Robotic Stereotactic Accelerated Partial-Breast Irradiation for Early-Stage Breast Cancer: 5-Year Results of a Single-Institution Pilot Study

Abstract

Outcomes following adjuvant accelerated partial breast irradiation (APBI) in select women with early stage breast cancer are comparable to whole breast irradiation. Robotic stereotactic accelerated partial breast irradiation (SAPBI) with fiducial tracking is an attractive treatment option, but limited data are available regarding the feasibility of this approach and 5 year outcomes have not yet been published. We report our institutional experience treating select women with SAPBI. Women with DCIS and early stage breast cancer treated from November 2008 to September 2015 were evaluated. Treatments were delivered with a robotic radiosurgical system. At least 4 gold fiducials were implanted around the lumpectomy cavity prior to the start of treatment for target tracking. The CTV was delineated on contrast enhanced CT scans using surgical clips and the lumpectomy cavity. In 2014, following the publication of the University of Florence phase III trial, the protocol was amended after 13 patients and the CTV was defined as the lumpectomy cavity with a uniform 1 cm expansion confined to the breast tissue. The PTV, prescribed 30 Gy in 5 fractions, was defined as the CTV with a 3-5 mm uniform expansion. AAPM TG-101 skin dose constraints were respected. Clinical examination and mammography were completed at 6-12 month follow up intervals. Breast cosmesis was scored using The Harvard Breast Cosmesis Grading Scale. Twenty women (median age 65 years) were treated over a median 7 days (range 5-13). Fourteen women had DCIS and 50% of the tumors were located in the upper outer quadrant. The median treated PTV was 63 cm3 (range 15-142), the median PTV/breast volume ratio was 8.3% (range 4.1-25.6) and the median prescription isodose line was 83% (range 75-87). Three or 4 fiducials were successfully tracked in 90% of patients. At a median follow up of 60 months (range 33-108), locoregional control, distant control, and overall survival was 100%. Five women with skin Dmax > 33 Gy (skin = CT surface minus 2 mm) developed chronic grade 1 dermatitis (faint hyperpigmentation), one of whom developed focal grade 1 telangiectasia at 52 months. Three women developed grade 1 fibrosis, one of whom was later diagnosed with fat necrosis on mammography. Two additional asymptomatic women were diagnosed with fat necrosis on routine mammography. There were no rib fractures. One woman required stereotactic biopsy to confirm benign tumor bed calcifications at 66 months. Fifteen evaluable patients maintained Good-Excellent cosmesis. Robotic SAPBI with fiducial tracking is a feasible, well tolerated and highly effective technique for the adjuvant treatment of select early-stage breast cancer patients. Cosmetic outcomes may be improved further by decreasing the skin Dmax to < 33 Gy. A confirmatory multi-institutional registry is actively enrolling patients. Given these encouraging results, future research in this patient population will evaluate robotic SAPBI as a definitive treatment.