Westerly David C, Schefter Tracey E, Kavanagh Brian D, Chao Edward, Lucas Dan, Flynn Ryan T, Miften Moyed
Department of Radiation Oncology, University of Colorado School of Medicine, Aurora, Colorado 80045, USA.
Med Phys. 2012 Aug;39(8):4812-9. doi: 10.1118/1.4736416.
Stereotactic body radiation therapy (SBRT) is a potent treatment for early stage primary and limited metastatic disease. Accurate tumor localization is essential to administer SBRT safely and effectively. Tomotherapy combines helical IMRT with onboard megavoltage CT (MVCT) imaging and is well suited for SBRT; however, MVCT results in reduced soft tissue contrast and increased image noise compared with kilovoltage CT. The goal of this work was to investigate the use of increased imaging doses on a clinical tomotherapy machine to improve image quality for SBRT image guidance.
Two nonstandard, high-dose imaging modes were created on a tomotherapy machine by increasing the linear accelerator (LINAC) pulse rate from the nominal setting of 80 Hz, to 160 Hz and 300 Hz, respectively. Weighted CT dose indexes (wCTDIs) were measured for the standard, medium, and high-dose modes in a 30 cm solid water phantom using a calibrated A1SL ion chamber. Image quality was assessed from scans of a customized image quality phantom. Metrics evaluated include: contrast-to-noise ratios (CNRs), high-contrast spatial resolution, image uniformity, and percent image noise. In addition, two patients receiving SBRT were localized using high-dose MVCT scans. Raw detector data collected after each scan were used to reconstruct standard-dose images for comparison.
MVCT scans acquired using a pitch of 1.0 resulted in wCTDI values of 2.2, 4.7, and 8.5 cGy for the standard, medium, and high-dose modes respectively. CNR values for both low and high-contrast materials were found to increase with the square root of dose. Axial high-contrast spatial resolution was comparable for all imaging modes at 0.5 lp∕mm. Image uniformity was improved and percent noise decreased as the imaging dose increased. Similar improvements in image quality were observed in patient images, with decreases in image noise being the most notable.
High-dose imaging modes are made possible on a clinical tomotherapy machine by increasing the LINAC pulse rate. Increasing the imaging dose results in increased CNRs; making it easier to distinguish the boundaries of low contrast objects. The imaging dose levels observed in this work are considered acceptable at our institution for SBRT treatments delivered in 3-5 fractions.
立体定向体部放射治疗（SBRT）是早期原发性和局限性转移性疾病的有效治疗方法。准确的肿瘤定位对于安全有效地实施SBRT至关重要。断层放射治疗将螺旋调强放疗（IMRT）与机载兆伏级CT（MVCT）成像相结合，非常适合SBRT；然而，与千伏级CT相比，MVCT导致软组织对比度降低和图像噪声增加。本研究的目的是探讨在临床断层放射治疗机上增加成像剂量以改善SBRT图像引导的图像质量。
在一台断层放射治疗机上创建了两种非标准的高剂量成像模式，方法是将直线加速器（LINAC）脉冲率分别从标称设置的80 Hz提高到160 Hz和300 Hz。使用校准的A1SL离子室在30 cm固体水模体中测量标准、中等和高剂量模式的加权CT剂量指数（wCTDIs）。通过对定制的图像质量模体进行扫描来评估图像质量。评估的指标包括：对比度噪声比（CNRs）；高对比度空间分辨率；图像均匀性；以及图像噪声百分比。此外，使用高剂量MVCT扫描对两名接受SBRT的患者进行定位。每次扫描后收集的原始探测器数据用于重建标准剂量图像以进行比较。
使用螺距为1.0的MVCT扫描，标准、中等和高剂量模式的wCTDI值分别为2.2、4.7和8.5 cGy。发现低对比度和高对比度材料的CNR值均随剂量的平方根增加。在0.5 lp∕mm时，所有成像模式的轴向高对比度空间分辨率相当。随着成像剂量增加，图像均匀性得到改善，噪声百分比降低。在患者图像中也观察到类似的图像质量改善，其中图像噪声降低最为明显。
通过提高LINAC脉冲率，在临床断层放射治疗机上实现了高剂量成像模式。增加成像剂量会导致CNR增加；从而更容易区分低对比度物体的边界。在本研究中观察到的成像剂量水平在我们机构被认为对于3 - 5次分割的SBRT治疗是可接受的。
