The effect of field strength on glioblastoma multiforme response in patients treated with the NovoTTF™-100A system

The NovoTTF™-100A system is a portable device that delivers intermediate frequency alternating electric fields (TTFields, tumor treating fields) through transducer arrays arranged on the scalp. An ongoing trial is assessing its efficacy for newly diagnosed glioblastoma multiforme (GBM) and it has been FDA-approved for recurrent GBM. The fields are believed to interfere with formation of the mitotic spindle as well as to affect polar molecules at telophase, thus preventing cell division. The position of the four arrays is unique to each patient and optimized based on the patient’s imaging. We present three patients with GBM in whom the fields were adjusted at recurrence and the effects of each adjustment. We believe there may be a higher risk of treatment failure on the edges of the field where the field strength may be lower. The first patient underwent subtotal resection, radiotherapy with temozolomide (TMZ), and then began NovoTTF Therapy with metronomic TMZ. She had good control for nine months; however, new bifrontal lesions developed, and her fields were adjusted with a subsequent radiographic response. Over the next five months, her tumor burden increased and death was preceded by a right insular recurrence. A second patient underwent two resections followed by radiotherapy/TMZ and NovoTTF Therapy/TMZ. Six months later, two new distal lesions were noted, and he underwent further resection with adjustment of his fields. He remained stable over the subsequent year on NovoTTF Therapy and bevacizumab. A third patient on NovoTTF Therapy/TMZ remained stable for two years but developed a small, slow growing enhancing lesion, which was resected, and his fields were adjusted accordingly. Interestingly, the pathology showed giant cell GBM with multiple syncitial-type cells. Based on these observations, we believe that field strength may play a role in ‘out of field’ recurrences and that either the presence of a certain field strength may select for cells that are of a different size or that tumor cells may change size to avoid the effects of the TTFields.