Dosimetric comparison of stereotactic body radiotherapy using 4D CT and multiphase CT images for treatment planning of lung cancer: Evaluation of the impact on daily dose coverage

Purpose: To investigate the dosimetric impact of using 4D CT and multiphase (helical) CT images for treatment planning target definition and the daily target coverage in hypofractionated stereotactic body radiotherapy (SBRT) of lung cancer. Materials and methods: For 10 consecutive patients treated with SBRT, a set of 4D CT images and three sets of multiphase helical CT scans, taken during free-breathing, end-inspiration and end-expiration breath-hold, were obtained. Three separate planning target volumes (PTVs) were created from these image sets. A PTV_4D was created from the maximum intensity projection (MIP) reconstructed 4D images by adding a 3 mm margin to the internal target volume (ITV). A PTV_3CT was created by generating ITV from gross target volumes (GTVs) contoured from the three multiphase images. Finally, a third conventional PTV (denoted PTV_conv) was created by adding 5 mm in the axial direction and 10 mm in the longitudinal direction to the GTV (in this work, GTV = CTV = clinical target volume) generated from free-breathing helical CT scans. Treatment planning was performed based on PTV_4D (denoted as Plan-1), and the plan was adopted for PTV_3CT and PTV_conv to form Plan-2 and Plan-3, respectively, by superimposing "Plan-1" onto the helical free-breathing CT data set using modified beam apertures that conformed to either PTV_3CT or PTV_conv. We first studied the impact of PTV design on treatment planning by evaluating the dosimetry of the three PTVs under the three plans, respectively. Then we examined the effect of the PTV designs on the daily target coverage by utilizing pre-treatment localization CT (CT-on-rails) images for daily GTV contouring and dose recalculation. The changes in the dose parameters of D₉₅ and D₉₉ (the dose received by 95% and 99% of the target volume, respectively), and the V_p (the volume receiving the prescription dose) of the daily GTVs were compared under the three plans before and after setup error correction. Results: For all 10 patients, we found that the PTV_4D consistently resulted in the smallest volumes compared with the other PTV's (p = 0.005). In general, the plans generated based PTV_3CT could provide reasonably good coverage for PTV_4D, while the reverse can only achieve 90% of the planned values for PTV_3CT. The coverage of both PTV_4D and PTV_3CT in Plan-3 generally reserves the original planned values in terms of D₉₅, D₉₉, and V_p, with the average ratios of 0.996, 0.977, and 0.977, respectively, for PTV_3CT, and 1.025, 1.025, and 1.0, respectively, for PTV_4D. However, it increased the dose significantly to normal lung tissue. Additionally, the plans generated using the PTV_4D presented an equivalent daily target coverage compared to the plans generated using the PTV_3CT (p = 0.953) and PTV_conv (p = 0.773) after setup error correction. Consequently, this minimized the dose to the surrounding normal lung. Conclusion: Compared to the conventional approach using helical images for target definition, 4D CT and multiphase 3D CT have the advantage to provide patient-specific tumor motion information, based on which such designed PTVs could ensure daily target coverage. 4D CT-based treatment planning further reduces the amount of normal lung being irradiated while still providing a good target coverage when image guidance is used.