PROTON THERAPY

Proton beam therapy is a type of radiotherapy that uses a beam of high energy protons, which are small parts of atoms, rather than high energy x-rays (called “photons”) to treat specific types of cancer.
Proton beam therapy enables a dose of high energy protons to be precisely targeted at a tumour, reducing the damage to surrounding healthy tissues and vital organs which is an advantage in certain groups of patients or where the cancer is close to a critical part of the body such as the spinal cord.

Proton therapy cancer treatment begins when each proton begins its journey at the injector located within an electric field. In the field, hydrogen atoms then separate into negatively charged electrons and positively charged protons. The protons travel through a vacuum tube within a pre-accelerator. This process boosts their energy to two million electron volts. The protons continue in the vacuum tube and begin their high-speed journey in the synchrotron. They travel around the synchrotron about 10 million times per second. Each time they circulate, a radiofrequency cavity within the ring delivers a boost of energy. This increases the protons' energy to between 70 and 250 million electron volts. The voltage achieved is enough to place them at any depth within the human body.

Fig shows the superconducting Synchrotron and the Proton Beam Transport System

Beam Transport System

 
After leaving the synchrotron, the protons move through a beam transport system, continuing in the vacuum tube through a series of steering and focusing magnets that guide them to the proton treatment rooms. Each proton treatment room has a beam delivery system, or nozzle, is the last device the protons travel through before entering the body. The nozzle shapes and spreads out the proton beam in three dimensions. 

Fig above shows Proton Magnet focus the beam and direct it into each treatment room.

Radiation oncologists must determine location, shape, and tissue density of the target tumor before determining the number of protons to deliver. They must also calculate the depth that the protons must travel in order to calculate the speed and shape of the beam. These decisions render a beam that is highly accurate and practically ‘tailor made’ for a specific treatments.

Treatment Gantry of a Proton Therapy System

After leaving the nozzle, the protons enter the patient's body.

The equipment in the proton therapy treatment rooms vary based on the conditions treated. One proton treatment room may have a stationary beam with two branches – one branch for irradiating eye tumors and the other for central nervous system tumors and tumors of the head and neck. The other treatment rooms may have gantries – wheels that are 35 feet in diameter that revolve around the patient to direct the beam exactly where needed. From the patient's perspective, all that is visible is a revolving, cone-shaped device.

Proton beam therapy is only suitable for certain types of cancer, such as highly complex brain, head and neck cancers and sarcomas as it does not lead to better outcomes for many cancer cases than using high energy x-rays, which is still considered the most appropriate and effective treatment for the majority of cancers.

Like high energy x-ray radiotherapy, proton beam therapy is painless, but patients may experience side effects similar to those experienced from other forms of radiotherapy.

How Does Proton Therapy's Effectiveness Compare to IMRT or Other X-ray Treatments?

Because proton beams can be delivered in higher doses and with far more accuracy, proton therapy typically can control cancer with fewer treatments than IMRT. This pinpoint accuracy also results in fewer long-term side effects (since the radiation does not spill over and damage healthy tissue and organs) meaning that patients treated with proton therapy experience a higher post-treatment quality of life as compared to IMRT and even conventional x-ray treatments.

Is Proton Radiation Therapy Ever Combined?

Yes. Conformal proton therapy is often used in conjunction with X-ray therapy. This method boosts the dose to sites of gross disease and allows irradiation of a large tissue volume. Depending on the amount of cancer within a particular lymph node and type of cancer that is present, a patient may be at risk for harboring microscopic nests of cancer cells within the nodes. These nodes may lie at some distance from the primary tumor and may not be irradiated if conformal proton treatment alone is delivered to the tumor.
The objective of the treatment plan is to treat both the primary tumor and any areas where a microscopic tumor might hide. X-ray treatment alone will limit the total dose of radiation that can be given due to the high doses it delivers to large amounts of healthy tissue. Therefore, conformal proton radiation therapy is used to treat the primary tumor, and is then followed by X-ray therapy to treat the regional nodes. By giving some of the treatment with conformal protons, the total X-ray dose can be reduced substantially.
This reduces the risk of complications and permits treatment of potentially involved lymph nodes. Microscopic cancer within these nodes might be missed if X-rays were not used.

Side-Effect

Since proton therapy allows the radiation to unfold directly in the tumour, the surrounding tissue and organs are protected to the best of their ability. If a reaction – i. e. a side effect – occurs, only the irradiated body region is usually affected. This can lead to irritation of the skin or mucous membranes, which usually recede completely within two to three weeks after treatment. Sometimes, however, a kind of permanent scarring can also occur as a late consequence.

Information on this page is provided for interest only on a "best efforts" basis and does not 
constitute personal advice. Always discuss medical conditions and related matters with your doctor.
 

Reference: https://protons.com/proton-advantage/how-does-proton-therapy-work