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What is the role of proton therapy in the treatment of children with medulloblastoma?
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It provides precise treatment with fewer late effects.
The major advantage of proton therapy is being able to direct the beam and have it stop where you want it to stop. In medulloblastoma, we must treat the cerebral spinal fluid pathway, the whole brain and the whole spine, to a certain dose based on the patient’s level of risk, followed by a boost to the primary area.
There are advantages to treating the whole spine with proton therapy. With conventional radiation, the spine is treated from the back and the beam exits through tissues in front of the spine. Those tissues, the heart, lungs, and in girls sometimes, the ovaries, get low doses of treatment, so protons would eliminate that risk.
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There is a small risk for heart damage later in life and dying from heart problems, but protons would eliminate that risk as well, given there is no dose to the heart with protons.
The risk for thyroid dysfunction or thyroid cancer would potentially be less also. There may be a small dose to the thyroid, but compared with conventional radiation, which is right in front of the spine, the dose is going to be significantly less, hopefully leading to less thyroid dysfunction and less thyroid cancer.
In terms of the brain treatment, when the boost is given to the site of the original tumor, proton therapy is more focused and will give less of a low-dose volume outside of the targeted area. With IMRT or any conventional radiation, you get low and medium doses to adjacent areas, such as the middle ear, which may possibly contribute to hearing and other difficulties.
Overall, proton therapy should decrease adverse, long-term effects to certain organs in a small group of patients who are destined to get them.
Robert B. Marcus Jr., MD, is professor of radiation oncology, University of Florida Proton Therapy Institute, Jacksonville.
Proton therapy is useful, but is best used in select patients.
There are numerous advantages of proton radiotherapy in medulloblastoma, unfortunately, however, there are only four centers in the country that currently provide the complex treatment setup for craniospinal proton treatment. It is time and labor intensive, and rather expensive. However, given all the normal tissues spared by using proton radiotherapy, the upfront increased costs are likely to pay off in the long run, as patients treated with protons are likely to require fewer medical resources to treat the late effects of therapy. Also, less normal tissue irradiated means fewer radiation-induced second tumors, which is a significant source of both morbidity and mortality in childhood cancer survivors.
The clinical outcomes data supporting the benefit of protons is just starting to be reported. In fact, MassGeneral Hospital for Children reported the preliminary clinical benefits of proton radiotherapy at this year’s ASCO conference: There appears to be a benefit in hearing, endocrine outcomes and neurocognitive outcomes while maintaining excellent disease control.
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Because proton therapy is such a finite resource, it currently needs to be rationed in a way that provides the greatest benefit to children requiring radiotherapy. Therefore, children in whom cure is less likely and/or for whom the burden of late effects is less, may not derive as much benefit from proton therapy. It is in these patients that the downsides of the disruption of leaving home and leaving support networks to travel to proton facilities for 6 to 8 weeks of treatment begin to outweigh the benefits.
Overall, proton radiotherapy provides a quantum leap improvement in the radiotherapeutic treatment of pediatric brain tumor patients and should be made more available in the future to this population at high risk for treatment-related side effects.
Torunn I. Yock, MD, is director, pediatric radiation oncology, MassGeneral Hospital for Children, Boston, and assistant professor, Harvard Medical School.