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Linear Accelerators: Radiotherapy Units, Cobalt

Excerpt from a Healthcare Product Comparison System (HPCS) report from ECRI Institute

Medical linear accelerators (linacs) and cobalt radiotherapy units are used inlinear accelerator external-beam radiation therapy to treat cancer. Linacs emit a well-defined beam of uniformly intense x-ray photon radiation of different energies, depending on the accelerator. Some linacs also produce electron beams. Cobalt radiotherapy units use a man-made radioisotope, cobalt-60, to produce gamma-ray photons. Cobalt units and low-energy linacs are used primarily to treat bone cancer and tumors of the head, neck, and breast. High-energy linacs are used to treat deep-seated neoplasms and tumors of the pelvis and thorax.

Since the development of radiotherapy units, external-beam radiation therapy has become a primary treatment modality, along with chemotherapy and surgery. Radiation is used to treat at least 50% of all cancer cases, and many patients receive a combination of all three modalities. Radiation therapy can be either curative or palliative, depending on the stage and prognosis of the disease. For successful treatment, the radiation field must be very carefully calibrated and well defined to avoid irradiating healthy tissue.

Principles of operation

Linear accelerators

Linacs consist of four major components—a modulator, an electron gun, a radio-frequency (RF) power source, and an accelerator guide (see Figure 1). The electron beam produced by a linac can be used for treatment or can be directed toward a metallic target to produce x-rays.

The modulator amplifies the AC power supply, rectifies it to DC power, and produces high-voltage DC pulses that are used to power the electron gun and RF power source. High-voltage cables electrically connect the electron gun and RF power source to the modulator, which can be located in the gantry, the gantry supporting stand, or a separate cabinet.

The electron gun injects electrons into the accelerator guide in pulses of the appropriate duration, velocity, and position to maximize acceleration. The electron gun can be attached to the accelerator guide by a removable vacuum flange, which allows easy replacement of the gun. In designs with a permanently attached electron gun, the entire accelerator must be replaced when the gun’s filament burns out.

 Figure 1.

The RF power source, either a magnetron or a klystron, supplies high-frequency electromagnetic waves (3,000 MHz), which accelerate the electrons injected from the electron gun down the accelerator guide.

Linacs are classified according to their energy levels. Low-energy units produce 4- or 6-megavolt (MV) photons, medium-energy units produce photons of 8 to 10 MV and electron beams of 9 to 15 million electron volts (MeV), and high-energy linacs produce photons between 15 and 25 MV and have electron energies ranging from 4 to 22 MeV. Most linacs are either dual-energy units offering a low-energy beam at 6 MV and a higher-energy beam of at least 10 MV or multiple-energy units offering a range of photon and electron energies.

All linacs have a dosimetry system in the treatment head that terminates the radiation at the preset dose. This system incorporates a compartmented, dual-system ionization chamber, which should be sealed against temperature and pressure fluctuations; the performance of this chamber must be checked frequently. Most dosimetry systems can detect asymmetries in the treatment beam and terminate irradiation if the asymmetry exceeds a preset value. Some linacs can reposition the beam after an asymmetry is detected. Some systems also have beam-steering circuitry to automatically compensate for changes in the angle or position of the beam caused by gantry or collimator rotation.

The radiation beam is shaped by the collimators, which are motor-driven, movable blocks of material that define the treatment field. A light field projected onto the patient outlines the area to be irradiated. Field sizes of up to 40 cm on a side are available, as are digital readouts of collimator position. Adjustable collimator jaws are available on most units. In addition, special size-specific collimators for electron-beam treatment are suspended below the fixed collimator system, and other freestanding blocks or shaping wedges can be placed below the collimator trays to further customize the beam shape. The entire collimator assembly rotates about an axis that passes through the center of the treatment field and the isocenter (the spatial point where the collimator’s axis of rotation intersects the gantry’s axis).

Cobalt radiotherapy units

Figure 3

Cobalt units provide low-energy (1.17 and 1.33 MV) treatment using cobalt-60 as a radiation source. Nickel-plated, high-specific-activity cobalt-60 pellets are encapsulated in two layers of low-carbon stainless steel sealed by heliarc welding in a cylinder. The source cylinder, approximately 1 to 2 cm in diameter, is mounted in the unit’s head; a pneumatically driven drawer moves the source from storage into the exposure position. Accurate source positioning is accomplished with limiting devices. The source is surrounded by lead in all directions for radiation shielding (see Figure 3). Like linacs, cobalt units are mounted isocentrically. The source-to-axis distance is either 80 or 100 cm. Adjustable collimators are used to define the treatment field, and special filters or beam modifiers are also available for individual therapy needs.

Cobalt radiotherapy units are operated similarly to low-energy linacs. The photon energy produced is 1.33 MV; this beam behaves much like a linac beam of 3.3 MV. Because cobalt radiation reaches its maximum dose at 0.5 cm below the skin surface, it is especially suited for radiotherapy of the head, neck, and breast, as well as for tumors within 5 cm of the skin surface that are located in other parts of the body.

Reported problems

A systematic review of reported problems has demonstrated that most errors and incidents are caused by user error. At least one study (Macklis et al. 1998) has reported that it is unlikely that a given patient will suffer a significant adverse medical event caused by a radiotherapy treatment error (routine toxicities and side effects were not documented, however). In this study, 15% of the errors were related to the use of the record-and-verify system (resulting from incorrect data entry). Errors can also occur at the planning stage or in equipment calibration. Missed clinical information at the planning stage has caused severe (even fatal) radiation injury, and poor calibration can lead to serious medical errors.

The Radiation Oncology Safety Information System, a database of errors in radiotherapy, has reported that most mistakes are due to human error. In most treatments, the large number of fractions means that most errors will be caught without serious repercussions. However, the move to hypofractionation and radiosurgery means that fewer fractions are being delivered and with a higher dose. So the impact of a small error will be much more pronounced. Also, there are a number of documented cases in which serious patient injury or death occurred because of overdose—and underdose presents an equally significant risk.

In several reported cases, electromagnetic interference from a linear accelerator caused infusion-pump failure when the pumps were being used on patients undergoing radiation therapy. ECRI Institute believes that this problem could affect other electronic devices as well. See citations from Health Devices in the bibliography.

Hardware failures can also result in the misadministration of a radiotherapy prescription. Because of the complexity of linacs and cobalt radiotherapy units, mechanical problems are common, although most injuries are caused by heavy equipment hitting patients and technologists. All units should have fault-detection systems that minimize the probability of an equipment-induced treatment error. Software, or programming, errors can have a serious impact on patient treatment. One small programming error can affect many patients. For example, several patient deaths from teletherapy overexposure occurred at Panama’s National Institute of Oncology; data entry and software errors, along with a lack of treatment plan verification, were cited as contributing factors by the U.S. Food and Drug Administration (FDA). There was no suggestion that a failure or malfunction of the teletherapy system contributed to the overexposure.

Risk management in radiotherapy requires a comprehensive quality assurance program (Nath et al. 1994). According to FDA, treatment plans should be verified by independent means, possibly including manual calculations or measurement of radiation dose.

This is an excerpt from a Healthcare Product Comparison System (HPCS) report from ECRI Institute entitled, Linear Accelerators; Radiotherapy Units, Cobalt. The full report is available to purchase individually, or with your subscription to HPCS or Health Devices Gold. The HPCS online database offers comparative charts covering thousands of brand-name model specifications for more than 450 types of devices.

 

 

 

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