Proton Therapy For Childhood Cancers

Authors: Dr. Ramakrishna Kamath & Dr. Srinivas Chilukuri, Apollo Proton Cancer Centre

Traditional X-ray based radiation therapy has been used for many decades to treat both cancerous and non-cancerous tumours in infants, children, and adolescents. Technological advancements in radiation therapy over the years have made it possible to treat tumours with reduced toxicity to the surrounding normal tissues. While traditional X-ray based radiation therapy can be effective, it also can produce documented long-term side effects in some children, such as a decrease in bone and soft tissue growth in the treated area, hormonal deficiencies, intellectual impairment, including neuro-cognitive deficits, and secondary cancers later in life. For children whose bodies are still growing it is very important we try to minimise the amount of radiation to the body.

Protons are positively charged particles found inside all atoms. These protons are extracted from liquid or gaseous forms of Hydrogen. The Cyclotron or Synchrotron are large generators in which Hydrogen atoms are split into Electrons and Protons. It is important that the protons are directed to travel at any depths within the body and this is done by accelerating the protons to increased energy in the Cyclotron or Synchrotron. The head or gantry of the Proton machine has a system of electromagnets placed perpendicular to each other. The Proton beam is shaped, guided, and bent to reach the depth of the tumour by this beam transport system. A customized beam, only few millimetres wide is directed to move throughout the tumour’s depth to paint the treatment volume with spots of radiation, conforming to the shape of the tumour and delivering thousands of spots of radiation in just a few minutes. This technique is used to precisely treat complex cancers and this is called Pencil beam scanning proton therapy. This can provide even greater benefits to young patients with tumours especially those that are oddly shaped or within or near critical structures or vital organs in the body.  

Once the proton enters the body, it slowly increases the energy deposited in the tissue until it reaches a particular depth where it deposits almost all its energy and comes to a halt. This effect is called Bragg’s Peak Effect. So, with Proton beam therapy we can deliver high doses to the tumour with better accuracy and precision and at the same time further reduce the toxicity to the surrounding normal tissues. Traditional radiation therapy beams enter from one side of the body, pass through the tumour, and exits from the opposite side, whereas, Proton beam enters and stops at the edge of the target without any exit radiation dose. 

Proton therapy has been proved to be extremely effective in treating tumours of the brain and spinal cord like Astrocytoma, Craniopharyngioma, Ependymoma, Glioma, Glioblastoma, Intracranial germ cell tumours (germinoma), Medulloblastoma, Meningioma, Neuroblastoma, Optic pathway/hypothalamic gliomas, and Primitive neuro-ectodermal tumour (PNET). It is also effective in treating tumours of the bone and soft tissue like Chordomas, Chondrosarcomas, Ewing’s Sarcoma, Osteosarcoma, Rhabdomyosarcoma, and other tumours like Lymphomas, Nasopharyngeal carcinoma, Wilm’s tumour, Retinoblastoma, Choriocarcinoma and Teratomas (1). Most tumours in children are treated with surgery, chemotherapy and radiation therapy and all treatment plans involve detailed discussions with multidisciplinary clinical team consisting of paediatricians, paediatric haemato-oncologists, paediatric surgeons, pathologists, radiologists, and radiation oncologists.

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The usual workflow for all patients undergoing proton beam therapy begins with a planning CT scan. Just before the planning CT scan, appropriate immobilization devices are used and markings are made to make sure that there is no movement during treatment and so that the tumour can be targeted with high precision and accuracy. The planning CT scan is followed by marking the tumour and surrounding organs at risk by the radiation oncologists. This is followed by radiation planning done by radiation oncologists and medical physicists together to get the best possible plan in which the tumour gets a high dose and the normal organs surrounding the tumour get a low dose of radiation. This is followed by a peer review of the entire radiation plan by the whole team of radiation oncologists, medical physicists, and radiation technologists. Treatment schedules and doses can vary for each patient’s specific case, but most children are treated daily, Monday through Friday, for five to six weeks. Treatment can take anywhere from 30 minutes to 1 ½ hours daily, depending on whether the child is sedated as some children find it difficult to lie still during treatment.

All children undergoing treatment are reviewed weekly to make sure that any treatment related side effects are identified and treated immediately. If the child is undergoing chemotherapy along with radiation, then the child is reviewed by paediatric haemato-oncologists as well. 
Proton beam therapy has made a major impact on the cure and survival rates of cancer and has improved the quality of life of children with reduced incidence of late radiation induced complications (2,3). It is very promising that large prospective clinical trials are being conducted by large groups like the children’s oncology group to further define its role in childhood cancers (4).    

References:
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