Is It Time to Reconsider the Need for the ITV In SBRT For Lung Tumors?
Published on: 2016-04-25
Purpose: The importance of tumor positioning for SBRT is self-evident. There has been marked evolution in the last decades from 2D through 3D and IMRT to the utility of IGRT and four dimensional tumor localization that has enabled prescription of high doses of radiation to small tumor volumes. 4D simulation reduces the likelihood of “geographic miss”. We developed a model involving fusion of images from 4D followed by free breathing CT. Methods and Materials: We analyzed 13 treatment planning datasets from our initial cohort of patients who underwent SBRT for primary lung cancer or oligometastases to the lung. We carried out 4-D simulation and constructed the ITV. We sought to determine whether the respective centers of the ITV and GTV are concordant. Comparison is predicated upon the fusion of the 3-D and 4-D CT simulation datasets. The deviation of the location of the isocenters of the ITV and the captured GTV was registered individually. Results: The mean distance from the isocenters of the GTV and ITV was 4.28 mm (SD 2.06.) in CC, 0.75 (SD 0.57) in LM and 1.08 (SD 0.95) in AP directions.The isocenters of the ITV were found in the central positions (from 4 to 5 segments) in 2 cases (15%). The tumor position in the peripheral segments (1 to 3 and 6 to 8) was found in 11 cases (85%)In 6 cases (46%) the range of motion was extremely high (segments 1 to 2 and 7 to 8) Conclusions: The center of the static GTV consistently deviates from the ITV center. It is mandatory to implement the ITVbased technique to be certain that in a non-forgiving, high-dose approach (i.e., SBRT) the appropriate targets are indeed encompassed.