Tangential VMAT vs. 3D-Conformal Radiotherapy for Left-Sided Whole Breast and Regional Lymph Node Irradiation.

Abstract

PURPOSE/OBJECTIVE(S)\nWhole breast and regional nodal irradiation presents a challenge in balancing the need for target coverage with avoidance of normal tissues. The novel technique termed tangential VMAT (tVMAT) utilizes beam angles tangential to the curvature of the chest wall without beams entering perpendicular to the chest wall in the direction of thoracic organs. In this study tVMAT was compared to standard 3D-conformal radiotherapy (3DCRT) in a dosimetric analysis. To our knowledge this is the first study to assess tVMAT in left-sided breast cancer patients requiring regional nodal irradiation.\n\n\nMATERIALS/METHODS\nTen patients with left-sided breast cancer previously treated with adjuvant RT to the breast, axillary, and supraclavicular nodal regions were selected for this study. For each patient two plans were created: 1) a standard dual-isocenter three-field 3DCRT plan (two tangents and a supraclavicular field) using field-in-field dose modulation and 2) a tVMAT plan. Each tVMAT plan utilized a single isocenter for two partial arcs with angles selected to keep the central axis of the beams tangential to the curvature of the chest wall at all points. The prescription for both techniques was 5000 cGy in 25 fractions to the breast, axillary levels I-III, and supraclavicular target volumes with a goal coverage of 95% of the breast and nodal target volumes receiving at least 95% of the prescription dose. Normal tissue dose constraints were adapted from NSABP B51. Plans were created using a treatment planning system and differences in mean values for dosimetric endpoints were assessed for statistical significance using a Student s t-test (α\u202f=\u202f0.05).\n\n\nRESULTS\nCompared to 3DCRT plans, tVMAT provided more uniform coverage to the breast and regional lymph nodes (mean conformity index was 1.38 for tVMAT versus 2.42 for 3DCRT, P < 0.01) and the max point dose for tVMAT was lower on average (112.8% for tVMAT versus 121.5% for 3DCRT, P < 0.001). Both plans met the breast target coverage goals, however coverage to the lymph nodes was superior for tVMAT (mean coverage to 95% of the axillary plus supraclavicular target volumes was ≥ 99.0% of the prescribed dose for tVMAT versus ≥ 94.8% for 3DCRT, P < 0.001). Cold spots within the breast and nodal target volumes were on average lower for 3DCRT (952 cGy for 3DCRT versus 2760 cGy for tVMAT, P < 0.001). OAR sparing was improved with tVMAT, with a lower average V20Gy for the left lung (16.5% for tVMAT versus 24.6% for 3DCRT, P < 0.01), and lower mean heart dose (168 cGy for tVMAT versus 200 cGy for 3DCRT, P\u202f=\u202f0.03).\n\n\nCONCLUSION\nTangential VMAT is a promising technique for the treatment of intact breast and regional lymph nodes and may provide better target coverage and OAR sparing compared to conventional 3D-conformal techniques. The use of a single isocenter for tVMAT could yield a simplified setup and more accurate treatment delivery, ideally with the use of deep inspiration breath hold or respiratory gating.