The radiation oncologist's mantra is to deliver the maximum dose of radiation to the malignant tumor, while limiting damage to healthy surrounding tissue. In proton therapy, this balance is achieved by using proton particles, accelerated to nearly the speed of light, to mimic the shape of a tumor and effectively deposit their energy within the confines of it with sub-millimeter precision.
New tools are enabling physicians at the Proton Therapy Center at The University of Texas MD Anderson Cancer to harness supercharged proton particles and conform them more precisely to the rugged landscape and uneven contours of a tumor. Using a technology known as pencil beam scanning, also known as spot scanning, protons are given the mission: Hone in on cancer cells and destroy. As much an art form as a war tactic, pencil beam proton therapy has the ability to treat the most complex of tumors, like those of the prostate, brain, base of the skull and eye, while leaving healthy tissue and critical structures virtually untouched. The powerful coupling of strength and accuracy offers unmatched capacity to treat a patient's tumor without compromising quality of life during and after treatment.
In nearly a decade since pencil beam's birth in a Swiss physics institute, the world's leading practitioners in radiation science at MD Anderson's Proton Therapy Center have integrated the tested technology into the institution's multidisciplinary approach to patient care and translational cancer research.
A New Frontier for Proton Therapy
Proton therapy derives its advantage over conventional forms of radiation from its ability to deliver radiation doses to a targeted tumor with remarkable precision that avoids the surrounding tissue, which results in fewer side effects and improves tumor control. Most proton patients are treated with a technique known as passive scattering, which uses apertures to shape the proton beam and deliver a uniform dose to the tumor. Since opening in the spring of 2006, MD Anderson's Proton Therapy Center has treated nearly 1,700 patients with this passive scattering technique.
Pencil beam proton therapy delivers a single, narrow proton beam (which may be less than a millimeter in diameter) that is magnetically swept across the tumor, depositing the radiation dose like a painter's brush strokes, without the need to construct beam shaping devices. The technology continues to build on the patient benefits already offered with proton therapy – more targeted, higher tumor dose, shorter treatment times, reduced side effects and increased treatment options – to treat complicated tumors perilously close to critical structures, such as the eye, brain and esophagus.
"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.
"This type of technology, along with our extensive experience in treating more types of childhood cancer than most other proton centers worldwide, continues MD Anderson's mission to provide pediatric patient care with the most advanced, research-based therapies as are available to our adult patients," he added.
MD Anderson has treated over 300 patients with pencil beam to date – both adult and pediatric patients.
Eloquent Treatment Planning Masters Complex Tumors
Pencil beam is only as good as the complex and intricate treatment planning systems used to direct the beam's motion, depth and strength. As these systems evolve to the extent of pencil beam's capabilities, the team at MD Anderson's Proton Therapy Center will tackle cancer's most difficult tumors based on their shape and location in the patient.
"The beauty of pencil beam is that we have the ability to target the tumor with exquisite accuracy and spare surrounding healthy tissue and structures," Mahajan said. "It's best utilized when we need to conform high doses of radiation to irregularly shaped tumors embedded near or wrapped around critical structures in the head and neck, such as the eye or brain." The advantage lies in the beam's capacity to approach the tumor from multiple directions, creating a "U" shape around these structures and avoiding them entirely during treatment. Side effects common after standard radiation therapy are reduced and healthy organs are preserved because the radiation is confined to the tumor.
The future introduction of intensity modulated proton therapy at MD Anderson will also be possible as pencil beam delivery is further developed. Intensity modulated proton therapy uses the same pencil beam configuration, but the energy or intensity of the proton beam can be adjusted at any time to penetrate the tumor at varying depths. "This is the holy grail of radiation therapy," Lee said. "Starting with pencil beam, and then working to develop treatment plans marrying the two together, is necessary to achieve this degree of sophistication for our patients."
Zeroing in on Advances for the Patient
A pioneer in radiation oncology, MD Anderson has paved the way for more effective radiation therapy around the world. The Proton Therapy Center will continue to make strides in the field by making the combination of precision and potency found in pencil beam technology accessible to increasing numbers of patients in a clinical setting. Each patient who receives pencil beam treatment will be part of a growing body of research protocols at MD Anderson, examining proton therapy's benefits over conventional radiation therapy and refining the technology to care for future generations of cancer patients with the best therapies available.
SIDEBAR: A BEST IN CLASS FACILITY
The Proton Therapy Center at MD Anderson stands as an international center of excellence for proton therapy, research and education. Within its 96,000 feet of space the Center houses three treatment rooms equipped with giant gantries – three stories tall, 35 feet in diameter, weighing 196 metric tons – each capable of maneuvering the proton beam to precisely target the patient's tumor. A fourth room utilizes a stationary beam for larger tumors in the body, including tumors of the pelvis. Pencil beam technology is currently installed in one of the gantry rooms and is used to treat adult and pediatric patients.
The Center offers patients:
- Access to the most advanced radiation therapy stateside and MD Anderson's world-renowned research, faculty and multidisciplinary patient care.
- Treatment for the most comprehensive range of disease sites including pediatric cancers and cancers of the head and neck, eye, prostate, brainstem, esophagus, lymphoma, liver and lung, among others.
- Reduced side effects and minimal damage to healthy tissue, which contribute to quality of life during and after treatment and enable patients to live longer, more fulfilling lives.
3D-printed model created to mimic cancer metastasis