|Figure. Verification image for patient treatment, three-dimensional conformal and intensity-modulated radiation therapy.|
The use of narrower radiation fields for the treatment of cancer, including the new techniques of intensity-modulated and other conformal protocols, has made diagnostic imaging an integral part of radiation oncology. The goal is to define the borders of a tumor closely, so that a lethal radiation dose is delivered to all of the tumor but little radiation (ideally, none) is delivered to the surrounding normal tissues.
Traditionally, plain portal films were used for this purpose, with the bones being the reference points. Many tumors can move in relation to the bones, however. This has driven a demand for high-resolution soft-tissue imaging to be available as part of the linear accelerator (so that the patient need not be moved after the tumor has been localized). The Siemens PRIMATOM” combines a linear accelerator with a CT scanner. The B-mode acquisition and targeting systems have an ultrasound probe mounted on an arm that tracks the position of the probe and the isocenter of the radiation field. With either approach, the treatment couch (with the immobilized patient) is moved according to the imaging findings. MRI, cone-beam CT, and even positron-emission tomography also are being employed for this purpose.
A new approach integrates portal imaging with the linear accelerator by using electronic image capture on flat panels of amorphous silicon,1 cesium iodide,2 or other materials. The images from the electronic portal imaging device (EPID) are read electronically in the single, continuous, or cone-beam mode, depending on the purpose of the study. Thus, single-mode readout is used for before-and-after depiction of the tumor, but images can be captured between linear accelerator pulses for during-treatment verification in the continuous or cone-beam modes. The electronic images can easily be stored and analyzed and can be transmitted for review by the radiation oncologist, increasing that person’s efficiency.
Implementation of EPID is challenging, and the American Association of Physicists in Medicine established a special task group to help users overcome the difficulties.3 The rewards are significant. Jack Yang, PhD, chief physicist, Monmouth Medical Center, Long Branch, NJ, reports, “We use EPID for most of our treatment verification if it is allowable. It is used for routine setup and verification in three-dimensional conformal radiotherapy, and we use it heavily for intensity-modulated radiation therapy (IMRT) QA and verification of intensity maps because it enables us to get our work done in very efficient way (see Figure).”
Pouliot4 has pointed out the value of EPID in dealing with an increasingly common problem: targeting for the obese patient. A skin tattoo can shift as much as 5 cm, making on-the-table confirmation of tumor targeting essential. With standard portal films, “By the time the film is exposed, processed, and looked at for patient position verification, the patient anatomy has already moved.”4 With EPID, on the other hand, after the patient is positioned, a flat-panel image is obtained for immediate viewing. If the position is not satisfactory, it can be changed and another image can be obtained (involving only a low dose of radiation). Through repeated iterations of imaging and movement, the patient can be positioned for accurate delivery of the radiation.
Studies5,6 using EPID have provided further documentation of the need to confirm tumor position at every treatment session. In a series of 20 women with low-stage breast cancer who were scheduled for postoperative radiation, a Canadian team5 demonstrated differences between simulation and the actual position of the tumor of as much as 23 mm. Using EPID, the team was able to reduce irradiation of the heart and lungs and compensate for ventilatory motion. The same researchers also showed that prostate cancers can move as much as 1.6 cm from one day to the text,6 again demonstrating the importance of daily confirmation of target positions.
Treatment planning is not the only function of EPID in a busy radiation therapy department. “Previously, we used film and other devices to gather the information to guarantee the quality of radiation delivery,” Yang says. “It took an average 2 to 4 hours each time from a complete set of IMRT QA including all processing time of films; however, with EPID, not only can we get the same information (such as the isocenter and multileaf collimator checks), we can set up the templates in the network, have the machine deliver the pattern that we desire, and electronically capture all the quality-assurance (QA) information for off-line analysis. If you are doing this work the old-fashioned way, with standard film dosimetry, it is very tedious, as you are running in and out of the room repeatedly to obtain and develop films. Our QA procedures now take about 30 minutes, reducing costs and the demands on staff time and making them available through electronic records.”
Yang expects QA procedures to expand as the equipment becomes more sophisticated, making radiation therapy even more precise. “At present, we do QA studies before and after a treatment session. Eventually, we hope to be able to do it during treatment,” he says. “We want to maintain high quality for each patient having an IMRT procedure, but we do not have the staff to perform each test on each patient individually. The problem is particularly acute because of the shortage of radiation physicists. EPID may help compensate for staff shortages.”
Judith Gunn Bronson, MS, is a contributing writer for Decisions in Axis Imaging News.
- Kubo HD, Shapiro EG, Seppi EJ. Potential and role of a prototype amorphous silicon array electronic portal imaging device in breathing synchronized radiotherapy. Med Phys. 1999;26:2410-2414.
- Mosleh-Shirazi MA, Evans PM, Swindell W, Symonds-Tayler JR, Webb S, Partridge M. Rapid portal imaging with a high-efficiency, large field-of-view detector. Med Phys. 1998;25:2333-2346.
- Herman MG, Balter JM, Jaffray DA, et al. Clinical use of electronic portal imaging: report of AAPM Radiation Therapy Committee Task Group 58. Med Phys. 2001;28:712-737.
- Pouliot J. Set-up of a 400-pound patient for prostate treatment. Available at: http://www.ucsf.edu/jpouliot/LargePatient.htm. Accessed May 29, 2003.
- Lirette A, Pouliot J, Aubin M, Larochelle M. The role of electronic portal imaging in tangential breast irradiation: a prospective study. Radiother Oncol. 1995;37:241-245.
- Vigneault E, Pouliot J, Laverdiere J, Roy J, Dorion M. Electronic portal imaging device detection of radioopaque markers for the evaluation of prostate position during megavoltage irradiation: a clinical study. Int J Radiat Oncol Biol Phys. 1997;37:205-212.