AbstractID: 9042 Title: Demonstration of Minimal Impact to Radiation Beam and Portal Image Quality Due to the Presence of an Electromagnetic Array A prototype array for precision target localization and continuous tracking has been evaluated for impact to the radiation beam and portal image quality (Calypso Medical Technologies, Seattle, WA). The array is used in conjunction with miniature, permanently implanted, wireless electromagnetic transponders (Beacon™ Transponders) to localize and continuously track targets in threedimensional space. The array is positioned above the patient during radiation therapy delivery. A target is marked by small, implanted wireless transponders. During treatment, the array may be in the path of the radiation beam for some gantry positions. The array has been designed with low-density materials to minimize radiation beam attenuation and the production of scatter radiation. Furthermore, the low-density array was designed to minimize degradation of portal image quality as the beam passes through the array during portal image formation. Ionization chamber measurements were performed with and without the array in the 6 MV photon beam in clinical settings. Port films and portal images were acquired at several angles to assess the geometry and image quality with the array in the beam path. Measurement of tissue maximum ratios (TMRs) confirmed that the increase in skin dose attributable to the array was within acceptable clinical limits. The radiation beam attenuation was measured to be less than 0.5% for radiation incidence normal to the array. In addition, degradation of portal image quality due to presence of the array in the beam path was similar to that of patient support devices, such as nylon-strung tennis racquet table inserts.