Abstract
The purpose of this study was to assess target repositional accuracy with respect to the bony structures using daily CBCT, and to validate the planning target volume (PTV) margin used in the lung SBRT. All patients underwent 4D CT scanning in preparation for lung SBRT. The internal target volume (ITV) was outlined from the reconstructed 4D data using the maximum‐intensity projection (MIP) algorithm. A 6 mm margin was added to the ITV to create the PTV. Conformal treatment planning was performed on the helical images, to which the MIP images were fused. Prior to each treatment, CBCT was taken after a patient was set up in the treatment position. The CBCT images were fused with the simulation CT based on the bony anatomy, in order to derive setup errors and separate them from the tumor repositional errors. The treating physician then checked and modified the alignment based on target relocalization within the PTV. The shifts determined in such a method were recorded and the subtractions of these shifts with respect to the corresponding setup errors were defined as the target relocalization accuracy. Our study of 36 consecutive patients, treating 38 targets for a total of 153 fractions shows that, after setup error correction, the target repositional accuracy followed a normal distribution with the mean values close to 0 in all directions, and standard deviations of 0.25 cm in A–P, 0.24 cm in Lat, and 0.28 cm in S–I directions, respectively. The probability of having the shifts ≥0.6cm is less than 0.8% in A–P, 0.6% in Lat, and 1.7 % in S‐I directions. For the patient population studied, the target centroid position relative to the bony structures changed minimally from day to day. This demonstrated that the PTV margin that is designed on the MIP image‐based ITV was adequate for lung SBRT.PACS number: 87.53.Ly
Highlights
42 Wang et al.: Target repositional accuracy and planning target volume (PTV) margin verification overcome the difficulties which occur during imaging acquisition due to respiratory motion
The tumor motion pattern may change from day to day[25,26,27,28,29,30] and even during treatment delivery.[31,32] This implies that a internal target volume (ITV) design based on one set of 4D computed tomography (CT) may not be accurate for treatment planning.[33] the PTV margin designed to account for setup error only may not be enough to account for the change of the centroid position of the ITV during the treatment course
We could calculate the probability of having setup errors less than 6 mm and found it to be 88.4% in A–P, 88.5% in Lat, and 80.8% in S–I directions, respectively
Summary
42 Wang et al.: Target repositional accuracy and PTV margin verification overcome the difficulties which occur during imaging acquisition due to respiratory motion. Using 4D CT technology, volumetric images can be acquired at various times during the respiratory cycle; the time-resolved imaging techniques eliminate respiratory-induced artifacts and allow tumor motion to be characterized .[9,10] This specific information can aid treatment planners in designing the patient-specific planning target volume (PTV), which could further improve targeting and planning accuracy,(11-14) as well as potentially reduce doses to surrounding normal structures, decreasing side effects This approach of individualized margins based on patient-specific tumor motion has been proposed for high-dose hypofractionated SBRT.[5,13,14,15,16,17,18,19]. From a clinical point of view, the positional change of the ITV with respect to the patient’s bony structures and the PTV margin are most relevant to the coverage of disease and requires further study
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