demonstrates a correlation between radiotherapy and various secondary malignancies,

including, but not limited to:

• Basal cell carcinoma
• Breast cancer
• Gastrointestinal cancer
• Meningioma
• Salivary gland cancer
• Thyroid cancer