Breast brachytherapy has advanced significantly over the past decade. Interstitial techniques evolved to single-entry devices, which in turn moved from single lumen, balloon based technology to multicatheter devices like the SAVI® applicator. This evolution of breast brachytherapy technology has provided radiation oncologists and physicists much greater control of radiation delivery and enables physicians to offer a shortened course of treatment to more women.
Despite these advances, some institutions still rely on dosimetric parameters that are based on single lumen balloon technology and do not reflect the enhanced dose customization capabilities of a device like SAVI. In this article, a panel of radiation oncologists and physicists discuss how the evolution of breast brachytherapy changed what constitutes a good treatment plan and the dosimetric criteria they currently utilize in order to achieve better patient outcomes.
What are the current dosimetric parameters you adhere to?
Rakesh Patel, MD:
V95>95% is the new benchmark and what we strived for years ago with interstitial brachytherapy; we had to relax the coverage to V90>90% when simpler techniques like single lumen balloon technology were developed, but with SAVI we can now return to more robust, more conformal planning goals. Less than 125% is acceptable for maximum skin dose, but the ideal is 100% of the prescription dose, which is often not achievable with any of the other current multi-lumen devices when there is a skin constraint. For chest wall dose, less than 125% is also acceptable, although there isn’t enough clinical data validating a dose limit; we should always aim to lower collateral radiation dose to healthy tissues as much as possible.
Lydia Komarnicky, MD & Jay Reiff, PhD:
Whenever possible our dosimetric objective is to cover 95% of the PTV with at least 95% of the prescription dose. We define the PTV as a 1.0 cm rind around the lumpectomy cavity. We limit the PTV to 0.5 cm from the skin surface and to the chest wall, which we define as the lateral and/or anterior aspect of the ribs. If the skin surface is at least 0.5 cm from the PTV, unless that part of the PTV is of particular concern based on the pathology report, we typically keep the skin dose to approximately 80% – 90% of the prescription dose. Otherwise we keep it as low as we can achieve, not to exceed 100% of the prescription dose. We limit the maximum dose to the chest wall to 100% of the prescription dose.
Robert Hong, MD:
I utilize the NSABP B-39 protocol requirements as my baseline criteria, which serves as the minimum dosimetric parameters I strive to achieve. However, on average SAVI allows me to treat the V90 to 97-100%. Because SAVI enables me to deliver dose to more of the regions at risk while minimizing hot spots, I find I’m using other dosimetric parameters, such as the V95. Currently, I try to achieve a V95 of 95% in every patient I see.
When developing a treatment plan for SAVI, what is your acceptable range of V200 limits? Are you able to correlate clinical outcomes with those levels?
Dr. Komarnicky & Dr. Reiff:
When planning a SAVI, even with a 10-1 device, we are able to keep the V200 at or below 20 cc with 95% of the prescription dose covering at least 95% of the PTV_EVAL. For smaller devices, we keep it as low as possible while striving to maintain the same target coverage. We keep our V200 below 20 cc for several reasons. The NSABP B-39 protocol recommends this value for interstitial based APBI treatments. With the struts abutting the breast tissue, we feel that in this respect a SAVI implant resembles an interstitial implant and therefore this is an appropriate plan evaluation criterion. Having followed this guideline in over 65 SAVI implants over the last 3.5 years, we have not seen any cases of fat necrosis.
In every case, there is a balance between covering the PTV and minimizing hot spots. We evaluate these hot spots as absolute volumes instead of ratios, which were used traditionally with interstitial brachytherapy and were correlated to late toxicities such as fat necrosis (i.e. DHI or dose homogeneity index). The SAVI is a hybrid between interstitial and intracavitary in my mind and thus dosimetric correlation with clinical outcomes needs to be better defined. The V200 and V300 will increase as the number of struts increase with larger devices but this has not translated to subcutaneous toxicities to date. We reduce the V200 to as low as possible–often less than 10cc and accept less than 20cc. Newer TPS allow improved and quick dose optimization which makes the planning process very simple.
How has the SAVI applicator challenged current guidelines on skin distance? Is your primary concern skin spacing or is it maximum skin dose? Or is it both?
It’s always been about the skin dose and perhaps surface area receiving a given dose instead of the spacing. Skin spacing is a legacy term used to qualify patients with single lumen balloon treatment and 2D planning methods. Doses could not be modulated significantly and the planning software was primitive at best. CT-based 3D treatment planning coupled with multi-lumen devices has allowed patients with much lower skin spacing to meet dose limits to the skin.
Dr. Komarnicky & Dr. Reiff:
Skin distance and skin dose are both concerns as the two are very closely related. However the effects of a small skin bridge (2-3 mm) can be mitigated by the appropriate use of a multilumen applicator in conjunction with appropriate treatment planning software. In our clinic we have treated patients where a strut could be visualized and palpated just under the skin surface. Despite this, the dose to the skin surface was below 100% of the prescription dose. Unless any part of the dermal layer is involved or has been designated a “close margin” by the pathologists, we do not feel it is acceptable to exceed 100% of the prescription dose to the skin.
How do you define optimal PTV coverage? What do you think of claims that balloon applicators can treat a larger PTV than SAVI?
I don’t think there is an inherent advantage to claiming that you’re treating more tissue than you think you’re treating. Based on pathology, it has been determined that the ideal margin of tissue is 1 cm from the lumpectomy bed, and this is used as the basis for breast brachytherapy in general, whether it involves multiple catheters, a single-entry balloon or SAVI.
Even if balloons are indeed treating at 1.5 cm as opposed to 1 cm, I would argue that you’re unnecessarily treating normal healthy breast tissue. If the basis for claims of a larger PTV is the fact that the tissue is compressed, I would also argue that irradiating compressed tissue and the subsequent scarring as a result of treatment explains why we’re seeing increased toxicity with balloons in terms of persistent seroma and fat necrosis.
The tissue at greatest risk of harboring residual microscopic disease after surgery is within 1 cm, although we often tailor the PTV to the clinical risk. This is based on the pathologic margins such as extent, volume of disease, orientation, etc. With greater dose modulation capabilities of the SAVI, we can tailor the PTV to each patient’s anatomic, pathologic and clinical parameters.
The more important question in my mind is not what can be treated but what volume should be treated and at what cost. It is clear that all APBI methods are not created equal and the volume irradiated varies significantly amongst brachytherapy methods, IORT and 3DCRT. In a recently published study from William Beaumont, they found that balloon brachytherapy treated an average effective cavity margin of 1 cm, less than the previously described 1.6 to 2.0 cm. There is also published data from both UCSD and MD Anderson suggesting that larger PTV’s of 1.5 cm can be treated with SAVI, but this isn’t often clinically necessary in favorable risk APBI patients.
With the breast brachytherapy technology available today, what constitutes a good treatment plan? Has the emergence of the SAVI applicator changed the ideal dosimetric parameters?
Dr. Komarnicky & Dr. Reiff:
As with all treatment plans in radiation oncology, the objective of brachytherapy is to treat as much of the target volume as possible with the prescription dose while minimizing dosimetric hotspots as well as the dose to the nearby critical structures. The “ideal” dosimetric parameters have always been to treat the entire target volume to the prescription dose with no dose delivered elsewhere. While this ideal has not changed over the years, the emergence of multilumen devices has allowed us to tailor the dose distribution to the target volume while at the same time not only minimizing the dose to the nearby critical structures, but also minimizing the volume of hot spots within the target volume. By optimizing the dwell times in the potential dwell position in the implant, HDR treatment planning has allowed patients to be treated with treatment plans coming closer to the “ideal” than ever before.
While we’re treating less tissue than conventional whole breast irradiation, we have to be diligent in treating the areas that are at the highest risk. The stakes are a lot higher when we’re doing breast brachytherapy, so it’s extremely important that the plans meet strict dosimetric guidelines. Ultimately what constitutes a good treatment is one that maximizes radiation dose to the areas with the greatest risk of recurrence while minimizing dose to normal tissue. The more flexibility we have with brachytherapy devices, the closer we can get to this ideal treatment plan. Without a device like the SAVI 6-1 or 6-1Mini, many of our patients would be biologically eligible for APBI but they would not be candidates because of the technical inability to deliver the proper dose.