Pencil beam scanning: The proton therapy innovation of the future

March 06, 2017
By Henry K. Tsai

As our ability to treat cancer improves, patients are living longer. This is an amazing medical accomplishment, but it does elevate an issue somewhat novel to modern cancer treatment: patients often experience consequences of their treatment later in life. Indeed, sometimes the very tool that cures patients can also harm them.

Conventional radiation, or X-ray, therapy is a very common form of cancer treatment for solid tumors that has been used for decades, and is effective for controlling many types of cancer. It’s composed of primary photons and secondary electrons that deposit their energy along the entire path of the executed beam. While effective at eradicating cancer, traditional radiation, in some cases, can cause long-term side effects. Depending on where the patient’s tumor is located, side effects can include heart and lung issues, hypothyroidism, intestinal problems, infertility, cognitive issues and increased risk of other cancers. Fortunately, there has been a groundswell of support among the medical community in recent years to adopt more tailored, precise radiation treatments to mitigate the risk of long-term harm to our patients.

On the forefront of this is proton radiation therapy. As with X-ray radiation, proton therapy destroys cancer cells by preventing them from dividing and growing. Unlike X-ray radiation, it uses protons — positively charged particles — to target tumors. As protons move through the body, they slow down, interact with electrons and release their energy. Protons penetrate tumors more precisely, spreading out its Bragg peak (the point of highest energy release) inside the tumor and reducing the chance that ancillary radiation will reach surrounding healthy tissue. If standard radiation is like a spotlight, proton therapy is a laser beam. But in the last few years, an even more precise radiation “laser beam” has emerged: pencil beam scanning (PBS).

Technique and application
Similar to regular proton therapy, pencil beam scanning places radiation more precisely within a tumor. Pencil beam scanning couples the unique properties of protons with a magnetically-guided scanning system that delicately and precisely moves a beam of pencil point sharpness back and forth across each layer of a tumor’s thickness, painting the tumor with radiation in three dimensions. When the energy of protons is delivered in this way — almost like the paint on the tip of a brush — it results in the most meticulous delivery of radiation available today.

Proton therapy has traditionally used a technique called uniform scanning or double scatter, which relied on physical structures (compensators and apertures) to shape the proton beam around the tumor and limit penetration through the tumor. PBS instead uses a magnet to scan the beamlet and fill in the shape of the tumor. This means a reduction in the time it takes to deliver proton therapy to patients. More importantly, it allows us to shape the proton beam around a target and even further avoid delivering unnecessary radiation to healthy organs nearby.

In other words, it helps to get cancer-blasting beams into the nooks and crannies of complex or irregularly-shaped tumors. While every cancer case is assessed on an individual basis to determine what treatment and radiation course is best, there are some tumor types that can particularly benefit from the ultra-precision of PBS.

• Tumors in the brain, skull base and near/on the eye are very complicated to treat because of the adjacent structures and tissue. PBS can enable radiation oncologists to effectively treat the tumor with less chance of impacting cognitive abilities, vision and motor skills.

• PBS is used a great deal for prostate cancer because the prostate is located in the highly sensitive spot between the bladder and rectum. Many men undergoing treatment for prostate cancer are concerned with side effects such as gastrointestinal issues, incontinence and impotence, which are all a risk with several other treatment options. PBS gives us a better shot at avoiding those issues, which can cause men a great deal of physical and emotional difficulty.

• PBS can be beneficial for some breast cancer cases, particularly when the tumor is in the left breast. Studies have shown that traditional radiation in left-sided breast cancer cases can sometimes cause cardiac issues later in life due to the tumor’s proximity to the heart. PBS’ precision can circumvent radiation to the heart, which is especially important for younger women who have many years ahead of them.

• Pediatric cancers are among the most sensitive cases we confront, since children are still developing. Radiation to the brain, one of the most common locations of pediatric tumors, poses a risk of developmental and cognitive delays. PBS, like with other cancer types, protects critical tissue, and also reduces the likelihood of secondary malignancies later in life.

• Another treatment path to consider for PBS is with cancer recurrence. For someone who has previously had cancer and been treated with traditional radiation, undergoing that radiation course again can either pose major health risks or be altogether not recommended. Proton therapy, and specifically PBS, can be an option for some of these patients because of its tissue-sparing capability.

What’s next for pencil beam scanning?
Pencil beam scanning is still a relatively new technology. The center I treat at is coming up on three years of having PBS available for our patients. Right now, only some of the proton therapy centers across the country have PBS. Cancer treatment facilities, however, are continuously looking for new technologies to bring to their patients, so adopting PBS-optimized systems is an initiative that I imagine most proton centers are eager to have. I am certain we will see more of an embrace of PBS in the coming years. Other changes on the horizon include:

• Updates and improvements to the treatment planning software used to calculate PBS. One specific change will be new features for dose calculation — helping improve the accuracy of proton delivery to patients.

• The possibility of combining PBS with apertures to help sharpen the edge of the pencil beam scan.

• Clinical research more closely examining PBS to assess the depth of benefits PBS can offer cancer patients. Right now, there is a lot of focus on assessing the efficacy of proton therapy in the research setting, but learning more about PBS specifically will enable us to treat a greater variety of tumor types with this tool.

• Increased use of intensity modulated proton therapy (IMPT), which requires PBS in order to operate. IMPT is a technique where, instead of using just a few beams to target a tumor, we’re integrating the radiation dose from multiple beams and coordinating the dose based on the combination of those multiple beams.

Pencil beam scanning is one of the most highly developed tools available to radiation oncologists today. Anytime you can treat cancer through a minimally invasive technology such as proton therapy, it’s a step in the right direction. It is our mission as oncologists to improve the lives of our cancer patients by focusing on curing the cancer and preserving our patients’ quality of life. Proton therapy and PBS enable us to bring this very important value to our patients, and we anticipate further adoption and continued innovation in the coming years.

About the author: Henry K. Tsai is a radiation oncologist at ProCure Proton Therapy Center in Somerset, N.J. Dr. Tsai graduated summa cum laude and Phi Beta Kappa from Harvard College before receiving his medical degree from Harvard Medical School in Boston, after which he went on to complete his residency at the Harvard Radiation Oncology Program where he was named chief resident. Dr. Tsai was trained at Massachusetts General Hospital, where he gained extensive proton therapy experience and knowledge.