Failure Patterns After 3d Conformal Radiotherapy For Whole Breast Irradiation In The Prone Position

Prone whole breast irradiation (WBI) has shown promise in reducing radiation-related toxicities in patients with large, pendulous breasts undergoing breast conserving therapy. However, there has been concern that volume of irradiated breast in the prone position is insufficient and may lead to more frequent treatment failures. Our institution has used CT volume based 3D conformal radiation therapy (3DCRT) to deliver prone WBI for large, pendulous breasts and/or large body habitus and has demonstrated favorable toxicity profiles and cosmetic outcomes. Here we closely analyze failure patterns after 3DCRT for prone WBI to ensure they are also acceptable. From 1998-2006, 110 women underwent prone WBI using 3DCRT. The breast, lumpectomy site, heart, and lung volumes were contoured. The chestwall was not included. The lumpectomy planning volume (PTV) reflected a total 1.5 cm expansion. The mean breast dose was 49 Gy to isocenter for a minimum of 45 Gy delivered to 95% of breast volume. 71% received a boost (mean 10 Gy/5 fx) such that 100% of the final dose covered 95% of the lumpectomy PTV. The median age was 61 (27-91); 75% were post-menopausal. Median BMI was 34 (19-51), and median breast volume was 1401 cc. Tumors were Tis-20%, T1-61%, T2-17%, T3-2%, and 14% had positive axillary nodes. Median follow-up was 40 mo. 4 patients were diagnosed with an ipsilateral breast tumor (3.6%, 5-yr actuarial rate 3.0%). Mean time to recurrence was 62 months (33-125 months). The original tumors were DCIS (1), infiltrating lobular (1), and invasive ductal (2). A close focal DCIS margin was present in 2/4. The average closest distance of lumpectomy PTV to the chestwall was 5 cm (2-9 cm). The original tumor location and location of recurrence respectively was: lower outer quadrant (LOQ)-failed in UOQ, UIQ-failed in UIQ, central-failed central, UOQ-failed in skin and with distant metastases. 3/4 recurrences had received a boost (total dose 6000 Gy). One patient experienced regional recurrence (0.9%, 1.5% 5-yr). 6 patients had distant metastasis (5.5%, 5.9% 5-yr). 5-yr actuarial disease specific and overall survival rates were 96% and 89%. Good to excellent cosmesis was achieved in 89% of patients. A higher BMI did not impact the rate of fibrosis or good-excellent cosmetic outcomes. In patients with large pendulous breasts or increased BMI, delivering WBI in the prone position using 3DCRT results in recurrence rates and failure patterns similar to those anticipated using supine WBI. The location of disease recurrence does not suggest failures related to exclusion of the chestwall with prone WBI. This series adds to growing literature demonstrating prone WBI may be advantageous with respect to cosmetic outcomes while providing local control rates comparable to supine WBI.