Coverage of axillary lymph nodes with high tangential fields in breast radiotherapy
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The aim of this study is to evaluate the coverage of axillary nodal volumes with high tangent fields (HTF) in breast radiotherapy and to determine the utility of customised blocking. The treatment plans of 30 consecutive patients with early breast cancer were evaluated. The prescription dose was 50 Gy to the whole breast. Axillary level I-II lymph node volumes were delineated and the cranial border of the tangential fields was set just below the humeral head to create HTF. Dose-volume histograms (DVH) were used to calculate the doses received by axillary nodal volumes. In a second planning set, HTF were modified with multileaf collimators (MLC-HTF) to obtain an adequate dose coverage of axillary nodes. The mean doses of the axillary nodes, the ipsilateral lung and heart were compared between the two plans (HTF vs MLC-HTF) using a paired sample t-test. The doses received by 95% of the breast volumes were not significantly different for the two plans. The doses received by 95% of the level I and II axillary volumes were 16.79 Gy and 11.59 Gy, respectively, for HTF, increasing to 47.2 Gy and 45.03 Gy, respectively, for MLC-HTF. Mean lung doses and per cent volume of the ipsilateral lung receiving 20 Gy (V20) were also increased from 6.47 Gy and 10.47%, respectively, for HTF, to 9.56 Gy and 16.77%, respectively, for MLC-HTF. Our results suggest that HTF do not adequately cover the level I and II axillary lymph node regions. Modification of HTF with MLC is necessary to obtain an adequate coverage of axillary levels without compromising healthy tissue in the majority of the patients.