Mapping of Level I Axillary Lymph Nodes in Patients With Newly Diagnosed Breast Cancer: Optimal Target Delineation and Treatment Techniques in Breast and Level I Axilla Irradiation.

Abstract

PURPOSE/OBJECTIVE(S)\nIn an era of increasingly use of axilla irradiation in high-risk breast cancer patients after breast-conserving surgery and sentinel lymph node (LN) biopsy, the optimal clinical target volume (CTV) for level I axilla (Ax-L1) LN is uncertain. We mapped the location of pretreatment LN metastases in the Ax-L1 of newly diagnosed breast cancer patients and propose modification of the CTV borders as defined by the Radiation Therapy Oncology Group (RTOG) breast cancer atlas. Further, we assessed the differences in dosimetric parameters of modified Ax-L1 planning target volume (PTV) and surrounding organs at risk (OAR) between whole breast irradiation with standard high tangential simplified IMRT (HT-sIMRT WBI) and whole breast plus Ax-L1 irradiation with VMAT (VMAT WBI+ Ax-L1).\n\n\nMATERIALS/METHODS\nWe identified 76 newly diagnosed breast cancer patients with 1-4 positive LNs after curative-intent surgery and axillary lymph node dissection treated between 2016 and 2018. All patients had a pretreatment diagnostic computed tomography (CT) chest scan demonstrating involved Ax-L1 LNs. The locations of 116 involved Ax-L1 LNs were mapped onto simulation CT images of a standard patient and the appropriateness of RTOG atlas was assessed. A modified Ax-L1 CTV with better coverage for involved Ax-L1 LNs was proposed, and PTV was generated for plans. Standard high tangential sIMRT plans for WBI and VMAT plans for WBI+ Ax-L1 on each side (left side and right side) were designed with a prescription dose of 50 Gy in 25 fractions, and dosimetric parameters were compared.\n\n\nRESULTS\nFor the RTOG atlas, the mapped LNs were 70.7% (82/116) inside, 13.8% (16/116) marginal and 15.5% (18/116) outside the CTV borders. The RTOG atlas missed 29.3% of LNs: 50.0% (17/34) in the anterior, 23.5% (8/34) in the caudal, 14.7% (5/34) in the cranial, 11.8% (4/34) in the medial direction. Modification by extending 1 cm caudal and 0.5 cm anterior to the CTV borders of the RTOG atlas allowed the modified Ax-L1 CTV to encompass 90.5% (105/116) of LNs. For HT-sIMRT WBI and VMAT WBI+ Ax-L1 plans, the mean V50 of modified Ax-L1 PTV was 69.5% and 97.8%; the mean dose, V20 and V5 of ipsilateral lung were 8.9 Gy, 16.4%, 32.5% and 10.5 Gy, 20.8%, 33.8%; the mean heart dose in left-sided plans was 4.1 Gy and 3.0 Gy; the mean dose and V5 of liver in the right-sided plans were 1.5 Gy, 3.6% and 2.8 Gy, 10.3%, respectively.\n\n\nCONCLUSION\nAccording to the distribution of Ax-L1 LNs, the RTOG atlas might be insufficient for their coverage. We propose a modified Ax-L1 CTV with expansion of RTOG Ax-L1 CTV borders, most notably in the caudal and anterior directions. Standard high tangential sIMRT WBI plans failed to deliver a therapeutic dose adequately to the modified Ax-L1 PTV. VMAT plans intended for whole breast plus Ax-L1 irradiation provided adequate dose to Ax-L1 PTV without increasing the doses to OARs.