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Inter-Fractional Tumor Motion Analysis Using 4D-CT and CBCT

M Xue

M Xue1*, S Tan1,2, A Reese1, B Yi1, W D'Souza1, S Feigenberg1, W Lu1, (1) University of Maryland Baltimore, Baltimore, MD, (2) Huazhong University of Science & Technology , Wuhan

SU-E-J-10 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantitatively evaluate the inter-fractional variation in tumor volumes with repeated 4D-CTs and repeated CBCTs for lung patients. To evaluate the uncertainties in patient set-up that uses internal target volume (ITV) of 4D-CT to match the soft tissue on CBCTs.
Methods: We retrospectively selected 5 lung cancer patients: each with three 4D-CTs (4D1, 4D2, and 4D3) and three CBCTs (C1, C2, and C3), and each CBCT was scanned within one week of a corresponding 4D-CT. All CT images are registered to 4D1, together with contours on each image: ITV for 4D-CT and gross tumor volumes (GTV) for CBCT. Then, these volumes are compared to ITV in 4D1 in terms of tumor volume, centroid distance, and volume overlap coefficient.
Results: In each CBCT/4D-CT pair, GTV in CBCT underestimate the ITV in 4D-CT by 41.22±1.39 %. When normalized to the ITV volume in the 4D1, other ITVs of 4D-CTs have an average volume of 1.07±0.13, and GTV in CBCTs have an average volume of 0.58±0.01. The centroid distance between ITV of 4D1 and a GTV of CBCT (5.6±11.9 mm) is larger than that between ITVs of 4D1and 4D2/4D3 (4.6±8.1 mm), while the CBCT GTV volumes are more included in the ITV of 4D1 (BinA: 0.863±0.018) than those of 4D2/4D3 (BinA: 0.735±0.033). From visual observation, the tumors presented in CBCTs are more similar to those in average projections compared to the ITVs of 4D-CTs.
Conclusions: The soft tissue alignment using 4D ITV on CBCT image has room for improvement. Although CBCT tumor seems more included, the centroid distance between CBCT-GTV and 4D1-ITV is larger than that between 4D2/4D3-ITV and 4D1-ITV. This, together with the underestimation of tumor volume from CBCT, makes current soft tissue alignment not as reliable as it seems.

Funding Support, Disclosures, and Conflict of Interest: This work is supported in part by Philips Healthcare, Inc.

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