Dosimetric Evaluation of Physical Radiation Delivery Limits of a Weak Magnetic Field Generator on GBM Dose Distributions

Abstract

<h3>Purpose/Objective(s)</h3> To demonstrate the feasibility of meeting dose prescription criteria when using a weak magnetic field generator to support glioblastoma multiforme (GBM) radiation therapy. <h3>Materials/Methods</h3> Weak magnetic fields extend free radical life, and recent experimental results have shown that adding weak magnetic fields (20G-40G) to radiation therapy delivery enhances free radical production, increases tumor cell death, and slows tumor growth. Based on these results, we have built the first clinical unit, consisting of two 63 cm ID Helmholtz coils, spaced 35 cm apart. The coil system is placed such that the patient and Linac couch are within the coils, and the Linac isocenter is coincident with the center of the coil system. We propose to use GBM for our first clinical trial. The limitation imposed by the coil system is a fixed Linac couch angle, constraining the gantry rotation to slightly more than 180°, from lateral to lateral. The impact on treatment plan quality of the corresponding reduction in available beam angles was unknown, so we conducted a treatment planning study to determine if we could provide clinically equivalent treatment plans with the coil in place. We evaluated the treatment plans for 10 GBM patients that had been treated with arc therapy. The prescription was 60 Gy (2 Gy x 30 fractions) for 6 patients, 55.8 Gy (1.8 Gy x 31 fractions) for 2 patients, 54 Gy (1.8 Gy x 30 fractions) for 1 patient, and 40.05 Gy (2.67 Gy x 15 fractions) for 1 patient. The dose was delivered to 95% of the planning target volume (PTV). The PTV margin was 1.3-2 cm surrounding the gross tumor volume. Critical structures, such as the brainstem, optic nerves, and optic chiasm, were contoured and assigned constraints of 62 Gy, 54 Gy, and 54 Gy, respectively. We repeated the treatment planning session using only coplanar arcs that spanned from 90° to 270° (half-arcs). <h3>Results</h3> All the treatment plans generated with the half-arcs met the clinical criteria. The minimum PTV doses for the half arc and clinical plans were 50.0 ± 6.1 Gy and 51.4 ± 7.1 Gy, and the maximum PTV doses were 61.9 ± 7.1 Gy and 62.7 ± 8.6 Gy, respectively. Table 1 summarizes the results of four reported critical structures. All the critical structures met the criteria, and the half-arc plans were neither better nor worse than the clinical plans. <h3>Conclusion</h3> The physical limits imposed by the magnetic field generator on the Linac beam angles do not degrade treatment planning for GBM radiation therapy. The prescribed dose can be adequately delivered while avoiding critical structures. With this groundwork, we can develop clinical trials to evaluate the impact of utilizing weak magnetic fields in radiation therapy.