Serial tomotherapy is an enhancement of the method previously described. An accelerator is equipped with mini-multileaf collimators that form a “slit” of radiation (normally 2 × 20 cm). The gantry is rotated through an entire arc around the patient while the mini-multileaf collimators are driven in and out of the field, thus modulating the intensity of the beam. The treatment couch is advanced by a few millimeters and the next arc is treated. An entire treatment is given once all the adjoining arcs have been delivered.
Instead of treating the patient on a normal linear accelerator, with helical tomotherapy the patient travels continuously through a modified CT ring. This CT ring has the capability of administering 6-mV x-rays, as in a standard linear accelerator, while at the same time performing a conventional diagnostic CT scan. Any anatomic or position changes that might require replanning can be performed before that day’s treatment. Following treatment, a daily, real-time image of the dose distribution can be obtained.
Protons, a form of particulate radiation, have been investigated recently as a means to improve tumor control. A proton has a charge of +1, is a stable particle, and, together with the neutron, makes up the atomic nucleus.
Protons are delivered to the tumor in the same manner as are photons and electrons. The dose deposited by protons remains relatively constant as they travel through the normal tissues proximal to the target.
The kinetic energy of the protons is transferred to the tumors by electrons knocked out of atoms. These electrons ionize DNA, and their biologic effectiveness resembles that of megavoltage photons.
Bragg peak At the end of the path, biologic effectiveness increases sharply as the protons slow down and eventually stop. This increase in dose is called the Bragg peak. The size of the Bragg peak is usually smaller than the tumor, however. This problem can be resolved by scanning the Bragg peak through the tumor volume by sequentially irradiating the target with lower energies. The dose falloff of the Bragg peak is sharp enough that the normal tissues distal to the tumor receive a negligible radiation dose.
Current clinical applications Uveal melanomas and skull-base sarcomas adjacent to CNS tissues, as well as prostate cancer, are areas in which proton therapy has been under clinical study, with promising results. Clinical studies are also examining its use in non–small-cell lung, hepatocellular, and paranasal sinus carcinomas.
Stereotactic radiosurgery is a 3D technique that delivers the radiation dose in one fraction. Specially designed collimators are attached to a linear accelerator, which delivers a high dose of radiation to a small volume, usually about 3 cm in diameter. Several stationary beams or multiple arc rotations concentrate the radiation dose to the lesion while sparing surrounding normal tissue. Although it is most often used to treat lesions within the brain, it can be used for selected extracranial sites (in which case it may be referred to as “body radiosurgery”).
Use in treating arteriovenous malformations Stereotactic radiosurgery is used to treat certain patients with arteriovenous malformations. These intracranial lesions arise from the abnormal development of arteries and venous sinuses. Surgical excision is the standard treatment of choice for operable lesions, but stereotactic radiosurgery has become a viable option for inoperable malformations.
Use in treating brain tumors As with conformal radiotherapy, clinical trials involving stereotactic radiosurgery for brain tumors are being conducted at major cancer centers. However, based on positive early results, many community centers have begun instituting a stereotactic radiosurgery program, either with a dedicated cobalt unit (Gamma Knife) or a linear accelerator-based system. Small (< 4 cm) tumors of the brain, whether primary, metastatic, or recurrent, may benefit from this treatment technique. The Gamma Knife is a form of stereotactic radiosurgery that uses multiple focused cobalt beams to treat lesions in the brain.
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