Introduction
Tumor Treating Fields (TTFields) are approved for the treatment of Glioblastoma (GBM). The efficacy of TTFields increases with field intensity at the tumor which depends on its highly heterogeneous electrical properties (EPs) distribution. Thus, an imaging technique, preferably with conventional sequences, rapid acquisition and fast processing, that is able to assess the EPs non-invasively might be of high interest for patient-specific TTFields treatment planning.
An approach termed water content electrical properties tomography (wEPT), estimates the EPs of brain tissue at 128 MHz from water content (WC) maps which are created with two spin echo sequences resembling a T1w and a PD image. Recently we performed experiments in tumor-bearing rats suggesting that wEPT could be adapted to map EPs in the brain at 200 kHz. Here we tested the feasibility of applying wEPT to map EPs at 200 kHz in GBM patients.
Methods
Analysis was performed for three patients that participated in the EF-14 trial. The image ratio was calculated as pixel-by-pixel division of T1 and PD images. The WC maps were estimated with a transfer function and two separate equations were used to calculate the maps of the electrical conductivity σ and the relative permittivity ϵr at 200 kHz. The table summarizes the median values in 5 tissues of normalized T1 and PD signals and the wEPT-estimations of WC, σ, and ϵr at 200 kHz.
Results
| median values | T1norm | PDnorm | WC[%] | σ[S/m] | ϵr[-] | |
| white matter | Patient 1 | 0,23 | 0,58 | 76% | 0,11 | 2575 |
| Patient 2 | 0,18 | 0,32 | 76% | 0,11 | 2462 | |
| Patient 3 | 0,24 | 0,35 | 74% | 0,10 | 2191 | |
| gray matter | Patient 1 | 0,20 | 0,60 | 83% | 0,22 | 2923 |
| Patient 2 | 0,16 | 0,33 | 81% | 0,17 | 2799 | |
| Patient 3 | 0,22 | 0,38 | 81% | 0,17 | 2919 | |
| necrosis | Patient 1 | 0,23 | 0,86 | 89% | 0,47 | 2495 |
| Patient 2 | 0,12 | 0,32 | 98% | 1,43 | 1381 | |
| Patient 3 | 0,24 | 0,44 | 84% | 0,24 | 2988 | |
| enhancing tumor | Patient 1 | 0,37 | 0,83 | 71% | 0,09 | 1546 |
| Patient 2 | 0,17 | 0,35 | 81% | 0,17 | 2961 | |
| Patient 3 | 0,23 | 0,42 | 85% | 0,26 | 2932 | |
| non-enhancing tumor | Patient 1 | 0,23 | 0,73 | 85% | 0,26 | 2979 |
| Patient 2 | 0,18 | 0,36 | 78% | 0,14 | 2769 | |
| median values | T1norm | PDnorm | WC[%] | σ[S/m] | ϵr[-] | |
| white matter | Patient 1 | 0,23 | 0,58 | 76% | 0,11 | 2575 |
| Patient 2 | 0,18 | 0,32 | 76% | 0,11 | 2462 | |
| Patient 3 | 0,24 | 0,35 | 74% | 0,10 | 2191 | |
| gray matter | Patient 1 | 0,20 | 0,60 | 83% | 0,22 | 2923 |
| Patient 2 | 0,16 | 0,33 | 81% | 0,17 | 2799 | |
| Patient 3 | 0,22 | 0,38 | 81% | 0,17 | 2919 | |
| necrosis | Patient 1 | 0,23 | 0,86 | 89% | 0,47 | 2495 |
| Patient 2 | 0,12 | 0,32 | 98% | 1,43 | 1381 | |
| Patient 3 | 0,24 | 0,44 | 84% | 0,24 | 2988 | |
| enhancing tumor | Patient 1 | 0,37 | 0,83 | 71% | 0,09 | 1546 |
| Patient 2 | 0,17 | 0,35 | 81% | 0,17 | 2961 | |
| Patient 3 | 0,23 | 0,42 | 85% | 0,26 | 2932 | |
| non-enhancing tumor | Patient 1 | 0,23 | 0,73 | 85% | 0,26 | 2979 |
| Patient 2 | 0,18 | 0,36 | 78% | 0,14 | 2769 | |
Conclusions
We adapted the wEPT approach to map WC and EPs at 200 kHz in three GBM patients. In healthy brain tissues the estimations are consistent with literature and also among patients, contrary to the highly variable tumor. This fast approach only needing conventional MRI holds some promise for patient-specific TTFields treatment planning. Yet the mapping of tumor tissues needs to be validated further, possibly including EP measurements of excised tumor samples and generally a higher number of patients analyzed.
Citation Format: Cornelia Wenger, Hadas Sara Hershkovich, Catherine Tempel Brami, Moshe Giladi, Zeev Bomzon. Using conventional imaging to predict water content and electrical properties at 200 kHz in brain and GBM tumor tissues: a feasibility study in three TTFields patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1413.