Proton Therapy for Pediatric Cancer

Epidemiology

There are approximately 12,000 new cases of pediatric cancers each year, of which around 3,000 will require radiation. The time course of therapy varies for each type of cancer, with most patients requiring a six-week treatment plan.  There are currently 14 functioning proton centers in the United States, and 11 more in development.

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http://www.proton-therapy.org/map.htm

 

Radiation Physics

Proton-beam therapy uses high-energy protons to deposit less radiation at their site of entry and most of their dose at the end of their range in the targeted tissue.  In contrast, photon therapy beams (x-rays) experience an exponential attenuation of dose along their traveled path.

Thus, from a physics standpoint, proton therapy can spare radiation to healthy tissue outside the cancer target, potentially reducing the risk of damage to additional tissue. In pediatrics, this is especially valuable, since most patients have the potential to survive to experience a secondary malignancy as a result of childhood irradiation.

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Clinical Evidence

The evidence for proton radiation in pediatric cancers varies by cancer site. In a retrospective cohort study of 558 patients treated at the Harvard Cyclotron with matched patients from the SEER database, secondary malignancies after proton therapy were significantly decreased compared to photon therapy (4.2% vs 7.5%, p=0.009).

For tumors of the central nervous system, including retinoblastoma, medulloblastoma, craniopharyngioma,and ependymoma, proton-beam therapy has been extensively studied. The data show that proton therapy is equally or more efficacious in local control while delivering less total radiation to surrounding normal structures. Particularly in cases where the hypothalamus can be spared, proton therapy is demonstrated to be cost effective in comparison to photon therapy with subsequent endocrine replacement treatments.

In Ewing sarcoma, proton therapy has been studied in conjunction with chemotherapy. Local control and overall survival rates were similar to those achieved with traditional photon therapy, but local toxicities such as radiation myositis and alopecia were not improved. Although the dose distribution to normal tissues was lower, the side effects persisted in the areas receiving higher doses.

For neuroblastoma, small case series and planning studies suggest that proton therapy may be useful in patients who require higher doses to the tumor. Proton therapy is able to improve sparing of renal parenchyma, liver, heart, and bowel; however the evidence for correlated improvement in clinical outcomes is limited.

Clinical trials for the potential use of proton therapy in high-risk Hodgkin lymphoma are underway. As the study of proton therapy and its strengths and weaknesses evolves, the improved understanding of treatment quality and late effects will guide clinical decisions for future patients.

 

Resources

Providing families with information is key when helping inform them of treatment options for their child's condition.  CLICK ON THE LINK below for one such resource from the National Association for Proton Therapy.

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http://www.proton-therapy.org/pediatric_section.htm

 

References

  1. Mailhot Vega, R., Kim, J., Hollander, A., Hattangadi-Gluth, J., Michalski, J., Tarbell, N. J., Yock, T. I., Bussiere, M. and MacDonald, S. M. (2015), Cost effectiveness of proton versus photon radiation therapy with respect to the risk of growth hormone deficiency in children. Cancer, 121: 1694–1702.doi: 10.1002/cncr.29209
  2. Merchant TE. Clinical controversies: Pediatric tumors. Seminars in radiation oncology. 2013;23(2):97-108. doi:10.1016/j.semradonc.2012.11.008.
  3. Patel S, Kostaras X, Parliament M, et al. Recommendations for the referral of patients for proton-beam therapy, an Alberta Health Services report: a model for Canada? Current Oncology. 2014;21(5):251-262. doi:10.3747/co.21.2207.