The Importance of Understanding the Physics of Ablation

At the 2024 Society for Interventional Oncology (SIO) Annual Meeting, Christos Georgiades, MD, PhD, Johns Hopkins University, Baltimore, Maryland, explained the physics of different ablation modalities for interventional oncologists.

In this summary, Dr Georgiades highlights the key points from his talk, including the differences in the physics of each ablation modality and how understanding those differences can help a clinician to improve efficacy of the procedure and reduce side effects.

Transcript:

I am Christos Georgiades. I'm a physician and professor at Johns Hopkins Hospital. My specialty isinterventional oncology. The talk is one that I gave in various forms at different conferences, and it focuses on the physics of ablation modalities. The reason I give this talk is because I think it's important for operators, clinicians to know a little bit of the physics, because I found out that it improves efficacy and minimizes the complications. During the talk, I specifically focused on thermal ablation mechanisms, microwave ablation, radiofrequency ablation (which as you may know utilizes electromagnetic radiation to heat and kill tissues), high intensity focused ultrasound, cryoablation (which freezes tumors), and histotripsy, which is a novel technology. And I went into the basic physics of each of these modalities.

How do the physics differ among the modalities of ablation?

Thermal ablative mechanisms, which include radiofrequency ablation, microwave ablation, and high intensity focused ultrasound, kill tissue by healing it up. It doesn't matter what the tissue is, whether it's healthy tissue or diseased tissue — cancer in this case. So it behooves the operator to ensure that the ablation volume includes the target tissue and only the target tissue, to avoid collateral damage. There are ablation or treatment modalities that don't suffer from that specific limitation. For example, histotripsy appears to kill cells specifically, while leaving the extracellular matrix relatively intact. High intensity focused ultrasound also kills tissue somewhat less indiscriminately as other heat-based modalities, but also suffers a little bit from that limitation. Cryoablation freezes tissue. It has certain advantages and disadvantages. For example, it's ablation shape is very well defined, well demarcated and seen very well on the imaging modality. So one can predict the eventual shape of the targeted ablation. On the other hand, because it freezes, it doesn't burn, it doesn't coagulate damaged vessels, so it suffers from a slightly increased risk of hemorrhage. These are the details that the operator needs to know to those complications.

Why is it important for clinicians to understand the physics of ablation?

I'll give you a specific example. This is common knowledge but worth reiterating. If the targeted tumor is next to a sizable blood vessel, the flow of blood can limit the effect of the ablation, whether that is cryoablation, freezing, or microwave ablation, heating. The term is called the “heat sink effect” or the “heat pump effect,” in the case of cryoablation, which counteracts the ablation itself. What that means is cancer cells that are right next to the blood vessel may not heat or cool enough to die. That’s one aspect that the operator needs to be aware of. Maybe more time or higher temperature or lower temperature, or a different ablation modality may be indicated. Another example is if it is next to a critical non-target structure, a nerve perhaps, we should avoid burning and use cryoablation, which is a bit more forgiving in those cases. So that's one of the physical aspects that we should be aware of.

Can better understanding the physics of ablation help a clinician avoid side effects?

Yes, both in absolute and relative terms, meaning comparing one to the other. If somebody is aware of the ablation size or the distribution of temperature, what exactly the modality will kill, one can theoretically construct the ablation volume in one's mind before one performs the ablation. I always say this: the ablation is finished before it even starts. If the operator is aware of the tool they use to ablate and goes through the steps of the ablation and marries those steps with the patient's distinct anatomy and the target tissue, that is a recipe for success. They will note the possible complications, how effective it will be. And even one step further, if you know the possible complications, you can be prepared to mitigate them, and that helps you be a little bit more aggressive to ensure efficacy and to be able to, like I said, mitigate those complications.

How does histotripsy work and how is it different from other ablative modalities?

Most interventional oncologists are familiar with high intensity focused ultrasound. Histotripsy utilizes ultrasound, but in a different way. High intensity focused ultrasound, or HIFU, kills tissues by focusing high energy ultrasound pulses to increase temperature, thereby killing tissue. Histotripsy avoids hitting the tissue. It constructs an ultrasound waveform that creates what we call microbubbles or cavitations. It's the same way that submarines cause cavitations, or if you move a body in water, you'll cause bubbles in water. And that's because the sudden drop in pressure brings the gasses out of solution momentarily, because they immediately disappear. That disappearing is called an implosion. There’s a low-pressure bubble surrounded by higher-pressure water, and once that disturbance goes away, that microbubble implodes. It's a mini explosion to describe it imperfectly, but adequately, I suppose, that creates mechanical forces that disrupt and kill the cells without increasing the temperature. That theoretically has some advantages: by not hitting the tissue, it will not destroy blood vessels or extracellular structures or the matrix, and only kill the cells. That is the theory. Preliminary results support that, but we need a lot more research and outcomes to make that part of our daily clinical practice.

Is there anything else you would like to add?

Inevitably, in this presentation, when we end the discussion, with people supporting one over the other, this is better for this, that is better for that, and we almost universally ignore the most important factor, and that is operator experience. If somebody is well-versed and has a lot of experience using microwave, for example, for a certain tumor, and they have excellent outcomes, then I suggest that's what they should stick with. I'm not adv advocating not learning a new modality but to follow blindly the published manuscripts that one modality is better for this tumor over another modality while ignoring the operator’s experience is something that we should avoid. Because it will decrease efficacy and increase complications. If one is starting from the beginning, they're at the beginning of their career, then they should read up what is more effective for what tumor and follow those guidelines. But operator experience is something that is very important and many times we ignore during these conversations.

Source:

Georgiades C. "The Physics of Ablation Modalities." Presented at Presented at the SIO 2024 Annual Scientific Meeting; January 25-29, 2024; Los Angeles, California.