Revolutionary focused ultrasound still needs to address limitations


In the past five years we have treated more than 400 patients for uterine fibroids, bone metastasis and osteoid osteoma in routine practice with significant clinical results and negligible adverse events, but therapeutic ultrasound still has some technological hurdles that need to be overcome, writes Alessandro Napoli.

Therapeutic ultrasound is an increasingly mature, image-guided treatment modality with an ever-widening number of practitioners providing effective, totally non-invasive and non-ionising solutions to treat uterine fibroids, bone metastases, prostate and breast cancers. With this modality, we are even able to offer alternatives to deep brain stimulation for patients who suffer from debilitating tremors.

In the past five years, in my own department, we have treated more than 400 patients for uterine fibroids, bone metastasis and osteoid osteoma in routine practice with significant clinical results and negligible adverse events.

As the technology improves briskly it has had to confront new limitations. While some seem relatively mundane—such as patient positioning to ensure adequate contact between skin and the transducer surface—others are more challenging.

The two primary guidance methods of delivering focused ultrasound have traditionally been ultrasonography-guided (US-HIFU) and magnetic resonance-guided (MRgFUS). Each of these modalities presents their own unique challenges that are compounded by the limiting factors of the technology at large. These are primarily: tracking tissue that moves with respiration, gaining an acoustic window when the target is blocked by bony tissue, and treating through intermediary tissue such as bowel loops.  When the abdominal organs are considered, all three of these factors come into play and have so far precluded large scale clinical investigations from taking place.

This is not to say that work has not been done in these areas. We have seen good success in treating hepatic, renal, and pancreatic neoplasms in animal models as well as human volunteers. Hepatic treatments in particular have shown promise, as well as the treatment of liver metastases via USgHIFU. In my own department, MRgFUS ablation has successfully been performed in one patient with unifocal hepatocellular carcinoma in the right liver lobe (segment VI). The patient refused surgery and was not eligible for other treatment options, including transarterial chemoembolization and radiofrequency ablation. The treatment was performed under general anaesthesia and we were limited by the need to position the patient to create optimal contact between the skin and the transducer surface. Despite this, post-treatment results showed a decrease in alpha fetoprotein compared to baseline.

We have also seen the treatment of seven patients who had histologically-proven unresectable pancreatic adenocarcinomas, with six of them exhibiting successful treatment for pain palliation. The visual analog scale score that measured pain decreased in all patients from a mean (SD) of seven to three after treatment with follow-up imaging showing negligible (n=1) or no (n=5) tumour regrowth within the ablated area. 

While my clinic has opted for MRgFUS, the platform has limitations that will need to be addressed if the technology is to reach its full potential. Compared to the real-time tracking provided by US-HIFU, MR-guidance is near real-time, with the potential to produce images several times a second. MRgFUS also suffers from the limitations of having to place patients within the MRI bore, as well as certain mechanical limitations. On the upside, MRgFUS has the advantage, absent in US-HIFU, of providing real-time awareness of ablated tissue via MR thermometry and of providing a more complete imaging solution. 

While US-HIFU is a more comfortable and economical modality, it suffers from its own limitations. Its imaging capabilities will never be as complete as MR-guidance—seeing past bony tissue will always be a limiting treatment factor, meaning lesions in some areas will simply not be viewable and remain inaccessible to treatment. And as mentioned, US-HIFU is also severely limited by being unable to provide the necessary real-time feedback regarding thermal rise at the target site, meaning ablation is often suboptimal and re-treatment is required.

While there are still technological hurdles to be overcome, several companies around the world are working hard to address these challenges and to increase the treatable population to ensure that therapeutic ultrasound has a bright and exciting future.

Alessandro Napoli, is an assistant professor at the Department of Radiological, Oncological and Pathological Sciences, Sapienza University of Rome, Italy. He has reported no disclosures pertaining to the article