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 3mm 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 5mm in the axial direction and 10mm 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 D(95) and D(99) (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 D(95), D(99), 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.