The initial registry trial findings reported in 2005 on the use of the MammoSite device in patients with DCIS showed the device to be well tolerated with favorable cosmetic results. This update largely confirms these initial results and also provides information about recurrence for patients treated with this method of APBI. Of the 63 patients with at least 5 years of follow-up, 92% had excellent to good cosmetic results. As previously reported, favorable cosmesis was associated with larger device-to-skin distance. The majority of adverse events, including infection and seroma, were self-limiting and did not impact cosmetic results. Importantly, the 5-year local–regional recurrence data reveal a 3.39% actuarial rate, which compares favorably with a 5-year recurrence rate of 7.5% associated with whole-breast radiation reported in the NSABP B-17 trial.7 Taken together, this study update continues to support the safety of MammoSite-delivered APBI for appropriately selected patients with DCIS.
The ASBS has been consistently following up and reporting on the use of the MammoSite device during the past 5 years.17
Both initial studies as well as studies evaluating 3- and 4-year follow-up results have found the device to be well tolerated and to result in a favorable cosmetic outcome.18,19
These early results have also been reported in smaller, single-institution studies.20,21
Concern regarding the durability of cosmetic results reported in earlier studies of MammoSite outcomes stem from the relatively long duration (approximately 3 years), of dynamic change associated with breast findings after treatment with lumpectomy and radiation. In light of this, 5-year results reported in this update, showing a majority of patients having excellent to good cosmetic results and minimal long-term device-related adverse events, adds further validation of the device’s utility.13,17–19
Additionally, a recent study comparing toxicity of APBI with MammoSite to whole-breast radiation found patients treated with APBI to have a higher performance status, less acute skin toxicity, and decreased fatigue.22
Increased seroma pain was the primary finding for patients treated with MammoSite.22
Seroma formation was the most commonly reported device-related adverse event in this study and was associated with the open-cavity placement technique. Additional single-institution studies have reported a similar correlation between the open-cavity technique and seroma formation.23,24
Use of prophylactic antibiotics has been suggested to help reduce the risk of device-related infection, which has also been linked to clinically significant seroma formation.24
Postoperative placement of the MammoSite has been recommended previously to allow selection of eligible patients based on findings from the final pathologic analysis.25
Collectively, these findings support the recommendation that, if possible, the MammoSite device be placed postoperatively using the closed-cavity technique.
Additional results from this registry trial update demonstrate the importance of adequate resection margins and appropriate patient selection for treatment with APBI, as close margins and younger patient age were associated with an increased risk of local recurrence. Furthermore, the 6 patients in this study who experienced a recurrence had intermediate- or high-grade disease, suggesting a decreased recurrence risk for lower-grade DCIS patients treated with MammoSite APBI. Up to this point, there has been an accepted paradigm of treatment for the management of DCIS that includes surgery and whole-breast radiation for patients undergoing breast conservation. However, the rationale for this standard treatment is open to further investigation, secondary to knowledge gaps that exist between the current favorable outcomes for patients with DCIS, the initiative to tailor therapy on a more patient-specific basis, and a fundamental lack of information about the biological differences of the DCIS subtypes. Consequently, these gaps encourage further study of less-extensive approaches for the treatment of DCIS, including MammoSite APBI.
The complexity of obtaining a standard 6-week course of whole-breast radiation remains unworkable for many patients diagnosed with DCIS. Since the initial ASBS report on the use of MammoSite for DCIS in 2005, patients with DCIS continue to be undertreated. Factors contributing to this undertreatment include employment issues, which may only be exacerbated by the current economic climate. Furthermore, transportation, insurance status, age, and physical limitations continue to be key obstacles to care.26,27
The favorable results for MammoSite reported in this update should encourage clinicians to offer APBI on a clinical trial to appropriately selected patients who would otherwise not get treated with radiation or who would potentially choose mastectomy to avoid the adverse effects of whole-breast radiation. The recently published ASTRO guidelines for the use of APBI outside of a clinical trial delineated patient groups as “suitable,” “cautionary,” or “unsuitable” for treatment with APBI.12
Of note, patients with DCIS were not included in the “suitable” group, as currently there is a dearth of data available regarding outcomes for these patients. As a result, patients with small (≤3 cm) DCIS lesions were listed in the “cautionary” group, and patients with larger-size DCIS were deemed “unsuitable.”12
Thus, while the use of APBI targeted to the tumor bed may ultimately provide a logical implementation of radiation therapy for patients with unifocal low-risk disease, this treatment approach has yet to achieve a “suitable” status for the management of DCIS outside of a clinical trial.12
This update of the ASBS MammoSite registry trial provides encouraging results regarding cosmesis and recurrence, with 5-year follow-up. The safety and efficacy of APBI continues to be assessed, and the role of this treatment for patients with DCIS and invasive disease is being actively studied through the NSABP-RTOG B-39 trial. As outcomes from the NSABP trial are years away, the favorable results from the ASBS trial are critical to clinicians who are actively using this radiation therapy device. While APBI may ultimately only apply to specific patient subsets, the shorter treatment time made possible by MammoSite may make radiation therapy, as well as breast conservation, feasible for patients who would otherwise go untreated or feel forced to opt for mastectomy. Finally, while data available from the ASBS registry trial are encouraging, these results are subject to the inherent limitations of a voluntary registry study. Implicit to the retrospective nature of the study are concerns regarding selection bias and the lack of uniformity of patient enrollment, which may affect the validity of the current study in terms of cosmetic results and adverse events.17,18
We look forward to additional follow-up results from the ASBS MammoSite registry trial and the results of the NSABP-RTOG trial to help further determine the safety and efficacy of APBI.