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

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

L. NAUMANN1, T. KAPREALIAN1, M. CAO1, R. SAVJANI1, K. IWAMOTO1, R. SANDSTROM2, L. STRAUSE2,3, and D. LOW1 1University of California Los Angeles, Los Angeles, California, USA 2Triplet State Technology LLC, Product Development, Longview, Washington, USA 3University of California San Diego, San Diego, California, USA

Weak magnetic fields (WMFs) extend free radical life and recent experimental results have shown that adding WMFs (20G-40G) to radiation therapy delivery enhances free radical production, increases tumor cell death, and slows tumor growth.

Triplet State has built the first human-sized, non-clinical WMF unit, consisting of two 63 cm ID Helmholtz coils, spaced 35 cm apart. The field generator constrains the gantry delivery angles to less than 95° and more than 265°.

The purpose of this work was to demonstrate the feasibility of meeting dose prescription constraints when using the magnetic field generator to treat glioblastoma multiforme (GBM).

A retrospective treatment planning study was conducted to determine if treatment plans with the gantry angle constraints imposed by the generator could be created that were clinically equivalent to the treated plans. These were generated using half-arcs. We evaluated the treatment plans for 10 GBM patients that had been treated with coplanar arc therapy.

Figure 1: Device set up for simulated treatment

The treatment plans generated with the half-arcs met the clinical criteria.

The minimum planning target volume (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.

All the plans delivered the prescribed dose, and all the critical structures met their respective physician constraints.

Table 1 summarizes the results for the four reported critical structures. 

Table 1: Differences (Gy) between dose constraints and achieved doses for GBM critical organs across all patients (mean ± standard deviation).

Table 2 compares the number of patients (out of 10) that fell within a certain percentage below the dose
constraint (none were above) between the clinical plans (360) and half-arc plans (180) for each critical structure.

Table 2: Number of patients within percentage of constraint

The half-arc plans were equivalent to the clinical plans.

Acceptable treatment plans for GBM radiation therapy can be generated in spite of the physical limits imposed by the magnetic field generator on the linac beam
gantry angles. 

The prescribed dose could be adequately delivered while avoiding critical structures.

Iwamoto KS, Sandstrom RE, Bryan M, et al. Weak Magnetic Fields Enhance the Efficacy of Radiation Therapy. Adv Radiat Oncol. May-Jun 2021; 6(3):100645.

O’Dea AR, Curtis AF, Green NJB, Timmel CR, Hore PJ. Influence of Dipolar Interactions on Radical Pair Recombination Reactions  Subject to Weak Magnetic Fields. The Journal of Physical Chemistry A. 2005/02/01 2005;109(5):869-873. doi:10.1021/jp0456943

This work was supported in part by Triplet State Technology, LLC.

Louise Naumann


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