Technical Note: Comparison of the internal target volume (ITV) contours and dose calculations on 4DCT, average CBCT, and 4DCBCT imaging for lung stereotactic body radiation therapy (SBRT)

Purpose To investigate the differences between internal target volumes (ITVs) contoured on the simulation 4DCT and daily 4DCBCT images for lung cancer patients treated with stereotactic body radiotherapy (SBRT) and determine the dose delivered on 4D planning technique. Methods For nine patients, 4DCBCTs were acquired before each fraction to assess tumor motion. An ITV was contoured on each phase of the 4DCBCT and a union of the 10 ITVs was used to create a composite ITV. Another ITV was drawn on the average 3DCBCT (avgCBCT) to compare with current clinical practice. The Dice coefficient, Hausdorff distance, and center of mass (COM) were averaged over four fractions to compare the ITVs contoured on the 4DCT, avgCBCT, and 4DCBCT for each patient. Planning was done on the average CT, and using the online registration, plans were calculated on each phase of the 4DCBCT and on the avgCBCT. Plan dose calculations were tested by measuring ion chamber dose in the CIRS lung phantom. Results The Dice coefficients were similar for all three comparisons: avgCBCT-to-4DCBCT (0.7 +/- 0.1), 4DCT-to-avgCBCT (0.7 +/- 0.1), and 4DCT-to-4DCBCT (0.7 +/- 0.1); while the mean COM differences were also comparable (2.6 +/- 2.2mm, 2.3 +/- 1.4mm, and 3.1 +/- 1.1mm, respectively). The Hausdorff distances for the comparisons with 4DCBCT (8.2 +/- 2.9mm and 8.1 +/- 3.2mm) were larger than the comparison without (6.5 +/- 2.5mm). The differences in ITV D95% between the treatment plan and avgCBCT calculations were 4.3 +/- 3.0% and -0.5 +/- 4.6%, between treatment plan and 4DCBCT plans, respectively, while the ITV V100% coverages were 99.0 +/- 1.9% and 93.1 +/- 8.0% for avgCBCT and 4DCBCT, respectively. Conclusion There is great potential for 4DCBCT to evaluate the extent of tumor motion before treatment, but image quality challenges the clinician to consistently delineate lung target volumes.