The challenges of paediatric protons

Research - June 19, 2013

Proton therapy is an ideal fit for paediatric patients, delivering high dose conformality while minimizing the dose to non-target tissues. But treating children comes with a unique set of challenges and constraints, as Anita Mahajan explained at the recent PTCOG 52 meeting in Essen, Germany.

Anita Mahajan

Anita Mahajan

"Children are not just little adults," said Mahajan, medical director for the University of Texas MD Anderson Proton Therapy Center (Houston, TX). "The proportions of their body are different, both on the outside and the inside." Brain development, for example, occurs rapidly during a child's first three years, and the increased radiosensitivity of the brain – and other maturing organs and tissues – must be accounted for when planning proton treatments.
Paediatric patients can present with a large variety of tumours, with varying radiosensitivity and location. Another issue is that the tumour size must be considered relative to the patient size, with a 5 cm mass, for example, representing a large proportional volume in a small child. The same goes for any planned margins: "A 1 cm margin may not be a big deal in an adult, but it could well be in a baby," explained Mahajan, who has been treating paediatric patients with protons at the MD Anderson Proton Therapy Center since it opened in 2006.

Minimizing uncertainties

The conformality of proton therapy comes hand-in-hand with the requirement for highly accurate tumour delineation, patient set-up and beam delivery. Treatment plans must pay particular attention to reducing dose to normal tissue, and any normal structures observed on imaging should be delineated for avoidance. For example, it's possible to map functional areas in the brain using functional MRI and then design a beam arrangement to avoid these areas. If possible, information regarding the different susceptibility of tissue at different development should also be incorporated into the treatment plan.

The proton gantry

The proton gantry

A key part of the set-up process for paediatric patients relates to reducing anxiety. This requires a dedicated therapy team and close liaison with anaesthetists and other specialists. At MD Anderson, around 60% of paediatric patients are under 10, with half of these five years or less. While anaesthesia is generally required for all under the age of five, reduced anxiety may remove the need for sedation in some older children.
Mahajan also pointed out that most external immobilization equipment is designed to fit adults, and that children under the age of five cannot use a bite block. It's important, therefore, to have access to masks and other body immobilization kit that can accommodate smaller patients.
The MD Anderson team uses daily kV imaging to increase the accuracy of patient set-up. Mahajan emphasises the importance of using appropriate structures – whether bones or fiducials – for alignment. "We need to be careful about what is used to set up the patient and its relationship to the tumour," she said.

Another option would be to use volumetric imaging for patient set-up. As such, Mahajan is hoping to install a diagnostic quality CT scanner in the treatment room. While daily CT can reduce the size of the planned target volume (PTV) margins, she emphasized that this must be weighed up against the increased dose delivered, as well as the additional set-up time and patient anxiety. It's important to use the lowest dose and smallest field size possible. Ultimately, it may be feasible to employ alternative, non-ionizing methods for image-guidance, such as surface mapping or MRI.     

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