Abstract
The aim of this study was assess the patient setup errors for various tumor sites based on clinical data from a sufficient number of treatments with volumetric‐modulated arc therapy (VMAT) using daily pretreatment CBCT imaging guidance. In addition, we calculated and compared the planning target volume (PTV) margins for all disease sites based on an analysis of specific systematic and random errors in our institution. All patients underwent pretreatment kV‐CBCT imaging. The various tumor sites were divided into four categories; 21 brain (438 fractions), 35 head‐and‐neck tumors (H&N, 933 fractions), 19 thorax and abdomen tumors (T&A, 313 fractions), and 17 prostate cancer tumors (546 fractions). Overall distributions of setup corrections in all directions, frequencies of 3D vector lengths, institution‐specific setup error, and PTV margins were analyzed. The longitudinal distribution for the T&A site represented an asymmetric offset in the negative direction. Rotational distributions were comparable for all treatment sites, and the prostate site had the narrowest distribution of ≤±2∘. The cumulative frequencies of 3D vector length of ≥ 7 mm were rare for brain lesions and H&N, but more common for T&A and prostate lesions at 25.6% and 12.1%, respectively. The overall mean error for all treatment sites were within ±1 mm and ±0.1∘, with the exception of the T&A site, which had overall mean error of 2 mm in the negative longitudinal direction. The largest magnitude of systematic error and random error for the brain lesions and H&N was 1.4 mm in the translational directions, and 3.3 mm for T&A and prostate lesions. The PTV margins required in this analysis are ≤ 4 mm for the brain lesions and H&N in all translational directions, but ranged from 4 to 10 mm for T&A and prostate lesions. Analysis of each institution's specific setup errors using daily CBCT is essential for determining PTV margins and reducing setup uncertainties, because setup errors vary according to each immobilization system and patient.PACS number: 87.55.km
Highlights
Acquisition of Cone-beam computed tomography (CBCT) images is a time-consuming process compared to the acquisition of conventional, two-dimensional (2D) portal imaging scans, and the total time needed for Intensity-modulated radiation therapy (IMRT) treatment including setup, pretreatment CBCT, repositioning, verification, and beam delivery is approximately 60 minutes.[30]. The prolonged treatment time may increase patient setup errors and intrafractional variations.[31]
The distributions were comparable for all treatment sites, and the prostate site had the narrowest distribution of ≤ ± 2°
Three-dimensional vector distances of ≥ 7 mm were rare for brain lesions and H&N, but more common for T&A and prostate lesions at 25.6% and 12.1%, respectively
Summary
86 Oh et al.: Assessment of setup uncertainties for various tumor sites organs at risk.[8,9,10,11,12] high geometric accuracy is important for the clinical application of IMRT.Image-guidance systems are commonly used to minimize setup uncertainties,(13-20) and have provided highly effective for quantifying patient setup errors and correcting target localization.[20,21] A reduction in setup uncertainties would allow for a reduction in treatment margins, leading to dose escalation and an improvement in local control at the tumor site.[22,23,24,25,26]Cone-beam computed tomography (CBCT) is an especially useful image-guidance modality for IMRT because it provides excellent visualization of the target with three-dimensional (3D) imaging and high-resolution, soft-tissue information.[27,28,29]acquisition of CBCT images is a time-consuming process compared to the acquisition of conventional, two-dimensional (2D) portal imaging scans, and the total time needed for IMRT treatment including setup, pretreatment CBCT, repositioning, verification, and beam delivery is approximately 60 minutes.[30]. VMAT technology is a novel extension of IMRT, capable of delivering highly conformal dose distribution over a much shorter beam-on time and requiring fewer monitor units than IMRT by the modulation of gantry rotation speed, multileaf collimator motion, dose rate, and the number of arcs.[32,33,34,35,36] In addition, a single rotation of the gantry takes less than 2 minutes at the maximum gantry rotation speed.[32] These improvements can significantly reduce the treatment time, thereby potentially reducing both the number of random daily setup errors in positioning and variations in patient anatomy during treatment. Multiple studies have addressed the setup uncertainties associated with CBCT imaging guidance in IMRT, only a few studies have included clinical data on the setup errors at various tumor sites using daily CBCT imaging guidance and the same immobilization system for each treatment site. In order to examine the setup errors more accurately, the same immobilization system should be applied for each treatment site and more patients should be studied
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