A patient lying in a high-precision robotic bed used for stereotactic radiosurgery.

Proton stereotactic radiosurgery is offered for the treatment of a variety of tumors and malformations. Some of these include:

• pituitary adenomas
• meningiomas
• acoustic neuromas
• cavernous sinus tumors
• arterial venous malformations (AVM)

The dose uniformity of protons is especially attractive for the treatment of AVMs as these malformations are not solid tumors but rather a tangle of vessels and normal brain. This means that when targeting an AVM it is impossible to avoid the normal brain contained within the malformation. A uniform dose means that there are no “spikes” of excessively high dose within the target. In some limited circumstances proton stereotactic radiosurgery can be used for lesions that are not contained within the head.

Below: An example of a proton stereotactic radiosurgery treatment plan for a large AVM. The PSRS treatment is delivered in two sessions using a total of six treatment directions to provide a conformal proton dose distribution.

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PSRS Planning & Treatment Process

Upon referral to our center for proton stereotactic radiosurgery, a senior neurosurgeon and radiation oncologist reviews each case, assessing the medical and treatment history, medications and prior imaging studies to determine the best treatment course for the particular condition. Further diagnostic imaging such as MRI or angiography may be required. An opportunity to meet with the physicians to discuss their treatment recommendations is part of the process.

On the PSRS treatment day, a member of the proton therapy team greets the patients making them comfortable, providing them with an overview of the treatment day and answering any questions they may have.

For proton stereotactic radiosurgery patients with lesions contained within the head, the treatment plan will require that several tiny beads, known as fiducials, be implanted in the surface of the skull. During the proton therapy session, the fiducials help to insure that the proton beam is very precisely aimed at the target. Using a small needle, a neurosurgeon puts the fiducials in place. Performed with a local anesthetic, the procedure is very simple and the patient should only feel a slight pressure from the needle. This procedure is only applicable if lesions are located within the head.

For both a CT scan and later during the actual proton therapy treatment, it is crucial that patients remain absolutely still. To insure this, we will need to fit the patient with an immobilization device. Some circumstances require a stereotactic head frame to remain in place from the time of the CT to the end of the PSRS treatment. If this is the type of immobilization that is indicated the physician will secure the frame to the patients head. While the head frame is a bit awkward, wearing it is not painful. Patients should feel only a slight pressure as the frame is attached. Patients fitted with this type of frame are planned and treated on the same day. If a different type of frame is used the process may be separated so that the planning occurs on a different day from the treatment.

Two different types of immobilization devices used for proton radiosurgery. The device on the left is a stereotactic frame, which requires that the planning and treatment be done on the same day. The device on the right enables treatments to be delivered on a different day than the planning CT study. This frame is useful for patients who require more than one treatment session. Many factors are considered in determining the most appropriate frame for treatment.

When the frame is securely in place, IV contrast is injected in preparation for a CT scan. The CT scan creates a precise three-dimensional picture of the area to be treated. This becomes the framework for calculating the radiation dose and designing the proton stereotactic radiosurgery treatment. For the CT scan, patients lie on a treatment bed with the immobilization device secured to prevent motion.

Physicians use the CT scan, in addition to other studies to outline the target(s) and important normal structures. The size of the target, as well as its relationship to these structures, is critical in calculating the prescribed radiation dose. It also determines the directions from which the proton beam is aimed through the body to the target.

Once the PSRS treatment plan is finalized, customized equipment is fabricated to shape the proton beam so that the radiation dose matches as closely as possible the shape of the target. This equipment is designed for each direction from which the beam is aimed.

The total time required for proton stereotactic radiosurgery treatment is around one hour. This time varies depending on the number of directions the proton beam is aimed. The proton beam is typically aimed from 3 to 6 directions. Treatments may be done using the gantry system or using the dedicated stereotactic “beamline”. Many factors are considered in determining the most appropriate treatment room.

A model poses to show how the proton beam devices are brought near the patient in preparation for treatment. In this picture the beam is rotated using the gantry system for a treatment from the front and left of the patient. In the dedicated stereotactic system the proton beam is fixed and patients are rotated to achieve the same directions.

As the PSRS treatment session starts the patient is greeted by radiation therapists. The therapists assist the patients onto the treatment bed and into their immobilization device. Once in position the therapists verify the patient’s position by taking x-ray images. Adjustments are made based on the images and when necessary new images are acquired to confirm the final treatment position. Prior to treatment a medical physicist reviews the x-ray images. With the patient in the treatment position the therapists request for radiation from the delivery system. The radiation is delivered to one of three treatment rooms sequentially therefore small delays are possible from the time the radiation is requested to the time it is ready to be delivered. When the delivery system is ready to deliver radiation the therapists leave the treatment room. Final radiation parameters are confirmed and the radiation is delivered while the therapists monitor the patient via closed circuit video cameras.

As soon as the proton stereotactic radiosurgery treatment is over, the immobilization device is removed. After treatment, the physician discusses further care with the patient, including any immediate precautions, follow-up recommendations and instructions in the event of further symptoms related to the illness and/or treatment. After a brief observation period patients are free to go home.