An Effective and Powerful Tool
Protons can be conformed to release their energy at precise depths. The peak of the proton-radiation dose (called the Bragg Peak) is set so it releases much of the radiation within the tumor. The radiation dose falls to almost zero beyond the treated volume. Compared with X-ray beams, protons deposit less radiation to the healthy tissue in front of the tumor and almost none to the healthy tissue behind the tumor, resulting in much less exposure to healthy tissue.1 Because proton therapy deposits more radiation directly in the tumor, a higher dose can often be delivered, leading to more effective treatment.1,2
Source: ProCure Training and Development Center
The physical properties of X-rays differ from the properties of protons. X-rays are electromagnetic waves that have no mass or charge and penetrate completely through tissue. Protons are large, positively charged particles that penetrate to a finite depth. When used to treat a tumor, X-rays and protons deliver radiation in different ways.
The Y axis in the chart above can be thought of as the surface of a patient’s skin and the beige rectangle as a tumor. The sloping black line shows how X-rays/IMRT deliver a dose. To deposit the proper amount of radiation into the tumor, X-rays/IMRT must irradiate much of the healthy tissue in front of it. The radiation continues to penetrate through the tumor and irradiates much of the healthy tissue behind it.
Protons deliver their dose much differently. As the blue line shows, they enter the patient at a low dose, then, at a precise depth, deposit a large burst and reach what is called the Bragg Peak. Immediately after this peak, they stop completely. To thoroughly irradiate the tumor, additional protons are sent in at lower doses to form a distribution like the turquoise line, the Spread Out Bragg Peak. In this way, protons completely irradiate the tumor while limiting the dose to healthy tissue.
The areas shaded in gray show the additional dose delivered to healthy tissue by X-ray/IMRT compared with protons. Proton treatments deliver a dose in a more accurate, more efficient way and spare more of the surrounding healthy tissue.
Proton therapy plays an essential role for tumor sites requiring a high degree of dose conformality due to their proximity to radiosensitive structures.3-5 Proton therapy achieves its conformal dose delivery through the use of only a few (often one or two) beams. Protons enable both a reproducibility of delivery (important for critical delivery plans) and a decrease of integral dose to non-intended targets. This means critical structures outside the treatment area are spared. Tumors of the brain, head and neck, prostate, pediatric, base-of-skull, juxtaspinal cord, non-small-cell lung, gastrointestinal, and melanoma of the eye are some of the types that may benefit from proton therapy.
Clinical benefits of proton therapy vs standard X-ray therapy include1,6-13:
- Less damage to healthy tissue
- Fewer treatment-related short- and long-term side effects
- Improved local tumor control
- Reduced risk of secondary tumors caused by treatment
In addition, proton therapy:
- Can be combined with chemotherapy
- Can be used to treat recurrent tumors even in patients who have already received radiation (standard radiation can only be used in the same area once)
- Can deliver an increased daily fraction dose, thus reducing the number of fractions required for treatment
Reduced Risk of Secondary Tumors
A retrospective study on proton therapy examined the incidence of secondary malignancies by comparing patients from the SEER registry treated with X-ray therapy to patients treated with proton therapy at the Harvard Cyclotron in Cambridge, Massachusetts. Patients were matched in terms of age at radiation treatment, year of treatment, cancer histology, and site of treatment. Of the patients receiving X-ray therapy, 12.8% went on to develop secondary tumors, while just 6.4% of patients receiving proton therapy did so.9
A retrospective study showed that patients treated with proton therapy were 50% less likely to develop a secondary malignancy compared with patients receiving X-ray treatment.9
Critical for Most Pediatric Cancers
Proton therapy is an especially important option for treating solid tumors in children because proton therapy limits the irradiation of normal tissues, helping to prevent serious complications and decreasing the incidence of secondary tumors later in life.6,8,14 When compared with standard X-ray therapy, proton therapy has been shown to reduce the risk of growth delays, developmental abnormalities, and secondary malignancies in most pediatric populations.8,15 For pediatric cancer, the rate at which secondary tumors occur is estimated to be 8 times lower with proton therapy than with IMRT (X-rays).8
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Improving Quality of Life in Prostate Cancer Patients
Men treated with standard X-ray radiation for prostate cancer commonly experience short-term side effects such as diarrhea and painful urination. Studies show that using proton-beam radiation to treat prostate cancer can result in minimal toxicities and reduce the risk of treatment-related side effects.2,5
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Targeting Brain Tumors With Precision
Because brain tissue is extremely radiosensitive, the risk of long-term side effects from radiation treatment is greater than for other parts of the body. Too much radiation to the brain has been known to cause neurological dysfunction and even death. Protons can target the tumor with more precision and do not continue past the tumor.1 Compared with other forms of radiation therapy, proton therapy results in less radiation to normal brain tissue, reducing the risk of side effects.13,15,16
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- Fowler JF. What can we expect from dose escalation using proton beams? Clin Oncol. 2003;15(1):S10-S15.
- Zeitman AL, DeSilvio ML, Slater JD, et al. Comparison of conventional-dose vs high-dose conformal radiation therapy in clinically localized adenocarcinoma of the prostate [published correction appears in JAMA. 2008;299(8):898-899]. JAMA. 2005;294(10):1233-1239.
- Meyer JJ, Czito BG, Willett CG. Particle radiation therapy for gastrointestinal malignancies. Gastrointest Cancer Res. 2007;1(suppl 2):S50-59.
- Rutz HP, Weber DC, Sugahara S, et al. Extracranial chordoma: outcome in patients treated with function-preserving surgery followed by spot-scanning photon beam irradiation. Int J Radiat Oncol Biol Phys. 2007;67(2):512-520.
- Vargas C, Fryer A, Mahajan C, et al. Dose-volume comparison of proton therapy and intensity-modulated radiotherapy for prostate cancer. Int J Radiat Oncol Biol Phys. 2008;70(3):744-751.
- Lee CT, Bilton SD, Famiglietti RM, et al. Treatment planning with protons for pediatric retinoblastoma, medulloblastoma, and pelvic sarcoma: how do protons compare with other conformal techniques? Int J Radiat Oncol Biol Phys. 2005;63(2):362-372.
- Steneker M, Lomax A, Schneider U. Intensity modulated photon and proton therapy for the treatment of head and neck tumors. Radiother Oncol. 2006;80:263-267.
- Miralbell R, Lomax A, Cella L, Scheider U. Potential reduction of the incidence of radiation-induced second cancers by using proton beams in the treatment of pediatric tumors. Int J Radiat Oncol Biol Phys. 2002;54(3):824-829.
- Chung CS, Keating N, Yock T, Tarbell N. Comparative analysis of second malignancy risk in patients treated with proton therapy versus conventional photon therapy. Int J Radiat Oncol Biol Phys. 2008;72(1):S8.
- Komaki R, Sejpal S, Wei X, et al. Reduction of bone marrow suppression for patients with stage III NSCLC treated by proton and chemotherapy compared with IMRT and chemotherapy. Particle Therapy Cooperative Group 47. 2008;O10:14.
- Mayahara H, Murakami M, Kagawa K, et al. Acute morbidity of proton therapy for prostate cancer: the Hyogo Ion Beam Medical Center experience. Int J Radiat Oncol Biol Phys. 2007;69(2):434-443.
- Miyawaki L, Dowd C, Wara W, et al. Five year results of LINAC radiosurgery for arteriovenous malformations: outcome for large AVMs. Int J Radiat Oncol Biol Phys. 1999;44(5):1089-1106.
- Vernimmen FJAI, Slabbert JP, Wilson JA, Fredericks S, Melvill R. Stereotactic proton beam therapy for intracranial arteriovenous malformations. Int J Radiat Oncol Biol Phys. 2005;62(1):44-52.
- Chin D, Sklar C, Donahue B, et al. Thyroid dysfunction as a late effect in survivors of pediatric medulloblastoma/primitive neuroectodermal tumors. Cancer 1997;80(4):798-804.
- Merchant TE, Hua C, Shukla H, Ying Xiaofei, Nill S, Oelfke U. Proton versus radiotherapy for common pediatric brain tumors: comparison of models of dose characteristics and their relationship to cognitive function. Pediatr Blood Cancer. 2008;51:110-117.
- Bolsi A, Fogliata A, Cozzi L. Radiotherapy of small intracranial tumours with different advanced techniques using photon and proton beams: a treatment planning study. Radiother Oncol. 2003;68:1-14.