Proton Beam Therapy for Cancer Treatment

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What is Proton Beam Therapy?

Proton beam therapy, also known as proton therapy is a type of external radiation treatment or radiotherapy that uses protons [positively charged particles] to treat cancer.

The proton beam therapy is different from the commonly used photon beam radiation which uses x rays or gamma rays to treat different types of cancers. The rays from a photon beam destroy both cancers as well as normal cells whereas proton therapy on the other hand, deposits much less radiation in the normal tissues, thus minimizing damage to healthy tissue. The radiation dose is deposited over a narrower range and the exit dose is also much lesser.

Proton beam therapy as a treatment for cancer was first proposed by Robert R. Wilson, a professor of physics at Harvard and designer of Harvard's cyclotron (a machine used to accelerate charged particles to high energies) in 1946.

Fundamentally, all tissues are made up of molecules with atoms as their building blocks. In the center of every atom is the nucleus containing protons (positively charged). Orbiting the nucleus of the atom are negatively charged electrons. The proton beam therapy uses this positively charged particle, the proton, accelerates its speed, and generates very high energy levels which are used in cancer treatment.

Proton therapy may be used alone or it may be combined with other treatments such as surgery, chemotherapy, and immunotherapy.

Studies have shown proton therapy to be effective in treating different types of tumors of the prostate, brain, head and neck, central nervous system, lung, and gastrointestinal system. Proton therapy is also used for treatment of cancers that cannot be removed completely by surgery.

How does Proton Beam Therapy Work?

Proton beam therapy uses special machines such as cyclotron and synchrotron.

The proton originates from the ion source, where hydrogen atoms are separated into negatively charged electrons and positively charged protons.

The separated positively charged protons are injected via a vacuum tube into the cyclotron where they are accelerated. Here the energy of the proton reaches seven million electron volts in just a few microseconds.

In the particle accelerator the charged particles are propelled by an alternating electric field between two large electrodes in a constant magnetic field created by two large magnets. The particles are injected at the center of the magnet and spiral outward as their energy increases.
Then these high power protons are sent through an energy selection system and a degrader that adjusts their energy. The beam transport system conducts the protons with the right energy and trajectory.

After leaving the synchrotron or cyclotron, the proton beam enters the treatment room via a gantry which is controlled by a network of computers and safety systems. The gantry can revolve 360 degrees, allowing the beam to be delivered at any angle. Hence a high accuracy of patient positioning is required.

The cyclotron can generate and accelerate protons to speeds up to 60 percent the speed of light and energies of up to 250 million electron volts.

At maximum energy, a proton beam travels 125,000 miles per second, which is equivalent to two-thirds the speed of light.

High energy protons travel deeper in the body compared to low energy protons. These accelerated protons are then beamed into cancerous cells where they deposit the specific radiation dose in the tumor, killing them.

Unlike regular radiotherapy, in proton beam therapy the beam of protons stops once it reaches the cancerous cells; the radiation does not go beyond the tumor. In contrast, with photon-based external beam radiation therapy, x-rays continue depositing radiation as they exit the body. This means that the radiation leaving the body may damage nearby healthy tissue. This damage can cause long lasting side effects.

Proton Beam Therapy

How is the Procedure Performed?

Proton beam therapy is typically performed on an outpatient basis. For most tumors the standard duration of course of treatment is two to eight weeks.

The treatment is delivered five days per week. The duration of each treatment varies according to the tumor type and stage. The delivery of the proton beam to the patient lasts only a few minutes. However, the total time spent in the treatment room is about 15 to 30 minutes for positioning the patient and adjusting the equipment settings.

After entering the treatment room the patient is fitted with an immobilization device to ensure that the patient is properly aligned and immobile.

During the treatment, radiation therapists continually monitor the progress of radiation delivery. Once the prescribed radiation dose has been delivered, the computer shuts off the proton beam.

Who can Benefit from Proton Beam Therapy?

People of all ages suffering from cancer can benefit from proton beam therapy.
  • However proton therapy is typically used for cancers that have well defined borders, have not spread to other parts of the body and for treating tumors located near organs or other structures that are sensitive to radiation.
  • It is particularly useful in cancer treatment in children so as to prevent any collateral damage to tissue that is close to the tumor. This reduces the possibility and intensity of side effects in their growing up period.
Proton therapy is most valuable in the treatment of tumors in areas such as:
  • Brain and spine
  • Breast
  • Lung
  • Bone
  • Gastrointestinal (colorectal, esophageal, stomach, pancreatic)
  • Prostate
  • Childhood cancer
Proton Therapy Is Effective in Treating Following Tumors

Is Proton Beam Therapy Effective?

According to a trial led by Massachusetts General Hospital (MGH) in Boston and published in The Lancet Oncology, proton therapy is as effective as standard photon or X-ray radiotherapy in treating the most common type of malignant brain tumor in children.

The current evidence suggests that proton beam therapy is unlikely to cure more people than current conventional radiotherapy techniques. However it appears to reduce adverse side effects.

Scientists have found that survival rates and the incidence and type of tumor recurrence were similar for the proton therapy patients to what has been reported for photon radiotherapy in other studies.

Follow up studies in prostate cancer patients show that incidence and long term effects of urine incontinence, bowel disturbances and sexual dysfunction were the same in patients who underwent either proton therapy or conventional radiation therapy.

But a significant finding is the absence of any secondary effects on the heart, lung, and intestine. All these side effects have been reported in photon therapy studies.
Although studies indicate that proton beam therapy causes less damage to the surrounding healthy tissue but more research is needed to establish the fact whether proton therapy is superior to photon therapy in destroying malignant tumors. There are no long-term follow-ups of patients who have undergone proton beam therapy.

In proton beam therapy, skin exposure at the entry point is greater; however, tissues beyond the tumor receive little or no radiation. X-ray therapy causes slightly less damage to the skin and superficial tissues, while proton therapy causes lesser damage to deeper tissues in front of and beyond the tumor tissue.

Also as proton beam therapy is usually reserved for some types of cancer, it is hard to gather systematic evidence about its effectiveness when compared to radiotherapy.

What are the Benefits of Proton Beam Therapy?

Proton therapy has several benefits which include the following:
  • Proton radiation, once inside the patientís body, has a very short half-life. After patients complete their treatment, they can leave the treatment room without any risks or radiation exposure to others.
  • It allows for a higher radiation dose to the tumor. This increases chances that all cancer cells are destroyed.
  • It causes fewer and less severe side effects during and after treatment.
  • It may deliver up to 60% less radiation to healthy tissue around the tumor, lowering the risk of damage to these tissues.
  • Clear benefits of proton therapy over photon radiotherapy have been shown for patients with resistant neoplasms such as chordomas (a rare type of cancerous tumor that can occur anywhere along the spine), chondrosarcomas (cancer of bones and soft tissues), and ocular melanomas (cancer of pigment cells), all of which require high radiation doses. These cannot be achieved with photon therapy owing to exit dose limitations.
  • Complex head and neck tumors which could not be treated earlier are now treatable using proton therapy.

What are the Side Effects of Proton Therapy?

  • Nausea, vomiting
  • Diarrhea
  • Hair loss
  • Headaches
  • Tiredness
  • Loss of appetite
  • Skin redness and soreness surrounding the body part which is being treated.

What are the Disadvantages of Proton Beam Therapy?

  • The highly specialized and expensive equipments required in proton therapy are available only at few hospitals and medical centers. Hence the treatment is not easily available.
  • It is more expensive than conventional radiation therapy.
  • Many insurance providers do not cover this treatment.
  • Studies indicate that more research with enough follow up data are required to rigorously evaluate the clinical advantage of proton therapy. This will help us to understand how it compares with other advanced external beam radiation therapy methods.
  • Proton therapy is mostly used to treat childhood cancers and cancers of the eye, brain, and spinal cord in adults. More data is required to see the effects on other types of cancer.

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