ESTRO 36 Abstract Book

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uniform treatment of nodal volumes. The CP recommended modified tangents (MT) including level I/II nodes for pts with micrometastases (pN1mi). For pts with macrometastases (pN1a), CP recommended including level I/II LN in MT and additional field to include level III, supraclavicular (SCV) LN +/- internal mammary nodes for pts with any adverse factor including T2 disease, LVSI, high grade, ER negative, ECE, or age <50. Adjuvant RT fields of pts undergoing BCS with +SLN but not ALND were retrospectively reviewed. Results The RT fields of 257 pts from July 2011 to August 2016 were reviewed, including 74 (29%) with pN1mi disease & 183 (71%) with pN1a. Of 127 pts treated prior to CP changes, 13 (24%) of 37 pts with pN1mi were treated with whole breast irradiation (WBI) alone and 18 (20%) of 90 pts with pN1a with WBI alone. Following CP changes, 130 pts were treated, including 5 (4%) pts treated with WBI alone, 63 (49%) with MT, and 62 (48%) MT + SCV field. Of 37 pN1mi pts, 3 (8%) were treated with WBI alone. Of 92 pN1a pts, 1 (1%) was treated with WBI alone. A summary of treatment fields relative to pathway change is included in Figure 1 . On multivariable analysis (MVA), pN1a disease and treatment after CP changes were associated with use of MT (Table 1) . Use of SCV field was associated with pN1a disease with adverse factors and treatment after CP changes. Conclusion CP’s are useful tools for translating published research and guidelines into pt management plans to promote evidence-based care and eliminate unnecessary variations in practice. Recognizing that adjuvant RT treatment volumes were heterogeneous following the publication of Z11 and AMAROS, we modified the CP in 2015 based upon the latest evidence for RNI. After the amendment, pts received standardized RT fields guided by the CP based upon clinical risk factors which will aid in tracking outcomes in future investigations.

Conclusion Hypofractionated hybrid FiF+VMAT SIB showed to be feasible and was associated with low acute toxicity burden. 1-year follow-up data demonstrated a noticeable decline in radiotherapy related QoL items.Long-term results are needed to assess late toxicity and clinical outcome. PO-0664 Standardized Nodal Radiation (RT) through a Breast Clinical Pathway (CP) within a USA Cancer Network B. Gebhardt 1 , Z. Horne 1 , G. Ahrendt 2 , E. Diego 2 , D. Heron 1 , S. Beriwal 2 1 University of Pittsburgh Cancer Institute, Radiation Oncology, Pittsburgh, USA 2 University of Pittsburgh Cancer Institute, Surgical Oncology, Pittsburgh, USA Purpose or Objective ACOSOG Z11 and EORTC AMAROS studies investigated patients (pts) with clinical T1-2 N0 invasive breast cancer (IBC) undergoing breast conserving surgery (BCS) with positive sentinel node (+SLN) biopsy and demonstrated the safety of omitting axillary nodal dissection (ALND). Adjuvant RT fields employed differed between the two trials as regional nodal irradiation (RNI) was mandated in AMAROS and RT fields were heterogeneous in Z11. Furthermore, MA-20 and EORTC RNI trials demonstrated a survival benefit with RNI in pts with positive nodes, leading to wide variation in RT treatment volumes. CPs standardize care when many therapeutic options exist and clinical practice varies unnecessarily. We sought to evaluate the impact of changes to a CP guiding adjuvant RT in pts with +SLNs on practice patterns throughout a large, integrated cancer network. Material and Methods We implemented a CP for management of IBC with adjuvant RT throughout a network of 22 centers that required entry of management decisions into an online support tool. The CP for treatment of pts with +SLN following BCS was modified in February 2015 to promote