An essay by Dr. Emma Holliday, PGY5 radiation oncology resident, Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, and Dr. Steven J. Frank, associate professor, deputy department chair for Strategic Programs and Proton Therapy Center medical director, Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center.

Radiation therapy is an important component of treatment often used for patients with head and neck cancer, and this is especially true when cancers arise in places where surgery would be difficult, dangerous or disfiguring. This is exemplified by cancers of the nasopharynx, the area between the soft palate and the nasal cavity which lies in close proximity to critical structures, such as the pituitary gland, optic pathway, spinal cord and parts of the brain. Although rare in the U.S., nasopharyngeal carcinoma (NPC) is quite common in Asia with an incidence of approximately 85,000 new cases diagnosed each year, resulting in 52,000 deaths from the disease worldwide.

Radiation, usually accompanied by chemotherapy, has been the mainstay of treatment for NPC for several decades. Since the 1990s, intensity-modulated radiation therapy (IMRT), a more conformal form of photon-based radiation, has been favored in the treatment of NPC because of its ability to preferentially treat the tumor while avoiding radiation dose structures, such as the major salivary glands. However, even with IMRT, an exit dose can cause side effects, such as nausea, mucositis and pain with swallowing that can lead to dehydration, malnutrition and possibly the need for a feeding tube. Recently, The University of Texas MD Anderson Proton Therapy Center has been investigating the use of proton therapy for head and neck cancers such as NPC to reduce unnecessary radiation to normal tissue even further and to reduce the side effects of patients undergoing treatment for NPC.
Proton therapy is different from traditional photon-based radiation because of the unique physical properties of protons when compared to photons. When photons are given a therapeutic energy, they deposit both entrance and exit dose on the way to and from the intended target because they have no mass to slow them down. Protons are heavier, so when they are given a therapeutic energy, they stop in tissue after delivering their intended dose to the target. This eliminates the exit dose and potentially reduces side effects associated with excess, unintended dose to normal tissue. Intensity-modulated proton therapy with multi-field optimization (IMPT) is the most technologically advanced and sophisticated way to deliver proton therapy. It utilizes a magnet to move a narrow “pencil beam” of protons in a pattern determined by CT-based 3D mapping of a patient’s individual tumor. Dose can be “painted” layer-by-layer at different depths, from different angles, and with different intensities. Currently, there are few centers in the U.S. that possess the capability to deliver IMPT, however, MD Anderson has been implementing this technology for head and neck cancers such as NPC since 2008.

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