Comparative analysis of delivered and planned doses in target volumes for lung stereotactic ablative radiotherapy

Abstract

Background: Adaptive therapy has been enormously improved based on the art of generating adaptive computed tomography (ACT) from planning CT (PCT) and the on-board image used for the patient setup. Exploiting the ACT, this study evaluated the dose delivered to patients with non-small-cell lung cancer (NSCLC) patients treated with stereotactic ablative radiotherapy (SABR) and derived relationship between the delivered dose and the parameters obtained through the evaluation procedure.

Methods: SABR treatment records of 72 patients with NSCLC who were prescribed a dose of 60 Gy (D_prescribed) to the 95% volume of the planning target volume (PTV) in four fractions were analysed in this retrospective study; 288 ACTs were generated by rigid and deformable registration of a PCT to a cone-beam computed tomography (CBCT) per fraction. Each ACT was sent to the treatment planning system (TPS) and treated as an individual PCT to calculate the dose. Delivered dose to a patient was estimated by averaging four doses calculated from four ACTs per treatment. Through the process, each ACT provided the geometric parameters, such as mean displacement of the deformed PTV voxels (Warp_mean) and Dice similarity coefficient (DSC) from deformation vector field, and dosimetric parameters, e.g. difference of homogeneity index (ΔHI, HI defined as (D_2%-D_98%)/D_prescribed*100) and mean delivered dose to the PTV (D_mean), obtained from the dose statistics in the TPS. Those parameters were analyzed using multiple linear regression and one-way-ANOVA of SPSS^® (version 27).

Results: The prescribed dose was confirmed to be fully delivered to internal target volume (ITV) within maximum difference of 1%, and the difference between the planned and delivered doses to the PTV was agreed within 6% for more than 95% of the ACT cases. Volume changes of the ITV during the treatment course were observed to be minor in comparison of their standard deviations. Multiple linear regression analysis between the obtained parameters and the dose delivered to 95% volume of the PTV (D_95%) revealed four PTV parameters [Warp_mean, DSC, ΔHI between the PCT and ACT, D_mean] and the PTV D_95% to be significantly related with P-values < 0.05. The ACT cases of high ΔHI were caused by higher values of the Warp_mean and DSC from the deformable image registration, resulting in lower PTV D_95% delivered. The mean values of PTV D_95% and Warp_mean showed significant differences depending on the lung lobe where the tumour was located.

Conclusions: Evaluation of the dose delivered to patients with NSCLC treated with SABR using ACTs confirmed that the prescribed dose was accurately delivered to the ITV. However, for the PTV, certain ACT cases characterised by high HI deviations from the original plan demonstrated variations in the delivered dose. These variations may potentially arise from factors such as patient setup during treatment, as suggested by the statistical analyses of the parameters obtained from the dose evaluation process.

Keywords: Adaptive CT; Deformable image registration; Dose evaluation; Lung SABR.

Fig. 1

Workflow of the dose evaluation procedure

Fig. 2

Dose comparison between planned (filled square) and estimated delivered (open square) dose to planning target volume (PTV) and their standard deviations: minimum (D_min), mean (D_mean), maximum dose (D_max) to the PTV, and the dose delivered to the 95% volume of the PTV (D_95%)

Fig. 3

Dose comparison between planned (filled square) and estimated delivered (open square) dose to internal target volume (ITV) and their standard deviations: minimum (D_min), mean (D_mean), maximum (D_max) dose delivered to the ITV

Fig. 4

(a) Number of days taken for the stereotactic ablative radiotherapy of NSCLS patients, (b) scattered plot of their averaged difference and its standard deviation of the internal target volume (ITV, %) described in the adaptive CTs compared to the volume of the ITV in the treatment planning CT (PCT) depending on the ITV volume in the PCT, and their averaged volume difference and its standard deviation of the (c) ITV and (d) planning target volume (PTV) per fraction

Fig. 5

(a) One dimensional distribution of the homogeneity index difference (ΔHI) and (b) scattered plot of the delivered dose to 95% volume of the PTV (D_95%) vs. ΔHI. Concerned outlier cases were highlighted in the scatter plot

Fig. 6

(a) One dimensional distribution of the PTV volume (cc) and (b) scattered plot of the delivered dose to 95% volume of the PTV (D_95%) vs. the PTV volume. Concerned outlier cases were highlighted in the scatter plot

Fig. 7

One-dimensional distribution of the independent parameters: (a) mean delivered dose to PTV (D_mean), (b) mean Warping distance (Warp_mean), (c) Dice coefficient of similarity (DSC), and a dependent parameter (d) delivered dose to 95% volume of the PTV (D_95%)

Fig. 8

Scattered plot of the residual of the dose prediction vs. the predicted valued of the regression model. Concerned outliers were highlighted in the scatter plot