SIMULTANEOUS DELIVERY OF ELECTRON-BEAM THERAPY AND ULTRASOUND HYPERTHERMIA USING SCANNING REFLECTORS - A FEASIBILITY STUDY

Purpose: The feasibility of simultaneously delivering external electro n beam radiation and superficial hyperthermia using a scanning ultraso und reflector-array system (SURAS) was experimentally investigated and demonstrated. Methods and Materials: A new system uses a scanning ref lector to distribute the acoustic energy from a planar ultrasound arra y over the surface of the target volume. External photon/electron beam s can be concurrently delivered with hyperthermia by irradiating throu gh 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 fo r maximum acoustic reflection and minimum radiation attenuation. Acous tically, the air reflectors were compared to brass reflectors (assumed ideal) for reflectivity and specular quality using several single tra nsducers ranging in frequency from 0.68 to 4.8 MHz, The relative refle ctivity 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 reflec ted by brass reflectors. Also, acoustic pressure fields generated by a SURAS prototype for two different arrays (2.24 and 4.5 MHz) were meas ured to investigate held 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. Fina lly, to determine the effect of the air reflectors on electron dose di stributions, these were measured using film in a water-equivalent soli d phantom after passage of a 20 MeV electron beam through the SURAS. T hese measurements were performed with the reflector scanning continuou sly across the electron beam and at rest within the electron beam. Res ults: The measurements performed using single ultrasound transducers s howed that the air reflectors had power reflectivities of 87-96% that of brass; and that for smooth surfaces the reflections from air reflec tors were as specular as those from brass reflectors. Acoustic pressur e fields measurements of the SURAS for two different arrays showed tha t the 50% pressure amplitude contours were well-distributed across the projected surface area of the array for different distances separatin g the array and the reflector. Finally, film dosimetry showed that the electron dose distribution was not affected by the air reflector of t he SURAS either for the scanning case or the stationary case. This ind icates that the reflectors as made are basically water-equivalent in t erms of high energy ionizing radiation. The measured isodoses also ind icate that the constructed SURAS prototype would allow the delivery of adequate radiation (90% isodose) to a depth of 2.0 cm. Conclusions: T he results presented show that the SURAS design has the potential to d eliver hyperthermia to large superficial tumors, white allowing simult aneous irradiation with 20 MeV electron beams without adverse effects on the radiation dose delivery.