Magnetic Resonance-guided Focused Ultrasound for Treatment and Palliation of Selected Cancers
January 27, 2014 | Technology Forecasts
MRgFUS is a noninvasive thermal (i.e., heat) ablation technique that FDA approved in 2012 for treating pain from bone metastases (and for benign uterine fibroids in 2004).1,2 Bone metastases occur in more than half of patients with solid tumors, and pain may not be responsive to pain medication or biologic therapy, such as denosumab. MRgFUS developers are seeking to expand its use for treating other solid cancers. MRgFUS is being studied for use in bone metastases and prostate, breast, lung, brain, liver, and kidney cancers.3 MRgFUS uses HIFU guided by real-time MRI in one unit. MRgFUS treats solid tumors by focusing a HIFU beam at a tumor to produce heat to cause necrosis and destroy the tumor. MRgFUS could potentially be used at many points in the treatment pathway for solid tumors and in combination with existing therapies, such as chemotherapy or targeted biologic therapies. Potential advantages of MRgFUS include its noninvasive approach, absence of ionizing radiation, short recovery time, and treatment accomplished in a single treatment session.4 MRgFUS therapy also can be used multiple times as needed, unlike EBRT and even after failure of EBRT, because no ionizing radiation is used.5
One potential drawback of MRgFUS therapy is inconsistent administration. Target volume has not been widely defined, so the boundaries of the treatment area can vary by practitioner. Submicroscopic disease at the tumor margins could be missed in some treatments;5 radiation therapy might offer an advantage over MRgFUS in this case. Additionally, MRgFUS does not allow tumors to be resected and analyzed for definitive staging. For abdominal tumors, the presence of ribs or movement caused by the diaphragm during respiration can complicate treatment.5 Other tumor locations could necessitate an invasive procedure (e.g., rib removal or artificial pleural effusion in liver cancer) before MRgFUS therapy to improve access to the lesion.3
MRgFUS is typically performed in the outpatient setting with the patient under conscious sedation and analgesia.3 The patient lies prone on the procedure table with the target lesion positioned over the ultrasound transducer and a coupling gel pad between the patient and transducer. The transducer, which produces the HIFU waves, is housed in a water-bath cooling system to prevent overheating and unwanted tissue damage. Planning accuracy is confirmed by initiating treatment with a subtherapeutic ultrasonic impulse and observing a mild temperature increase in the target. HIFU operates like a magnifying glass in that it directs and concentrates multiple ultrasonic waves to a small target on a lesion. Waves move through interceding tissue purportedly without adversely affecting the tissue in the pathway. The waves converge at an MRI-localized target and heat the tissue to cause protein denaturation and necrosis.6 The ultrasonic impulses, called sonications, are followed by cool-down periods so that nearby tissue is not unnecessarily heated.5 Multiple sonications at different points are required to treat a larger tumor. Treatment typically takes one hour per lesion but can vary with tumor size and location.7
During treatment, quantitative MR thermometry—with phase map imaging—monitors temperature changes, ensuring that tissues are heated between 57°C and 95°C.3,4,6 Treatment parameters (e.g., power, frequency, sonication duration, sonication target size) can be adjusted during treatment based on MR imaging. Contrast-enhanced, T1-weighted MRI can follow treatment immediately or for up to several weeks later, after side effects have abated, to determine treatment efficacy (i.e., tumor cell ablation).4 For pain palliation of bone metastases, the bone cortex on the surface becomes heated from MRgFUS, and the HIFU indirectly ablates the...