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Purpose: assess the percentages of irradiated lung volume and tissue during DIBH driven breast irradiation including axillary and supraclavicular nodes
Methods: effects during DIBH radiotherapy technique in breast cancer patients are well established. Whereas impact on the lung dose remains obscure. The absolute irradiated lung volume increases, however its percentage to the total lung, or lung tissue in the field might not. We chose ten patients, with available free-breathing (FB) CT scans, and ran breast irradiation plans that included axillary and supraclavicular nodes. The volumes and the average HU numbers for the ipsilateral lung and the in-field lung were analyzed. This study is follow-up of the study with tangential field arrangements only1.
Results: volumes of tissue for the ipsilateral lung and for the in-field lung can be obtained, as HUavg=(Vair*HUair+Vtissue*HUtissue)/( Vair+ Vtissue ). HUair can be calculated assuming that VTissue in the whole lung remains the same in DIBH and FB cases. The average HUair was -970 and HUtissue=45. The average lung volume for FB and DIBH were 1129.9 (stdev: 163.3) and 1939.1 (422.6) cc, and the average percentage of tissue in the whole lung was 22.8% (3%) and 14.1% (1.9%), respectively. The in-field volumes were 253.2 (101.235) and 448.46 (136.9) cc, and the in-field tissue 44.2 (16.5) and 52.4 (13.2) cc for FB and DIBH respectively. For all 10 cases the DIBH irradiated lung tissue was more than that of FB. The average ratio of the infield tissue to the total lung tissue is 17.2% (6%) and 19.5% (4.3%).
Conclusion: The DIBH technique irradiates more lung volume and lung tissue than FB even in the breast irradiation including axillary and supraclavicular nodes. However, the difference is smaller than in the study with tangential fields only¹.
¹ MO-E115-GePD-F3-01 Medical Physics, 46 (6), June 2019
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