2999 - Five-Year Outcomes of Robotic Stereotactic Accelerated Partial Breast Irradiation for Early-Stage Breast Cancer

Purpose/Objective(s):

Adjuvant accelerated partial breast irradiation (APBI) offers comparable outcomes to whole breast irradiation in select early-stage breast cancer patients. Robotic stereotactic APBI (RSAPBI) with fiducial tracking is a precise and efficient option, though long-term safety and efficacy data remain limited. This study presents 5-year outcomes from a single institution participating in a prospective registry, assessing the efficacy, safety, and tolerability of RSAPBI in women with early-stage breast cancer.

Materials/Methods:

Eligible patients were women aged =50 years with early-stage breast cancer (T1-T2, N0, M0), including histologically confirmed ER+/PR+ invasive ductal carcinoma (IDC) or ductal carcinoma in situ (DCIS). Tumors requirements were =2 cm with negative surgical margins of =2 mm. Exclusion criteria included lymph node involvement, metastatic disease, prior ipsilateral breast cancer or breast irradiation, multifocal/multicentric disease, or contraindications to radiation or fiducial placement. A robotic radiosurgery system was utilized for treatment. Four gold fiducials were implanted around the lumpectomy cavity for tumor bed delineation and tracking. CTV was defined as the cavity with a uniform 5-15 mm expansion limited to breast tissue, and PTV was CTV with 0-5 mm expansion. The prescribed dose was 30 Gy in 5 fractions. Outcomes included overall survival (OS), ipsilateral breast tumor recurrence (IBTR), toxicity (RTOG grading), and cosmetic result (Harvard Scale), assessed at 4 weeks, 3, 6, 12, 18, 24 months, and then yearly for 5 years. Kaplan-Meier curves were used for actuarial outcome analysis.

Results:

Fifty-four women (median age 66.5 years, range 51–83) were enrolled from March 2016 to May 2021. Median follow-up of the cohort is 60 months (range 0-100 months). 83.3% of women had IDC while 16.7% had DCIS. 87% were Caucasian, 9.3% African American, 1.9% Asian.

All patients received 30 Gy in 5 fractions. The median treatment duration was 9 days (range 7–14). The median CTV expansion was 10 mm (range 5-10) and PTV expansion was 3 mm (range 2-5 mm). The median PTV volume was 116 cm³ (IQR 69-156), and the median prescription isodose line was 81.5% (IQR 80-83).

There was 1 ipsilateral breast tumor occurrence with no local, regional, or distant failures. 5-year actuarial survival was 87%. No deaths were directly attributed to breast cancer. Two patients withdrew from the protocol. Grade 1 skin toxicity occurred in 5 patients (9.3%) at 1-month follow-up; no grade 2-4 toxicity was observed. Cosmetic outcomes at 5 years were rated excellent in 60% and good in 40%. Neither fat necrosis nor breast pain were reported at last follow-up.

Conclusion:

Five-year outcomes suggest that RSAPBI with fiducial tracking is a safe, effective, and well-tolerated treatment for early-stage breast cancer, demonstrating excellent local control and cosmetic outcomes.