2937 - A Dosimetric Comparison of Breast Regional Nodal Irradiation with and without the Use of an Axillary Apical Clip

Purpose/Objective(s): Adjuvant radiotherapy to the undissected axilla is recommended when macroscopic nodal involvement is detected at axillary dissection. The undissected axillary targeted volume is delineated based on information from the operative note, contouring atlases and visible postoperative change. Defining the true extent of the undissected axilla can pose a challenge due to possible incongruity between surgically defined axillary nodal levels and those defined by radiotherapy contouring atlases. Visible postoperative change may also resolve if there is a long time interval between surgery and radiotherapy. An axillary apical clip, placed intraoperatively at the most superomedial aspect of the dissected axilla, may help guide the delineation of the undissected axilla. The aim of the study was to evaluate the impact of axillary apical clip on target volume delineation and dosimetric outcomes.

Materials/Methods: A single institution retrospective study was conducted on thirty-five patients who underwent apical clip placement at the time of axillary dissection. The ESTRO contouring guidelines were utilized. The nodal Clinical Target Volume (CTV) was defined in 2 ways: (1) with the inferior extent commencing 2.5mm superior to the apical clip and (2) created without reference to the axillary clip, curtailed inferiorly based on the documented extent of axillary dissection and postoperative changes on the CT. Two corresponding radiotherapy plans were created and compared. Paired t tests and Wilcoxon signed rank tests were used to compare parameters.

Results: All apical clips were identified in level 2. The documented extent of dissection was level 2 and level 3 in 33 and 2 patients respectively. After a documented level 2 axillary dissection, the median distance from the apical clip to level 3 was 22mm (3-48mm). The apical clip was located a median of 2.5mm superior to the inferior extent of the axillary vein. The volume of axilla included in the CTV was significantly larger when the clip was used for delineation (Mean 46cc vs 28cc, p<0.001). On plan comparison, the humerus maximum dose (Dmax) was significantly higher when the clip was used (Median Dmax 23.7Gy vs 1.8Gy, p<0.001). There were no differences in evaluated lung or heart constraints.

Conclusion:

This study highlights an inconsistency between the superior extent of the surgically defined level 2 dissection and level 2 as defined by the ESTRO atlas. To minimize the clinical impact of this, either apical clip placement or detailed interdisciplinary discussions, between the surgeon and radiation oncologist, may be warranted. Defining the undissected axilla with the use of an apical clip was associated with an increased extent of axillary radiation. Further studies are warranted to evaluate if this translates to a clinical impact including potentially reducing the risk of nodal recurrence or increasing the rate of lymphoedema or shoulder morbidity.