Proton beam therapy is an advanced radiation treatment for certain cancers. It uses a targeted beam of protons (positively charged particles) to deliver radiation to a tumor while reducing exposure to surrounding healthy tissue. The complex machinery involved generates protons, accelerates them to high energies, and guides them precisely to the target within the patient’s body.
Understanding Proton Energy Delivery
The primary advantage of proton therapy lies in the physical properties of protons, which differ from the X-rays used in conventional radiotherapy. When a proton beam enters the body, it travels to a specific depth, deposits most of its energy, and then stops. This point of maximum energy release is known as the “Bragg peak.”
By adjusting the beam’s energy, medical professionals can position the Bragg peak directly within the tumor, ensuring it receives the highest radiation dose. Unlike X-rays that deliver an “exit dose” to tissues beyond the tumor, the proton beam’s abrupt stop minimizes damage to healthy organs and structures behind the target. This precision is a fundamental reason why proton therapy is an explored treatment option.
Decoding the Proton Therapy Machine
A proton therapy system illustrates the path a proton takes from creation to the tumor. The system has several large, technical components working in sequence. The process begins with a proton source, derived from hydrogen gas, where an electrical field separates protons from electrons. These protons are then injected into a particle accelerator.
The most common accelerators are cyclotrons or synchrotrons. A cyclotron uses a large magnet and a radiofrequency field to accelerate protons in a spiral path to a fixed high energy. A synchrotron uses a ring of magnets with varying field strength to accelerate protons to a variable energy level, which can be adjusted during the process.
After leaving the accelerator, protons travel through a vacuum tube called the beam transport system. This corridor is lined with electromagnets that steer and focus the proton beam toward the treatment room. Some systems use a degrader or energy selection system to reduce the beam’s energy to the precise level needed to reach the tumor’s depth.
The final stages involve the gantry and the nozzle. The gantry is a massive, rotating structure, often three stories tall, that allows the beam to be delivered from any angle around the patient. This 360-degree capability is important for targeting the tumor from the most effective direction.
At the end of the gantry is the nozzle, the final component that shapes and delivers the beam. The nozzle can use custom devices like apertures and compensators to conform the beam to the tumor’s specific shape and depth. More advanced systems use pencil beam scanning, where magnets in the nozzle “paint” the tumor with a narrow proton beam layer by layer with millimeter accuracy.
Navigating the Treatment Process
The patient’s journey begins long before the first treatment with detailed imaging using computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET) scans. These scans create a precise three-dimensional map of the tumor and surrounding healthy tissues for treatment planning.
Using this 3D map, a team of radiation oncologists, physicists, and dosimetrists develops a treatment plan. This plan dictates the exact angles, energies, and doses of the proton beams. To ensure accuracy, patients are often fitted for custom immobilization devices, like masks or molds, to maintain the same position for each session.
A treatment session lasts between 15 and 45 minutes, though the proton beam delivery only takes a few minutes. Most of this time is spent positioning the patient on the treatment table using lasers and imaging systems to align with the plan. Therapists then leave the room and operate the machinery from a control room, monitoring the patient via cameras and microphones as the gantry delivers the painless, invisible beam according to the predetermined plan.
Clinical Uses of Proton Therapy
Proton beam therapy is utilized for tumors located near sensitive structures where precision is needed. This treatment is also a common option for pediatric cancers. Because children’s bodies are still developing, reducing the radiation dose to healthy tissues is a primary concern to lower the risk of long-term complications and secondary cancers later in life.
The decision to use proton therapy depends on the type, stage, and location of the cancer. It is used for cancers in areas such as:
- Brain and spinal cord
- Head and neck
- Eyes
- Lungs
- Liver
- Prostate
- Some lymphomas and sarcomas