1480 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 39, NO. 5, OCTOBER 1992 zyx High Contrast, CdTe Portal Scanner for Radiation Therapy G. Entine, M.R. Squillante, R. Hahn, L.J. Cirignano, W. McGann and P.J. Biggs* Radiation Monitoring Devices, Inc., Watertown, MA *Massachusetts General Hospital, Boston, MA zyxw Abstract One of the most promising new technologies for improving the quality of radiation therapy is the use of real-time systems to produce portal images. In our approach, we are constructing a linear array of 256 CdTe photovoltaic detectors attached to a very compact linear scanner, all of which will be mounted in a cassette shaped package to be located under the patient table. The high stopping power of the CdTe allows a high contrast image to be made using only a single Linac pulse per line, resulting in a high contrast image in under 5 seconds. I. INTRODUCTION Radiation therapy is an extremely important therapeutic modality for the cure of malignant tumors. It is estimated that over 100,OOO patients in the United States alone undergo radiation treatment each year, and changes in linear accelerators are likely to provide improved cure rates. One of the most important considerations in radiation therapy is the precision with which the radiation dose can be delivered to the tumor. This is due to the well documented fact that a variation in dose of 5% can make a large difference in the tumor control probability and in the likelihood of serious complications [l, 21. The traditional tool for verifying the position of the beam, portal film, has extremely poor contrast at megavoltage beam energies and are often very difficult to interpret. Because of this limitation, alternative techniques have been explored to enhance the contrast of portal images. These techniques include the use of digital enhancement after the portal film has been developed, the use of fluorescent screens in conjunction with video cameras and computerized enhancement, and the use of electronic imaging systems based upon active nuclear detectors. All of these approaches offer significant advantages over the use of standard portal films and have led to improvements in the ability of therapists to assess the accuracy of individual treatment setups. However, the response time of several of these techniques is such that no useful information is presented in real time and therefore no corrections can be made during the course of a single This work was supported in part by grant zyxwvutsr # N44-CM-07601 from the National Cancer Institute. treatment. The cost of more advanced systems which can produce images in real time are currently very expensive ($130,000 - $150,000) and often not easy to retrofit to existing accelerators. One of the most promising approaches to portal imaging is a scanning system developed at Johns Hopkins Medical School a few years ago [3]. This system used a linear array of diodes to generate an image as it was scanned across the treatment portal in a manner which is conceptually similar to that used to scan baggage for security purposes at airports. The approach has several appealing aspects. The system, in principle, produces a good resolution image in real time using about 256 inexpensive solid-state diodes. The data are taken in electronic form and are thus compatible with computerized techniques for enhancement. Finally, the array and the associated mechanical scanning mechanism can be made sufficiently compact that it can be used with most existing medical linear accelerators. The fundamental problem with the Hopkins diode array scanning system is that the inherent sensitivity of the silicon diodes is insufficient to allow a useful image to be constructed in under zyxwv 40 seconds. The key to increasing the speed of response of this imaging system lies in improving the sensitivity of the detectors, for it is at that level that the entire performance of the system is limited. In particular, while the existing silicon-based system can generate an image within a short time by using only a single pulse from the accelerator at each position of the scan, the quality of the image is inadequate. Therefore in actual use, the system typically must average 16 radiation pulses at each scan position to obtain a good image by allowing the detectors to absorb enough photons to provide the statistical averaging needed to achieve useful contrast. We have begun construction of a new portal scanning system based on the use of high-Z, high density, cadmium telluride diodes. We have designed detectors specifically for use in the radiation fields associated with treatment accelerators and their sensitivity has been measured to confirm that it is indeed possible to obtain very good signal to noise ratios without the requirement to average more than one pulse from the accelerator. We are now in the process of constructing a full clinical prototype portal scanning system using a 256 element linear array of 2x2~2 mm3 CdTe high flux sensors. To date, we have completed a fully functional 48 channel system and have taken initial test images with it to obtain a first estimate of its performance. The entire 256 detector array has also been completed and the final electronics for the full system is now being assembled. 0018-9499/92$03.00 zyxwvutsr 0 1992 IEEE