Catheryn Yashar, MD
Though single-entry brachytherapy catheters target 1 cm of tissue surrounding the surgical tumorbed, some studies claim balloon devices can actually treat an equivalent of 1.5 cm of tissue as a result of tissue stretching, more closely approximating interstitial brachytherapy. Radiation oncologist Catheryn Yashar, M.D. of UC San Diego discusses her thoughts on such claims based on her research of ideal and published irradiation treatment volumes of various breast brachytherapy devices.
What do you think of claims that balloon applicators can treat a larger PTV than SAVI?
The standard for single-entry brachytherapy devices is to target 1 cm of tissue surrounding the lumpectomy cavity. However, two published studies claim that by stretching the tissue, balloon catheters actually have the ability to target 1.5 cm of tissue and therefore radiate more tissue. These studies, which examined a combined total of approximately 12 patients, took the total volume of treated tissue following device removal and cast the total volume on CT scan in order to measure the amount of treated tissue in centimeters.
I advocate that we stop discussing the centimeters of tissue treated and instead focus on the total volume of breast tissue treated, which will be more comparable among methods of partial breast irradiation. If the volume of treated tissue with SAVI equals the volume of treated tissue with a balloon applicator, it doesn’t matter how you measure it – the volumes are still the same. Whether it is 1 cm or 1.5 cm, if the total volumes are equal, it stands to reason that an equivalent amount of tissue being irradiated.
I would also advise against extrapolating the conclusions of these two papers to the general population, because when you’re working with such a small sample size, it’s extremely difficult to predict the results for an expanded number of patients.
So according to your data, do balloon applicators treat a larger volume of tissue than SAVI?
Based on our review of the literature, balloons treat a volume of tissue that ranges from 74-120 cc, with an overall average of 95 cc. In our experience, the larger SAVI devices (8-1 and 10-1) treat average volumes of 78.25 and 113.83 cc respectively, with the 8-1 ranging from 33.7-138.9 cc and the 10-1 ranging from 62-212.9 cc. Therefore, the large SAVI devices treat the same range of volumes as balloon devices.
We have found that, in general, the smaller SAVI devices – the 6-1 and 6-1Mini – treat a smaller volume of tissue than balloons. However, these specific devices are typically used in patients that require us to direct radiation away from the skin, chest wall or other critical structures – in other words, you are trying to treat a smaller overall amount of tissue. When you treat a lower volume of tissue, it’s because you’re taking advantage of the flexibility of SAVI to exclude healthy tissue in patients that require this consideration.
How do you define optimal PTV coverage?
Ideally, you’re aiming to get as much of your target volume covered by as much of your dose as possible. According to the NSABP-39 criteria, optimal PTV coverage is V90 over 90 percent; however if you can get V95 over 95 percent, or even 100 at 100 percent, that’s even better. Our results are normally around V90 at 97-99 percent and our V95 is often very close to 94 percent. However, those numbers all depend on patient selection, device deployment and how well the device fits in the tumor bed.
How do you determine the PTV eval?
I follow the B39 criteria and subtract 3-5 mm from the skin, chest wall, ribs and muscle. However, when I’m using SAVI, I don’t have to use the 10 percent air rule that is also part of the B39 criteria. With balloon devices, the B39 criteria recommends contouring the air in the cavity when contouring your PTV. If more than 10 percent is air, they recommend that you wait or remove the device. But in my experience, SAVI is so flexible it can successfully treat 1 cm of tissue beyond the air. Unlike balloon devices, there is almost never a need to remove the SAVI if there is air beyond the struts – you can typically treat the tissue beyond the air as long as you have the ability to stay within the V150 and V200 toxicity guidelines.
What do you think is an acceptable dose to critical structures? Do you always try to keep it to 100 percent if possible?
I try to keep it to 100 percent if possible. I know some of the balloon devices will treat 125 or even 145 percent, but SAVI is flexible enough that I rarely need to go anywhere near 125 percent. This also explains why the V95 with SAVI is sometimes smaller than with balloon applicators – because we’re keeping our skin doses lower. It’s always a tradeoff between coverage and skin dose. We try to keep our skin dose down in order to avoid long-term cosmetic toxicities. According to the data, 125 percent is an acceptable dose, but I would unlikely find 145 percent acceptable as I believe there are still risks of late cosmetic toxicities with that dose. I would likely feel comfortable going to 125 percent depending on the volume, but I’ve never needed to with SAVI. If you can minimize dose to critical structures without sacrificing your target volume, why wouldn’t you do that for the safety of your patients?
Dr. Yashar is an associate professor of radiation oncology at the UC San Diego School of Medicine and chief of breast and gynecological radiation services at the Moores UCSD Cancer Center.