Study on dosimetric changes between initial and second treatment plans in IMRT for large volume NSCLC

Objective: To evaluate the cumulative dose of the target volume and organs at risk (OARs) in intensity-modulated radiation therapy (IMRT) for large volume non-small cell lung cancer (NSCLC) based on rigid and deformation registration methods. The dosimetric changes between the initial and second treatment plans were compared. Methods: Thirty patients treated with IMRT for large volume NSCLC with twice 4DCT scans acquired before radiotherapy and after 20 fractions of radiotherapy were recruited. The initial treatment plan (Plan1) based on the average density projection CT (CT_1-avg) of the first 4DCT images and the second treatment plan (Plan2) based on the average density projection CT (CT_2-avg) of the second 4DCT images were calculated. Then, the dose distributions of Plan1 and Plan2 were accumulated based on rigid and deformation registration methods to obtain Plan_rig and Plan_def, respectively. Finally, the volume changes of gross tumor volume (GTV) and OARs between two CT scans were compared. The dose-volume parameters between Plan1 and other plans (including Plan2, Plan_rig and Plan_def) were also statistically compared. Results: Compared with the initial CT scan, the mean volume of GTV and heart on the second CT was decreased by 44.2% and 5.5%, respectively, while the mean volume of ipsilateral lung, contralateral lung and total lung was increased by 5.2%, 6.2% and 5.8%, respectively (all P < 0.05). Compared with Plan1, the D_95%, D_98% and V_100% of target volume IGTV (GTV fusion of 10 4DCT phases) and PTV in Plan2 did not significantly change (all P > 0.05), and those in Plan_rig and Plan_def were decreased (all P < 0.05). The dose-volume parameters of spinal-cord, heart, ipsilateral lung and total lung in Plan2, Plan_rig and Plan_def were significantly lower than those in Plan1 (all P < 0.05). Among them, the V_30Gy and D_mean of heart were decreased by 27.3%, 16.5%, 15.3% and 15.2%, 6.6%, 5.6%, respectively. The V_20Gy and D_mean of total lung were decreased by 15.6%, 4.5%, 3.7% and 15.7%, 6.2%, 5.1%, respectively. Some dose-volume parameters (including D_95% and D_98% of target volume, V_40Gy of heart, V_20Gy and D_mean of the ipsilateral lung and the total lung) of Plan_def were higher than those in Plan_rig (all P < 0.05). The Dice similarity coefficients (DSCs) of OARs after deformation registration were significantly higher than those after rigid registration (P < 0.05). Conclusions: The dose-volume parameters of OARs significantly differ between Plan1 and Plan2. Hence, all these parameters have a large degree of deviation in predicting radiation-induced injury of OARs. Nevertheless, the dose-volume parameters obtained by deformation registration can enhance the prediction accuracy. (authors)