Did MammoSite have less toxicity than interstitial brachytherapy in patients who had not received chemotherapy?

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In interstitial brachytherapy (IB), cosmetic outcomes and toxicity correlate with the volume of tissue irradiated, the dose homogeneity index (DHI), and adjuvant doxorubicin/cyclophosphamide-based chemotherapy (ACCT). MammoSite brachytherapy (MSB) irradiates smaller volumes than IB, and lower dose homogeneity does not appear to affect toxicity. However, clinical experience suggests that other factors may also play an important role in cosmetic outcomes and toxicity with MSB. To investigate, researchers reviewed their prospectively maintained database of women who underwent accelerated partial breast irradiation (APBI).

Beginning in September 1995, 115 women were enrolled in a trial evaluating APBI as the sole treatment after lumpectomy. The first 75 eligible patients received IB, and the most recent 28 eligible patients received MSB. All patients received 34 gray (Gy) in 10 twice-daily fractions using high-dose-rate iridium-192 brachytherapy; 19% of the IB group and 0% of the MSB group received ACCT.

At one year after treatment, grade 2–4 subcutaneous fibrosis was significantly less common with MSB than with IB (10.7% vs. 32%; P = 0.04), as graded by a radiation oncologist using the Radiation Therapy Oncology Group/Eastern Cooperative Oncology Group system. However, the difference was no longer statistically significant when only ACCT-naïve patients in the IB group were compared with the MSB group. In patients receiving MSB, significantly smaller volumes were irradiated and DHI was lower.

The authors noted that current studies suggest a better toxicity profile with MSB than IB, attributed to the smaller irradiated volumes with MSB. However, toxicity and cosmetic outcomes were similar when only chemotherapy-naïve patients were compared, suggesting a more complex interplay between irradiated volumes, DHI, and chemotherapy. The authors considered the relationship between ACCT and toxicity in this setting intriguing and in need of further investigation.