ExAblate (Magnetic Resonance-guided Focused Ultrasound Surgery) Treatment of Brain Tumors

Although the initial treatment of malignant gliomas is well established, the best treatment for progressive disease remains undefined. Patients with newly diagnosed gliomas are typically treated with surgery followed by conformal radiation and concomitant chemotherapy. Even though these tumors are not curable, prolonged survival can be achieved in selected patients. Despite improved multimodal therapies, almost all of the patients experience recurrence at the site of the primary tumor where they have already received maximal surgical resection and radiation therapy. Typically, these patients have a life expectancy of approximately 6 months, with less than a third of patients alive at one year. Salvage therapy for these patients usually take the form of various systemic chemotherapeutic agents because localized therapies such as reirradiation may not be possible in the previously irradiated site. As progressive disease has very poor response rates to current systemic therapies, efforts to define the role of novel local therapies, such as MR guided focused ultrasound (MRgFUS), is necessary.

There are very few local therapy options for progressive gliomas. Traditionally, reirradiation and surgery are considered last resort treatments for symptomatic recurrence. In the initial treatment of gliomas, the tumor and surrounding brain receive near tolerance doses and usually preclude a second course of radiation therapy. If clinically advisable and feasible, a second operation may be undertaken but is usually reserved for younger patients with a good neurological status who are developing neurological symptoms related to the mass effect of the tumor. The use of MR guided focused ultrasound represents a new, noninvasive therapeutic option that overcomes some of the limitations of secondary surgery or reirradiation and provides the patient with an option for local ablative therapy. The lesioning of the tumor is done accurately under MRI guidance with real-time monitoring of the ablative ultrasonic hyperthermia. As the cytotoxic affects are due to hyperthermia, there is no cumulative radiation affect which limits the use of radiotherapy in recurrent disease. The potential benefits for the patient include local control of the disease/tumor, and prolonging the time to subsequent salvage therapies.

A similar situation exists for brain metastases. Historically, the development of brain metastases was considered a terminal event, however, with better local therapies as well a systemic therapies, patients are living longer. Initial management of patients with brain metastases usually involves surgery, whole brain radiation, radiosurgery or a combination of these modalities depending on the clinical situation. The goal of treating brain metastases is to prevent the patient from succumbing to CNS disease. As improved systemic therapy has lengthened the survival of patients with metastatic tumors, more patients are surviving and developing recurrent or progressive metastatic CNS disease. Progressive disease usually requires more local therapies as chemotherapeutic agents do not adequately cross the blood brain barrier to have a large impact on CNS metastases. MRIgFUS could play and important role in ablating brain metastases in patients who have already been maximally radiated and are otherwise without options.