Tumor treating fields: concept, evidence and future

Local control is fundamental, both for the curative as well as the palliative treatment of cancer. Tumor treating fields (TTFields) are low intensity (1 ? 2 V/cm), intermediate frequency (100 ? 200 kHz) alternating electric fields administered using insulated electrodes placed on the skin surrounding the region of a malignant tumor. TTFields were shown to destroy cells within the process of mitosis via apoptosis, thereby inhibiting tumor growth. TTFields have no effect on non-dividing cells.

This article reviews in vitro and in vivo preclinical studies, demonstrating the activity of TTFields both as a monotherapy as well as in combination with several cytotoxic agents. Furthermore, it summarizes the clinical experience with TTFields, mainly in two indications: one in recurrent glioblastoma multiforme: in a large prospective randomized Phase III trial TTFields was compared with best standard care (including chemotherapy): TTFields significantly improved median overall survival (OS) compared with standard therapy (7.8 vs 6.1 months) for the patients treated per protocol. Importantly, quality of life was also better in the TTFields group. The second indication was a Phase II study in second-line non-small cell lung cancer, where TTFields was administered concomitantly with pemetrexed. This combination resulted in an excellent median OS of 13.8 months. Interestingly, the progression-free survival (PFS) within the area of the TTFields was 28, however, outside the TTFields the PFS was only 22 weeks.

The proof of concept of TTFields has been well demonstrated in the preclinical setting, and the clinical data seem promising in various tumor types. The side effects of TTFields were minimal and in general consisted of skin reaction to the electrodes. There are a number of ways in which TTFields could be further evaluated, for example, in combination with chemotherapy, as a maintenance treatment, or as a salvage therapy if radiotherapy or surgery is not possible. While more clinical data are clearly needed, TTFields is an emerging and promising novel treatment concept.