SBRT of lung cancerDosimetric 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
Introduction
It is well known that thoracic tumors exhibit significant intra-fractional motion due to respiration [1], [2], [3], [4], [5]. Moreover, respiratory motion during CT acquisition can introduce severe motion artifacts, resulting in distortion of tumor shape, size and inaccurate assessment of tumor location [6], [7], [8], [9], [10], [11]. Therefore, the conventional approach for defining target volumes employs large margins, based on typical site-specific motion amplitudes (on the order of 1–2 cm). These margins are applied to a gross tumor volume (GTV) generated from a free-breathing CT data set to create a planning target volume (PTV). This increases the amount of normal tissue in the field, which could lead to an increased toxicity. It also limits dose escalation, which is necessary to improve local control for an early stage non-small cell lung cancer [12], [13], [14], [15]. Recently, some investigators have suggested that individualized margins should be generated based on patient-specific tumor motion [16], [17], [18], [19], [20], [21], [22], especially when using high dose, hypofractionated SBRT [23].
Early methods to account for patient-specific tumor motion use multiphase CT scans taken during free-breathing, end-inspiration and end-expiration breath-hold. These scans are then fused for treatment planning [15], [24]. This approach assumes that images taken during the extremes of respiration define the extremes of target motion, thus accounting for tumor position during all respiratory phases. Recent advances in imaging technology using four-dimensional (4D) CT have provided us with a more reliable tool to study patient-specific motion [9], [25], [26], [27], [28], [29], [30], [31]. The 4D CT technique synchronizes image acquisition with respiratory phase. By sorting a sequence of 3D images according to the phase of the respiratory cycle during which they were acquired, one can reconstruct multiple CT sets (generally 10 sets for 10 respiratory phases) at different phases of respiratory cycle. Thus, respiration-correlated CT data contain information about temporal changes in tumor position and shape.
Directly using 4D CT images for treatment planning requires that the GTV be contoured in each of the 10 phases to generate an internal target volume (ITV). This creates a tremendous workload for clinicians, dosimetrists and physicists. Thus, investigators have proposed using other post-processing tools [26], [32]. One such tool is the maximum intensity projection (MIP) reconstructed 4D data set. Briefly, MIP projections reflect the highest data value encountered along the viewing ray for each pixel of volumetric data, giving rise to a full intensity display of the brightest object along each ray of the projection image. Underberg et al. [32] have studied the use of MIP for target volume definition in 4D CT scans for lung cancer and verified the accuracy of MIP in a motion phantom. They concluded that MIP is a reliable clinical tool for generating ITVs from 4D CT data sets. Another study [33] also confirmed that MIP is superior in defining an internal target volume.
In our clinic, we are now using MIP reconstructed 4D data sets to define an ITV. Prior to 2006, when we purchased the GE Light Speed CT scanner™ (General Electric Healthcare Technologies, Waukesha, WI) and its 4D CT image processing and reconstruction software, we used three multiphase CT images for ITV definition. In this study, we compare the target volume, treatment planning, and daily tumor coverage resulting from different PTV designs using 4D MIP images and multiphase 3D images. Considering that the conventional approach using helical CT images with population-based margins is still widely used clinically, we also include this approach in our comparison study. Our purpose is, by studying the impact of different target definitions on daily target coverage, to determine which approach more accurately accounts for patient-specific tumor motion during SBRT for lung cancer.
Section snippets
Methods and materials
A Phase I dose-escalation trial for malignant lung tumors (either primary non-small cell lung cancers (NSCLC) or limited metastatic tumors to the lung) measuring ⩽5 cm has been ongoing at our institution since 2003. The dose fractionation was initially 40 Gy in four fractions, and gradually increased to 48 Gy and then to 56 Gy in four fractions. Initially, we used three sets of multiphase CT for defining the target [24]. The purpose of this approach was to eliminate any artifacts due to respiratory
Target volume variation
We first compared ITV volumes from the three sets of CT scans with those outlined on the MIP reconstructed 4D images. As seen in Fig. 1a, the ITV3CT volumes are consistently larger than the ITV4D volumes if one ignores volume segmentation uncertainty. A paired t-test confirmed that this difference was statistically significant (p = 0.049). This reveals that multiphase CT simulation over-estimates target motion compared to 4D CT, assuming that 4D CT best captures moving target trajectory [32]. The
Discussion and conclusion
4D CT imaging represents an exciting new development for treatment planning due to its ability to capture patient-specific tumor motion [22], [26], [31], [35], [36], [37], [38]. Post-processing tools such as the MIP protocol make 4D CT a practical tool in the clinic for defining individualized ITVs without substantially increasing the clinician’s workload. Before such an approach can be adopted clinically, it requires validation and comparison with conventional approaches. Several other studies
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