ESTRO 37 Abstract book

S736

ESTRO 37

asses chronic heart toxicity and dosimetric parameters with a longer follow-up. EP-1346 Stereotactic Partial Breast Irradiation for Early Stage Breast Cancer: Early Outcomes S. Kataria 1 , O. Obayomi-Davies 1 , I. Paydar 1 , L. Campbell 1 , S. Collins 1 , S. Rudra 1 , B. Collins 1 1 Georgetown University Hospital, Radiation Medicine, Washington DC, USA Purpose or Objective In carefully selected women with early-stage breast cancer, the preliminary outcomes following accelerated partial breast irradiation (APBI) are comparable to whole breast irradiation. Stereotactic body radiation therapy (SBRT) with fiducial tracking is an attractive APBI treatment option, but limited data are available regarding the outcomes associated with using this technique. We report our institutional experience treating select women with SBRT. Material and Methods Women with DCIS and early-stage (cT1N0) breast cancer treated from November 2008 to September 2015 with a minimum follow up of 2 years were evaluated. Treatments were delivered utilizing the CyberKnife (CK) radiosurgical system. Four to six gold fiducials, tracked in real-time using the CK Synchrony tracking system, were implanted around the lumpectomy cavity prior to the start of treatment. Prior to 2014, the clinical target volume (CTV) was delineated on contrast enhanced CT scans using surgical clips and the visible lumpectomy cavity. Beginning in 2014, the CTV was uniformly expanded by 1 cm, confined to the breast tissue. A 3-5 mm uniform expansion to the CTV was added to generate the PTV. A dose of 30 Gy in 5 fractions was delivered to the PTV. Dosimetry was assessed per institutional protocol, the National Surgical Adjuvant Breast and Bowel Project B-39 guidelines, and TG-101. Clinical examination and mammography were completed at 6-12 follow up intervals. The Harvard Breast Cosmesis Scale was used to assess cosmesis. Fat necrosis was diagnosed on mammography. Results Twenty women (median age: 65 years) were treated with CK over a median 7 days (range 3-13). Treatments began a median of 74 days (range 41-166) after breast conserving surgery. Fourteen women had DCIS and 50% of the tumors were located in the upper outer quadrant of the affected breast. The median treated PTV was 63 cm 3 (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). At a median follow up of 40 months, locoregional control was 100%. Acute toxicity was rarely observed. A focal Grade 1 telangiectasia was identified in one woman at 52 months. Fat necrosis was diagnosed on mammography in 2 asymptomatic women at 28 and 59 months, respectively. No rib fractures have occurred. Overall, all patients tolerated the treatment without any adverse events and the long term assessment of cosmetic outcomes is ongoing.