While ordinary ultrasound is well known for its diagnostic use, high intensity focused ultrasound can be used to destroy tissue, including cancers.
Traditionally, surgery has been the only curative modality for many solid tumours. Recent technological advances have resulted in a shift towards less invasive techniques. High intensity focused ultrasound (HIFU) has promised to deliver the goal of non-invasive, highly targeted tumour ablation. Only recently however, with improvements in imaging, has it finally emerged as a real clinical possibility.
HIFU involves a high-energy focused ultrasound beam (10,000 times the power of diagnostic ultrasound) directed harmlessly across the skin and intervening tissues towards the target tumour. Only at the focus of the beam is the energy level great enough to cause a temperature rise to about 800C sufficient for instantaneous cell death. Unlike radiotherapy the mechanism of action of HIFU is not tumour-specific and so a wide variety of tumour types may be targeted. Furthermore the treatment can be repeated as there is no upper limit of tissue tolerance to ultrasound exposure. There are very few side-effects of treatment, and serious adverse events are very rare.
HIFU treatment may be guided by ultrasound or MRI imaging. A typical focal region is ellipsoidal in shape and may be 15 mm along the beam axis and 1.5mm in diameter. Two main devices are in clinical use for cancer treatment, a trans-rectal device for the treatment of prostate cancer and an extracorporeal device for the treatment of abdominal tumours.
HIFU has been used to clinically treat:
- Liver tumours
- Kidney tumours
- Pancreas tumours
- Bone tumours
- Breast tumours
- Soft tissue sarcoma as well as 3.
Much of the experience originates from China although with few good trials. One randomised study has looked at trans arterial embolisation of hepatocellular carcinoma with and without HIFU treatment and has shown and increased survival with HIFU treatment. There is now increasing experience of its use in the western world.
Improvements in technology and imaging have already resulted in equipment modifications that will improve treatment outcomes but only through appropriately planned and conducted trials will we develop an understanding of how best to include HIFU in the overall management of malignant tumours.