An Effective and Powerful Tool

Proton therapy is an effective radiation treatment for many types of tumors. Unlike standard X-ray radiation therapy, protons deliver less harmful radiation to healthy tissue, which is of particular importance when a tumor is close to a critical organ or structure such as the brain or spinal cord. Patients who are treated with protons may experience fewer short- and long-term side effects compared to the standard forms of radiation therapy.

Clinical Benefits

Proton therapy plays an essential role for tumor sites requiring a high degree of dose conformality because of their proximity to radiosensitive structures. Tumors such as brain, head and neck, pediatric, and prostate are a few of the type of tumors that may benefit from being treated with protons.

The clinical benefits of proton therapy over standard X-ray therapy include:¹

• Improved tumor control
• Reduced incidence of secondary tumors
• Fewer treatment related short- and long-term side effects
• Extended survival rates
• Potential of increasing the daily fraction dose, thus reducing the number of fractions required for treatment

Proton therapy is generally preferred for treating solid tumors in children, where great emphasis is placed on reducing any irradiation of normal tissues to prevent serious complications and reduce the incidence of secondary tumors. When compared to standard X-ray therapy, proton therapy has been shown to reduce the risk of growth and developmental abnormalities as well as secondary malignancies in certain pediatric populations.²

In addition, clinical studies report that more than half of men treated with standard X-ray radiation for prostate cancer experience short-term side effects such as diarrhea and painful urination. By comparison, proton beam radiation, when used in the treatment of prostate cancer, can result in minimal toxicities and morbidity.³

• Clinical Literature

Clinical Indications

The improved dose distribution of proton therapy makes it simpler to conform the radiation pattern to the tumor site. Many patients with cancers and solid tumors can benefit from proton therapy.

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Protons Vs. X-rays (Photons)

The options in external beam radiation include photons and protons. Proton therapy is ionizing, high-energy radiation that uses the same mechanism to attack cancer cells as standard X-ray (photon) radiation.

Higher doses of radiation are associated with better cure rates. However, as the dose increases in X-ray radiation, the number of complications rises dramatically. This is not the case with protons.  The unique dose distribution of proton therapy, coupled with its ability to precisely conform radiation dose to the tumor site, allows for higher doses of radiation during treatment without a substantial increase in side effects.

The Bragg Peak

X-rays (photons) deliver substantial doses of radiation both anterior and posterior to any tumor volume. For even the most energetic X-ray beams available, the depth at which the maximum dose of radiation is delivered ranges from as little as 0.5 mm to a maximum of 3 mm. Because a tumor is almost always located deeper than this range, a higher dose is invariably delivered to the normal tissues anterior to the tumor.

This can be overcome to some extent by bringing in X-ray beams from multiple directions, allowing the dose to sum within the tumor volume. Since the X-ray beams travel throughout the entire thickness of the body, all normal tissues from the entrance area to the exit of the beams are affected. The integral dose to the patient is substantially greater.

In contrast, the dose deposited by a proton beam increases gradually with increasing depth until close to the maximum proton range, and then suddenly rises to a peak, known as the Bragg Peak. A proton beam can be directed so that the Bragg Peak occurs precisely within a volume of about a 3-5 mm radius, something that can almost never be done with X-rays. This spot can be moved anywhere in three dimensional space within the body. Protons also can be accelerated at different energies and intensities, which widens the Bragg Peak so that a larger volume of tumor can be treated. This phenomenon is known as the Spread Out Bragg Peak (SOBP).

With proton therapy, the net dose to healthy tissue surrounding the tumor volume can be much less than to the tumor, sparing normal tissue in the area. The dose immediately beyond the Bragg Peak is essentially zero, which also allows for the sparing of normal tissues distal to the tumor volume. Side effects, both acute and long-term, typically seen with X-ray therapy can be markedly reduced with proton therapy.

Challenges with Proton Therapy

Protons provide significant therapeutic benefits due to their precise dose distribution. However, protons also present several challenges - challenges that for the most part are being addressed with improved technology and best practices.

• The precision of proton therapy demands that tumor location and tumor volume be well defined. Improvements in imaging technology - high resolution CT, MRI and even PET - address this. ProCure centers will soon use 4D-CT scanners that provide high resolution images and reveal the motion of a tumor that cannot be defined by other imaging technologies.
• Delivery of protons to organs or structures that move pose a challenge given the excellent conformality required in this therapy, an issue identical to that encountered in IMRT. Lung malignancies, for example, are a particular concern, although most internal organs in the chest cavity also are subject to significant motion. The 4D-CT technology addresses this with its high resolution and many images acquired rapidly over time. New technologies that track tumor motion in real time are in the process of being incorporated.
• Treatment planning requires accurate calculations of the actual energy lost to tissue the proton encounters on its path to the tumor, in order to be able to predict where the maximum dose is to be delivered. New CT scanners are designed for this purpose.