SU‐E‐J‐10: Inter‐Fractional Tumor Motion Analysis Using 4D‐CT and CBCT

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. This work is supported in part by Philips Healthcare, Inc.