A New Planning Tool for Cryoablation: An Interview With Franz Edward Boas, MD, PhD

Franz Edward Boas, MD, is an interventional radiologist at Memorial Sloan Kettering Cancer Center. Dr. Boas and his colleagues have developed a tool for planning cryoablation procedures, and he presented details about the tool at the 2016 Annual Scientific Meeting of the Society of Interventional Radiology in Vancouver, British Columbia, Canada. He sat down with Interventional Oncology 360 at the meeting to discuss the need for and development of the tool.

IO360: What was the impetus for creating this planning tool? 

Boas: If you look at radiation oncology, they have great planning tools. They know in advance how much radiation they’re going to deliver to the tumor vs the surrounding nontarget tissue. We don’t have tools like that for cryoablation.

We have limited data from the manufacturer; the manufacturer might tell you, if you use a single probe and you freeze for 10 minutes, you’ll get a 3 cm × 4 cm ice ball. That’s great if you want a 3 cm × 4 cm ice ball, but if you want a different size, then you have to adjust the ablation time or use multiple ablation probes. At that point you’re pretty much flying blind. You’re using guesswork and intuition to try to get an ice ball that covers the entire tumor with good margins. So we’ve developed a cryoablation planning tool where you specify what size ablation zone you want, and the tool will tell you where to place the ablation probes and how long to ablate. We think this tool could potentially take the guesswork out of cryoablation procedures. 

IO360: Could you share some details about how the planning tool was developed?

Boas: We got some experimental data from Galil Medical (acquired by BTG in May 2016) on ice ball sizes in gel, and I wrote some software to do cryoablation simulation. It actually involved looking through the literature, digging up old papers; there are a lot of data published on thermal properties of kidney and liver. It’s in old papers, but you can dig them up. I used that information from multiple sources, brought it together, and wrote software that would simulate these cryoablation procedures. 

Once we had the simulation software, we first validated it using the gel data to make sure we were accurately predicting the ice ball sizes, and we also validated it in some clinical cases to show that the ice ball sizes we got in clinical cases matched the predictions. Once we were able to predict the ice ball size, the next step is to develop the planning software. One challenge is that although the simulation software is very accurate, it’s also very slow, so we had to develop a technique that would allow us to do planning more quickly. We developed a precomputed database where we calculated thousands of different cryoablation procedures. 

The database includes simulations of ice ball size and shape using the Pennes bioheat equation on a 1 mm isotropic 3D grid, using published values for tissue-specific thermal conductivity, heat capacity, water content, and perfusion. We performed 5,670 different simulations of 1-6 cryoablation probes (IceRod Plus or IceSphere [Galil Medical/BTG]), each placed in an axial plane, with 1-2 cm spacing between probes. Ablation was simulated in different tissues (kidney, liver, muscle, fat, and water), with a variety of different freeze and passive thaw cycle times. The resulting ice ball was measured (0°C and -20°C isotherms) along three perpendicular axes and recorded in a database.

The database covers a wide range of ice ball sizes and shapes up to 9.8 cm, with aspect ratios (long axis length/short axis length) ranging from 1 to 3.7. For any desired ice ball size a × b × c cm, where each dimension is between 3 cm and 6 cm, there is always a match in the database within 0.9 cm, and the average match is within 0.2 cm. This means that if you want a 3 cm × 4 cm × 5 cm iceball (or any other desired size), you’ll find a match in the database.

The database is available now for anyone to use; it involves entering in the type of tissue you’re ablating and the size of the ice ball you want, and it will tell you what type of probe to use, how many probes to use, where to place the probes, and for how long to freeze. I’ve started using it in my own procedures. I should note that the software is not yet FDA approved, and should only be used in the setting of an IRB-approved research protocol.

IO360: What kind of results have you seen after having used it? Was there anything surprising about going from the planning tool to the actual procedure? 

Boas: Average error between the simulated and experimentally measured ice balls was 1 mm in gel experiments, and 4 mm in clinical cryoablation cases.  The simulations accurately predicted the degree of synergy in multiple-probe ablations. There definitely have been cases where it’s been very helpful for doing the procedures. We have these rules of thumb where, for example, we try to cover the entire tumor using probes spaced 1 to 2 cm apart and 1 cm from the edge of the tumor. I recently did a renal cryoablation procedure where I thought, based on these rules of thumb, that I could ablate the tumor with just two probes, but the planning tool told me that I needed to use three probes and ablate for less time. When I actually did the procedure, the planning tool was right: I needed to use three probes.

IO360: You didn’t have a planning tool when you started doing cryoablation. How will this change practice for new interventional oncology clinicians doing cryoablation?

Boas: This tool came out of my own need when I started doing a lot of cryoablation procedures. I felt that I didn’t really know what I was going to get and that I was guessing at times. Someone with a lot of experience with cryoablation may not need this tool because they may already have a good intuition that’s been built up over years of experience, but for someone who doesn’t have as much experience or for someone who has a lot of experience but is using a new type of probe that they don’t know as much about, this tool might be very handy. 

IO360: What’s the takeaway message?

Boas: We’ve developed a cryoablation planning tool that is accurate, fast, and comprehensive. It’s accurate because we model tissue-specific perfusion and thermal properties. It’s fast because we use a precomputed database of ice ball sizes for different probe configurations and ablation times. It’s comprehensive because we have thousands of cryoablation procedures in the database.

Editor’s note: Dr. Boas reports patent ownership and stock ownership of Claripacs, LLC.

Suggested citation: Ford J. A new planning tool for cryoablation: an interview with Franz Edward Boas, MD, PhD. Intervent Oncol 360. 2016;4(5):E86-E88.