RADIATION THERAPY TARGETING ACCURACY WHEN COMBINED WITH ALTERNATING ELECTRIC FIELD THERAPY

Tumor treating fields (TTFields) is a noninvasive, regional antimitotic treatment approved for the treatment of newly-diagnosed and recurrent glioblastoma (GBM) including use concurrent with temozolomide. As anti-mitotic therapies may have synergy with radiotherapy, we have developed a plan to implement use of this therapy during radiotherapy and to test safety and tolerability. In preparation for the clinical trial, we plan tests evaluating impact of placement of the applicator (transducer array) on RT targeting and dosimetry using an anthropomorphic phantom, aquaplast immobilization, and intensity-modulated radiotherapy. In this first test, several strategies were employed to ensure minimal degradation of targeting accuracy: 1) the CT scan artifacts due to the array were reduced by use of the Metal Artifact Reduction for Orthopedic Implants (O-MAR) algorithm, 2) radiation was delivered to a tumor location that did not require the radiation beam to pass through the array, by using an arrangement of six noncoplanar intensity modulated radiation therapy (IMRT) beams entering inferiorly, 3) the cone beam CT (CBCT) alignment was performed with a region of interest box that excluded any of the arrays. The end result was an X,Y,Z targeting error of 0.58 mm, -0.59 mm, 0.40 mm, for a 3D Euclidean error of 0.92 mm, well within the acceptable range for GBM irradiation. An extensive series of systematic tests of comprehensive geometries is planned to determine the accuracy in various tumor locations and geometries including dosimetric effects. The data obtained from these preclinical investigations, when completed, will support optimal testing of concurrent TTFields and radiation for safety/tolerability and ultimately efficacy.