Top Line: As discussed, breast contouring atlases vary with RTOG being a bit skimpy on the level 1 axilla.
The Study: In this dosimetric study, locations of 116 gross level 1 axillary nodes detected (retrospectively) on pre-op diagnostic computed tomography (CT) were mapped onto CT simulation scans for 76 patients post-axillary dissection with pathologic conformation of 1-4 involved axillary nodes with no known involvement of supraclavicular or internal mammary nodes. Turns out, contouring the level 1 axilla per RTOG guidelines on each of these simulation scans missed prior sites of gross tumor in roughly 30% of cases. In learning from this, the authors propose extending the level 1 axillary target volume 0.5 cm anterior (where half of nodes were missed) and 1 cm inferior (where another quarter were missed) to RTOG-proposed borders. When doing this, only 10% of gross node locations were missed. They went a step further with comparison plans and, unsurprisingly given the benign location of the level 1 axilla, adequate coverage of the newly-proposed level 1 axilla did not increase the dose to organs at risk compared with coverage of the smaller RTOG volume.
TBL: It may behoove you to be more generous in the anterior and inferior directions when referencing the RTOG atlas for contouring the level 1 axilla in the setting of post-op radiation for pN1a disease. | Zhao, Pract Radiat Oncol 2022