How is Proton Radiation Therapy Planned and Delivered?

The precision of the proton beam requires equal precision in planning and delivery. This is done in two steps:

a three-dimensional reconstruction of the tumor and its relationship to surrounding structures and
a reproducible treatment position that minimizes movement errors.

The three-dimensional information is usually obtained by performing computed tomography (CT) scans through the region of interest (chest, pelvis, etc.) with images taken at 2- to 3-millimeter intervals. Before performing the CT scan, an immobilization device is made for the patient to reproduce the patient's treatment position each day. Typical immobilization devices include full-body moulds (form-fitting foam liners surrounded by rigid plastic shells) for patients with tumors below the neck. Custom fit masks are made for patients with eye, brain, and head tumors or abnormalities. The CT scan is obtained with the patient immobilized in the device and in the treatment position, so that the thickness of the immobilizing material can be taken into account for treatment planning.

Once the immobilization device has been made and the CT scan has been performed, the treating physician traces on the tumor and the surrounding normal tissues on a computer. This tracing is done image by image, to get the most precise map.

Next, physicists and dosimetrists create a treatment plan by outlining on the computer a series of proton beams entering at various angles. From these, they calculate the radiation dose being given to the tumor and the normal tissues.

This plan is reviewed by the physician and, after it is approved, is transferred electronically to a series of automatic machines that make the special devices (apertures and tissue-compensating filters) required by the plan. All of these devices are calibrated by the physics support staff before the patent's first treatment. This ensures that the planning and manufacturing have been done correctly.