Abstract 676: Preliminary investigation into the dosimetric impact of tumor treating field arrays on concurrent radiotherapy for newly-diagnosed glioblastoma
Authors: Stachelek Gregory C. , Grimm Jimm , Moore Joseph , Jackson Juan F. , Redmond Kristin , Bettegowda Chetan , Kleinberg Lawrence 
Published: August 2018

Abstract
Rationale:
Glioblastoma (GBM) is the most common and deadly primary CNS malignancy in adults. The addition of tumor treating fields (TTFields), a non-invasive anti-mitotic therapy approved for use in newly-diagnosed or recurrent GBM, following standard resection and chemoradiation has been shown to significantly improve both overall and progression-free survival in a randomized phase III clinical trial.
Preclinical evidence suggests that TTFields may disrupt mitotic spindles and exhibit a similar synergism with radiotherapy (RT) to other microtubule inhibitors. The ability to treat patients concurrently with RT and TTFields is thus of considerable clinical interest. The TTFields device array applicator worn on the scalp may affect radiation dose distribution, an effect that may be mitigated but also be made more difficult to account for by the routine repositioning of the applicator during therapy to avoid skin toxicity. In preparation for a clinical trial, we have therefore undertaken a phantom investigation to evaluate the effect of varying TTFields array position on RT dosimetry and planning target volume (PTV) coverage.
Methods:
Radiation beams and contours were copied to an anthropomorphic phantom from the treatment plan of a newly-diagnosed GBM patient receiving standard adjuvant RT. Four CT scans of the phantom were employed: one lacking the TTFields array and three with the array placed in varying positions over the cranium of the phantom to simulate routine replacement and repositioning of the array as would occur in clinical use.
Dosimetric recalculations were then performed for each scan using the Philips Pinnacle treatment planning system and the dosimetric average of the three array positions was calculated using Varian Velocity software.
Results:
Recalculation of the treatment plan of our test patient onto the phantom wearing the TTFields array resulted in reduction of the isodose covering 95% of the PTV by 5.5 cGy/day over the course of treatment as compared to the phantom with no array (to 51.6 Gy from 53.4 Gy). This small difference in PTV coverage is unlikely to be of clinical significance, obviating the need for time-intensive replanning each time the skin position of the array has changed.
Conclusion:
We plan to confirm this novel finding by further investigating the impact of repositioning TTFields arrays on the radiation treatment plans of a larger, sequential cohort of newly-diagnosed GBM patients. These preliminary studies will ultimately be the foundation of a clinical trial evaluating the concurrent use of TTFields during standard radiotherapy.
Citation Format: Gregory C. Stachelek, Jimm Grimm, Joseph Moore, Juan F. Jackson, Kristin Redmond, Chetan Bettegowda, Lawrence Kleinberg. Preliminary investigation into the dosimetric impact of tumor treating field arrays on concurrent radiotherapy for newly-diagnosed glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 676.