(Monday, 7/15/2019) 1:15 PM - 1:45 PM
Room: Exhibit Hall | Forum 3
Purpose: To assess the percentages of irradiated lung volume and lung tissue volume during DIBH driven tangential breast irradiation.
Methods: It is well known that the DIBH treatment can radically decrease the dose delivered to the heart in supine tangential breast irradiation. The effect on lung dose is analyzed less thoroughly. The absolute lung volume inside the radiation field increases, however its percentage to the total lung, or lung tissue in the field might not. We chose 12 DIBH patients, with available free-breathing (FB) CT scans, and run plans with extended (to include IMN) tangential field arrangements. The volumes and the average HU numbers for the ipsilateral lung and the in-field lung were measured.
Results: Since HUavg=(Vair*HUair+Vtissue*HUtissue)/( Vair+ Vtissue ) the volumes of tissue for the ipsilateral lung and for the in-field lung can be obtained, in both the DIBH and FB plans. We used HUtissue=45. HUair can be calculated assuming that VTissue in the whole lung is the same for DIBH and FB. The average HUair was -970.The average lung volume for FB and DIBH were 987.0 (stdev: 357.4) and 1755.3 (650.2) cc, and the percentage of tissue in the whole lung in average was 26.2% (4.7%) and 15.2% (3.9%), respectively. The in-field volumes were 129.9 (67.9) and 292.0 (110.4) cc, and the volume of in-field tissue 24.5 (15.7) and 32.9 (14.8) cc for FB and DIBH respectively. For all 12 cases the DIBH lung tissue was more than that of FB. The average ratio of the infield tissue to the total lung tissue is 8.9% (5.5%) and 11.7% (4.1%).
Conclusion: The DIBH technique irradiates more lung volume and lung tissue than FB. However, the infield lung mass is still only 9% to 20% of the total ipsilateral lung mass.
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