Data Continue to Illustrate Potential Role of Partial Breast Irradiation in Early-Stage Breast Cancer

Robert W. Mutter, MD

Whole breast irradiation vs partial breast irradiation remains a topic of debate among radiation oncologists for patients with hormone receptor (HR)–positive, early-stage breast cancer, according to Robert W. Mutter, MD.

“There has been a lot of recent research that has come out on partial breast irradiation. We have long-term follow-up on a number of randomized trials now,” said Mutter, who presented on partial breast irradiation and early-stage breast cancer at the 2024 ACRO Summit. “There is a lot of interest in how we should interpret these trials and how they should impact our practice. The talk I gave largely [focused on] interpreting that data and discussing potential future directions of where research might be going.”

In an interview with OncLive^®, Mutter discussed partial breast irradiation in early-stage breast cancer; conveyed the significance of recent research findings for this approach and their implications for clinical practice; and addressed unmet needs in the field, particularly focusing on ongoing research regarding radiation omission in HR-positive, node-negative breast cancer and the necessity for longer-term follow-up in clinical trials.

Mutter is the chair of research in the Department of Radiation Oncology and an associate professor of radiation oncology and pharmacology at Mayo Clinic in Rochester, Minnesota.

OncLive: What trials have helped determine the feasibility of partial breast irradiation in the breast cancer treatment paradigm?

Mutter: I discussed the phase 3 IMPORT LOW trial [ISRCTN12852634]. This was a study that compared whole breast irradiation vs partial breast irradiation, and this trial is especially interesting because, unlike many of the other trials, they used the same dose and fractionation [in both arms], but changed the volume of breast that was treated.

There is a lot that can be learned from that trial. The conclusion of the study was that there was no difference in disease control between whole breast irradiation and this generous partial breast irradiation technique that was used in that study. It was a practice-changing, impactful trial for a number of reasons.

What unmet needs exist in the early-stage breast cancer space that need to be addressed?

There is a lot of research ongoing that's looking at omitting radiation in patients with HR-positive, stage I, node-negative breast cancer. That is in part because these tumors do recur at lower rates than some of the other subtypes of breast cancer.

What's unique about these [early-stage, HR-positive] breast cancers is that they recur at the same annual rate for 20 years or more. Therefore, we have to be very careful about interpreting these clinical trials, as many of them have just 5 years of follow-up. There is some concern that we may see more recurrences with longer follow-up from these trials. Additionally, in endocrine therapy omission trials, there is often greater adherence to endocrine therapy in the context of a clinical trial than what we see in routine clinical practice.

We've been advocating for consideration of a more personalized, combined-modality therapy approach. By this, I mean trying to optimize all the benefits of endocrine therapy and radiation. [This involves] just giving enough radiation to provide improved disease control. In all the studies so far that have been conducted, radiation has consistently been shown to reduce the risk of both local recurrence and recurrence in the axilla.

In a perfect world, we want to maximize that benefit [to reduce the risk of recurrence] and have the optimal disease control; however, we also want to minimize adverse effects of treatment in the long term.

What could be done in the future to evaluate combined-modality approaches in early-stage breast cancer?

We're proposing investigations into new endocrine therapy approaches, such low-dose tamoxifen, which may be associated with fewer AEs, [which could lead to] better adherence for endocrine therapy.

Rather than omitting radiation altogether, there are new approaches such as partial breast irradiation, where the treatment can be given over just 5 days and be very focused to allow for minimal normal tissue exposure. Combined modality therapy with tailored endocrine therapy and tailored radiation therapy may be the best way going forward to treat these patients with favorable tumors over the long term.

These are patients who may have 20 to 40 years of life expectancy to develop these recurrences. Therefore, we want to be cautious about de-intensifying their therapy based on short-term follow-up [data] from these studies.

Reference

Mutter RW. Partial breast irradiation and early-stage breast cancer: known knowns, known unknowns and unknown unknowns. Presented at: 2024 ACRO Annual Meeting; March 13-16, 2024; Orlando, FL.