Simultaneous delivery of electron beam therapy and ultrasound hyperthermia using scanning reflectors: a feasibility study

Abstract Purpose: The feasibility of simultaneously delivering external electron beam radiation and superficial hyperthermia using a scanning ultrasound reflector-array system (SURAS) was experimentally investigated and demonstrated. Methods and Materials: A new system uses a scanning reflector to distribute the acoustic energy from a planar ultrasound array over the surface of the target volume. External photon/electron beams can be concurrently delivered with hyperthermia by irradiating through the scanning reflectors. That is, this system enables the acoustic waves and the radiation beams to enter the target volume from the same direction. Reflectors were constructed of air-equivalent materials for maximum acoustic reflection and minimum radiation attenuation. Acoustically, the air reflectors were compared to brass reflectors (assumed ideal) for reflectivity and specular quality using several single transducers ranging in frequency from 0.68 to 4.8 MHz. The relative reflectivity was determined from acoustic power measurements using a force-balance technique. The specular quality was assessed by comparing the acoustic pressure fields reflected by air reflectors with those reflected by brass reflectors. Also, acoustic pressure fields generated by a SURAS prototype for two different arrays (2.24 and 4.5 MHz) were measured to investigate field distribution variations as a function of the distance separating the array and the scanning reflector. All pressure fields were measured with a hydrophone in a degassed water tank. Finally, to determine the effect of the air reflectors on electron dose distributions, these were measured using film in a water-equivalent solid phantom after passage of a 20 MeV electron beam through the SURAS. These measurements were performed with the reflector scanning continuously across the electron beam and at rest within the electron beam. Results: The measurements performed using single ultrasound transducers showed that the air reflectors had power reflectivities of 87–96% that of brass, and that for smooth surfaces the reflections from air reflectors were as specular as those from brass reflectors. Acoustic pressure fields measurements of the SURAS for two different arrays showed that the 50% pressure amplitude contours were well-distributed across the projected surface area of the array for different distances separating the array and the reflector. Finally, film dosimetry showed that the electron dose distribution was not affected by the air reflector of the SURAS either for the scanning case or the stationary case. This indicates that the reflectors as made are basically water-equivalent in terms of high energy ionizing radiation. The measured isodoses also indicate that the constructed SURAS prototype would allow the delivery of adequate radiation (90% isodose) to a depth of 2.0 cm. Conclusions: The results presented show that the SURAS design has the potential to deliver hyperthermia to large superficial tumors, while allowing simultaneous irradiation with 20 MeV electron beams without adverse effects on the radiation dose delivery.