Radiographic film (port films) using film cassettes with lead or copper filters which improve the radiographic contrast of the port films are typically used to verify patient isocenter position and for portal shape (Fig. 36). Devices to support the port film cassettes and to help insure that the film plane is orthogonal to the beam direction and close to the patientís surface from which the beam exits are also important accessories (Fig. 37). In addition, radiopaque graticules (Fig. 38) that can be inserted into the treatment beam are invaluable for the evaluation of port films (30). Typically, such devices consist of platinum or tungsten wire embedded in plastic and molded in a frame that can be attached to the treatment machine accessory mount.

The generally poor quality of the film images and the inconveniences of the film processing and physician reviewing procedure have spurred development of computer-aided enhancement and digital imaging techniques in radiation therapy. Computed radiography (CR) systems, such as the Kodak 2000RT CR system shown in Fig. 39, is an example of such a system that allows digital DICOM images to be distributed electronically throughout the department.

In addition, electronic portal imagers (EPID) have made great strides this past decade and such devices are poised to replace film over the next few years (31,32). More recently, linac manufacturers have integrated EPID and tomographic imaging systems on their linacs (Fig. 40) for localization of bone and soft-tissue targets and have set the stage for image-guided radiation therapy to move into routine practice (33,34). Such systems will make quantitative evaluation of immobilization and repositioning of the patient much more achievable by allowing daily imaging of the patientís treatment.

In addition to imaging verification, there is sometimes a need to verify actual dose delivered to the patient. Simple point dose verification can be achieved using TLDs, diodes,

Figure 37. Port film cassette holder. Devices used to support a port film cassette behind the patient in any orientation device are especially useful for oblique treatment angles as the plane of film can be adjusted so as to produce normal incidence of the radiation field. (Courtesy of MEDTEC.)

Figure 39. Kodak 2000RT CR system. Computed radiography system that allows digital DICOM images to be distributed electronically throughout the department. (Courtesy Eastman Kodak Co.)

Figure 38. Example of a port film fiducial grid. Device identifies the central axis of the radiation beam and provides a scale at the calibration distance, thus providing a magnification factor for the port film. (Courtesy of MEDTEC.)

Or MOSFET dosimeters placed on the patientís skin surface or in body cavities. The most recent development in these types of devices is the OneDose patient dosimetry system (Fig. 41) developed by Sicel Technologies, Inc. It consists of a wireless handheld reader which interacts with self-adhesive external MOSFET dosimeters placed on the patient. To use, the therapist simply places the precalibrated dosimeters on the patientís skin in the treatment field, treats the patient, and then slides the dosimeter into the reader

Figure 40. Elekta Synergyģsystem. Linac with electronic portal imager (EPID) and conebeam tomographic imaging system for localization of bone and soft-tissue targets. (Courtesy of Elekta AB.)

Figure 41. Patient dosimetry monitoring system. Semiconductor detectors are typically used and applied on the patientís surface using surgical tape. (Courtesy of MEDTEC.)

Figure 43. DoubleCheck A matrix detector, consisting of multiple ionization chambers embedded in a plastic phantom, used to check the constancy of radiation beam output, symmetry, and flatness. (Courtesy MED-TEC, Inc.)

For an immediate display. The reader automatically provides a permanent record of dosage, time, and date with minimal data entry. Another dose verification device is shown in Fig. 42; because IMRT treatments presently require verification of the dose delivered and pattern for each patient, special phantoms and/or check devices have been developed to facilitate the IMRT verification measurements.

Also, there are numerous QA devices available to check the constancy of the linacís beam calibration, symmetry, and radiation-light field alignment. Generally, the QA radiation detection devices consist of several ionization chambers or semiconductors positioned in a plastic phantom that can be placed in the radiation beam (Fig. 43).

Finally, one of the latest advances is a system (still under development) that will be capable of performing continuous objective, real-time tracking of the target volume during treatment (Calypso Medical Technologies, Inc.) (35). The system is based on alternating current (ac) magnetic fields utilizing permanently implantable wireless transponders that do not require additional ionizing radiation and do not depend on subjective interpretation of

Figure 42. MapCHECK device provides 2D therapy beam measurements intended for quick and precise verification of the dose distribution resulting from an IMRT plan. (Courtesy of Sun Nuclear.)

Images. The system is currently undergoing clinical evaluation and is not yet available for clinical use.

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