Proton therapy is still a fairly new treatment method in the fight against cancer. It differs from standard radiation treatments in both its method and results. To learn more about how this cutting edge treatment process works, view our illustrated guide to understanding proton therapy.
 

It is easy to appreciate why proton therapy is so effective: proton therapy delivers dose to the target while sparing healthy tissue and avoiding critical structures. Consequently, effective clinical application of proton radiation requires state-of the-art beam delivery systems, patient imaging and treatment planning systems, and patient immobilization and setup techniques.

Proton beam radiation therapy is motivated by the need to:

1. Spare normal tissue, organs at risk, or previously irradiated tissue.
2. Eliminate exit dose.
3. Minimize entrance dose.
4. Minimize integral dose.

General examples include:

1. Brain tumors and inoperable brain lesions.
2. Pediatric tumors (to reduce the risk of secondary malignancies, and to avoid the irradiation of growth plates).
3. Tumors near the optic apparatus.
4. Tumors next to the spinal cord or brainstem.
5. Prostate cancer (to spare the rectum and bladder).
6. Treatment after local failure.

Prostate Cancer

Prostate cancer is typically treated with opposed lateral beams (right and left laterals) that overlap on the anatomical midline. The prostate gland lies adjacent to the anterior rectal wall and inferior to the bladder and beam cross sections are chosen to meet dose constraints to these structures.

Patient simulation and treatment utilizes a rectal balloon filled with contrast solution. The balloon serves to aid imaging, to reproducibly fix the position of the prostate gland, and to provide separation of the anterior and posterior regions of the rectal wall. In this manner, the posterior portion of the rectal wall is displaced from the radiation field and receives significantly reduced dose. As in the figure (UR), the 7524 cGy isodose line contacts the anterior rectal wall but the 3000 cGy isodose line is removed from the posterior rectal wall.

Typical prescribed doses for prostate cancer are 5040 cGy in 28 fractions to the prostate gland and seminal vesicles, with an additional 2880 cGy in 16 fractions to the prostate gland while sparing the bladder and anterior rectal wall to constraints. The total dose delivered is 7920 cGy in 44 fractions. The DVH constraints on the anterior rectal wall are <30% to 7740 cGy and <10% to 7020 cGy. The bladder constraint is <20% to 6000 cGy.

Based on 7524 cGy (95% of the prescription dose of 7920 cGy), the prostate gland is typically treated at the 100% level, while the prostate gland plus margin is typically treated at the >97% level.

Compared to IMRT prostate plans, proton therapy provides a more uniform dose distribution (no hot spots), significantly lower dose to normal tissue (lower integral dose), and greater sparing of the rectum. 

At MPRI, patients are immobilized for treatment using a plastic form molded to the pelvis. Treatment positioning is verified daily via orthogonal X-ray images: a Visicoil and a seed implant are standard. Additionally, patients drink 16 ounces of water 30 minutes prior to treatment to reproducibly fill the bladder.

Head and Neck Disease: Clival Chordoma

Clival Chordomas are problematically located in proximity to the optic nerves and optic chiasm, the brainstem, the spinal cord, and various bone-air interfaces.

Therapeutic doses are high, 7740 cGy, while tolerance doses to critical structures are significantly lower:

• Brainstem 5400/6200 cGy (center/surface)
• Spinal Cord 5040/5500 cGy (center/surface)
• Optic Nerves 4500-5400 cGy (point)
• Optic Chiasm 4500-5400 cGy (point)
• Globes (Eyes) <4500 cGy (point)
• Lens of Eye <1000 cGy (point).

The lower integral dose achieved with proton therapy is crucial to maintain an acceptable dose level to these critical structures surrounding the target.

Patients are immobilized with an Aquaplast mask and Alpha Cradle mold on a carbon fiber frame thereby allowing access to the skull base for treatment.

Orbital Atypical Meningioma

With the target enveloping the right optic nerve and abutting the right eye, and the need to spare the contralateral eye and optic nerve, the optic chiasm, and brain stem, a highly conformal treatment was developed using a three-field technique utilizing precise gantry and couch rotations. The patient is immobilized on a carbon fiber table fitted with a bite block. This system, along with fiducial markers surgically placed in the skull, facilitates daily positioning with ±1.0 mm accuracy. Orthogonal X-ray images are used to determine the setup position, and small day-to-day variations are compensated with the robotic positioning system.

Depending on age, treatment site, and temperament, younger patients may require daily anesthesia (with LMA) to avoid movement during treatment.

As in the DVH, the chiasm, brainstem, and contralateral optic apparatus receive no dose, yet 97% of tumor volume receives at least 5640 cGy (95% of prescribed dose).

(See also the figures showing the treatment plans for a Metastatic Brain Lesion, and a GBM.)

Patch Field Technique

Since a proton beam has a defined range of penetration, it is possible to “patch” two fields together by joining the distal penumbra of one field to the lateral penumbra of another. In effect, the two fields are positioned so the end of one abuts the side of the other. This creates an “L” shaped dose distribution which surrounds – yet avoids – a critical structure. This is distinct from “matching” techniques, where two fields are joined edge to edge.