"The difference between passive scattering and pencil beam is like painting something with a can of spray paint versus using an airbrush," said Andrew Lee, M.D., M.P.H., associate professor in the Department of Radiation Oncology at MD Anderson, and the director of the Proton Therapy Center. "Pencil beam is more like a very fine airbrush. Instead of needing a brass template to define the shape, the proton beam is made ultra fine to conform to the contours and landscape of a tumor. When all these small beams are combined, they can cover the entire tumor volume with a high degree of conformality. If the tumor is shaped like an egg, then the proton dose will look like an egg."

Rapid Fire with Exquisite Precision


The Proton Therapy Center, which began treating patients with pencil beam in May 2008, continues to be the first in North America and one of only three clinical centers in the world to treat patients with this technology. Because pencil beam does not require any external shaping devices, the treatment is less time consuming on a daily basis than passively scattered beams, with most treatments only taking a few minutes.

Using rapidly fired pulses, the pencil beam hits each planned spot within the tumor with the prescribed amount of radiation, starting at the deepest layer and working in succession, layer by layer, until the whole tumor is covered. Lee estimates that a typical tumor has between 1,000 to 2,000 separate spots arranged in up to 24 layers in a single pencil beam treatment. "We are able to maximize the protons generated and deposit more cancer-fighting energy directly into the tumor," Lee said.

MD Anderson has used pencil beam proton therapy to treat patients with cancers of the brain, prostate, liver and esophagus - and has extended its use to begin treating tumors in pediatric cancer patients. Anita Mahajan, associate professor in the Department of Radiation Oncology at MD Anderson, who treats many of the Proton Therapy Center's pediatric patients notes that it is an especially attractive option for solid tumors in children, who are generally more sensitive to the short- and long-term adverse effects of radiation. "Without the apertures, pencil beam deflects fewer neutrons into healthy tissue, which have been shown to increase the risk of second malignancies in young, still growing patients."

As the only center in the nation treating patients with pencil beam proton therapy, Lee said that MD Anderson can offer children with cancer an even more targeted option to fight cancer and limit damage during and after treatment.

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