The Acute and Late Toxicities of MRI-Guided External Beam Partial Breast Irradiation Delivered Using a Once-Per-Day Regimen

Abstract

Background and PurposeThe use of external beam accelerated partial breast irradiation (APBI) using a twice-per-day regimen has raised concerns about increase rates of late toxicities. We compared toxicity outcomes of external beam APBI using a once-per-day regimen and accelerated hypofractionated whole breast irradiation (AWBI) in patients with early-stage breast cancer.Materials and MethodsThis was a single-institution, retrospective cohort study. Patients aged ≥50 years with pTisN0 or pT1N0 breast cancer who underwent breast-conserving surgery and adjuvant radiotherapy were included. APBI was delivered at 38.5 Gy in 10 fractions once daily using magnetic resonance imaging (MRI)-guided radiotherapy only to patients who were strictly “suitable”, according to the ASTRO-APBI guidelines. AWBI was delivered at 40.5–43.2 Gy in 15 or 16 fractions with or without a boost.ResultsBetween October 2015 and December 2018, 173 and 300 patients underwent APBI and AWBI, respectively. At a median follow-up of 34.9 months (range 7.1 to 55.4 months), the 3-year recurrence-free survival rates of the APBI and AWBI groups were both 99.2% (p=0.63). Acute toxicities were less frequent in the APBI than AWBI group (grade 1: 95 [54.9%] vs. 233 [77.7%] patients; grade 2: 7 [4.0%] vs. 44 [14.7%] patients; no grade ≥3 toxicities were observed in either group, p<0.001). Late toxicities were less common in the APBI than AWBI group (grade 1: 112 [64.7%] vs. 197 [65.7%] patients; grade 2: 9 [5.2%] vs. 64 [21.3%] patients; grade 3: 0 vs. 5 [1.7%] patients, p<0.001). Multivariate analysis showed that APBI was significantly associated with fewer late toxicities of grade ≥2 compared with AWBI (odds ratio 4.17, p=0.006).ConclusionOnce-per-day APBI afforded excellent locoregional control and fewer toxicities compared with AWBI. This scheme could be an attractive alternative to AWBI in patients who meet the ASTRO-APBI guidelines.

Implementing stereotactic accelerated partial breast irradiation using magnetic resonance guided radiation therapy

“…In both treatment settings, our 1.0cm surgical bed to PTV margins were smaller compared to contemporary studies which used 1.0-1.5cm, depending on resection margins, and 1.3cm margins, respectively 21,22 . In the context of non-MRgRT studies, it has been shown that increased irradiated volumes potentially lead to higher risk of toxicities 17,19 .…”

“…In both treatment settings, the surgical bed+1cm is the CTV, meaning that our PTV = CTV. This is a tighter margin approach compared to contemporary studies 21,22 . Our previous work ranging 2014-2015 provided reassurance that a 1cm margin from surgical bed was appropriate 6 .…”

“…To reduce irradiated breast volumes, brachytherapy can be advantageous due to reduced margins compared to EBRT but suffers from higher invasiveness due to the insertion of interstitial needles or single-entry catheters into the patients' breast 20 . With MRgRT, it is our goal to utilize excellent soft-tissue contrast to reduce margins similar to that of brachytherapy 6,21,22 .…”

“…These trials inspired our single-fraction and threefraction MRgRT stereotactic accelerated partial breast irradiation (S-APBI). Our MRgRT S-ABPI is also novel in the fact that our institution uses tighter margins compared to other contemporary MRgRT nonstereotactic APBI programs 21,22 .…”

Magnetic resonance linear accelerator technology and adaptive radiation therapy: An overview for clinicians

“…There have been several recent publications focused on the use of MR guidance in the treatment of breast cancer 136‐138 . This area is both promising and interesting for future exploration.…”