Elsevier

Lung Cancer

Volume 56, Issue 1, April 2007, Pages 59-67
Lung Cancer

Four-dimensional measurement of lung tumor displacement using 256-multi-slice CT-scanner

https://doi.org/10.1016/j.lungcan.2006.11.011Get rights and content

Summary

The concept of internal target volume is of marked importance for radiotherapy to lung tumors as respiration-induced motion is important. Individualized assessment of motion is required as tumor site may not predict the extent or pattern of tumor motion. We performed volumetric cine scanning using the 256-multi-slice CT (256MSCT) to study tumor motion during free breathing in 14 inpatients who were treated with carbon-ion radiotherapy. Motion assessment in 16 respiratory phases of the cine CT revealed most tumors to show hysteresis-like behavior. Isocenter displacement between peak exhalation and inhalation for the average of the right and left lungs were 7 mm, 7 mm and 15 mm for the upper, middle and lower lobes, respectively. Cine CT with the 256MSCT improved the evaluation of tumor displacement and overcomes some of the limitations associated with current CT methods. Volumetric cine CT data provides useful data on motion for planning in all radiation approaches for lung tumors.

Introduction

In the 1970s and 1980s, the technology of radiotherapy was based on the use of conventional simulators. Usage was suitably handled by the International Commission on Radiation Units and Measurements (ICRU) recommendations in ICRU Report 29 [1]. With the rapid development of computer hardware and software in the mid-1980s to early 1990s, however, conventional simulators were replaced by CT scanners, which provided three-dimensional (3D) anatomical views for radiotherapy. This innovative technology improved accuracy in both determining the shape and location of tumors, and in dose delivery.

Radiotherapy under free breathing requires clear determination of the internal target volume (ITV). An example is provided by respiratory-gated radiotherapy (RGRT), which irradiates at a particular phase of the respiratory cycle (generally at peak exhalation) using a device which gates the respiratory signal. Four-dimensional radiotherapy has been defined as the explicit inclusion of temporal changes in anatomy during imaging, planning and delivery of radiotherapy delivery (4DRT) [2]. This assumes that no additional internal margin is needed as the radiation fields used will incorporate all positions of the moving tumor during delivery. Therefore observation of respiration is required to ensure conformation of the beam field with the movement of the tumor.

Phase-based observation of respiration is necessary because tumors moving under respiration do not behave similarly, but rather move in different paths during inhalation and exhalation, showing hysteresis-like behavior [3] (Fig. 1). Evaluation of ITV at peak exhalation or inhalation is therefore insufficient. Most investigations of respiratory motion to date have used fluoroscopy [4], radiography [5], US, MRI, CT or PET. Since CT scans can image moving organs with higher spatial and temporal resolutions than with the use of other techniques, current research in organ motion is mainly directed toward CT scanners. The reproducibility and accuracy of gross tumor volume (GTV) delineation for most treatment sites is not sufficiently understood because it is largely based on images obtained clinically, which tend to taken at thick-slice, short acquisition time settings. Moreover, the shape and size of the GTV can vary significantly depending on imaging modality.

Of these modalities, CT is the principal source of imaging data used for defining the GTV for radiotherapy planning (RTP). CT scanning under free breathing, however, does not represent either the time-averaged position or total respiratory moving distance of the tumor [6], [7], [8], and moreover the artifacts induced by respiration can mimic disease symptoms in clinical settings [9]. It would be better, therefore, if CT accurately captured the moving tumor at each respiratory phase.

Since higher target conformity requires accurate definition of the target, many investigators have adopted the respiratory-gated CT (RGCT) [10] and 4DCT scan techniques [11] under free breathing. RGCT scanned at the most stable point of the gated respiratory cycle, usually peak exhalation. While to obtain tumor displacement at each respiratory phase, 4DCT scan method is performed in cine mode, which operates the scanner without couch movement using respiratory signals from surrogate systems to obtain all respiratory phases at each couch position, moves the couch to the next position, and sorts CT images at the same respiratory phase. The latest multi-slice CT, however, now incorporates 64 segments with a segment size of 0.5–0.625 mm at the center of rotation, representing a substantial improvement over conventional multi-slice CT (MSCT), especially in cardiac imaging. The maximum axial field of view is less than 40 mm, however, making the location of investigation critical as only a limited section of the organ-of-interest can be examined. Given that many institutions prefer wide SI (superior–inferior) coverage over spatial resolution to improve examination efficiency in RGCT or 4DCT, residual uncertainties in the SI direction have resulted in dose delivery errors. Selection of a thinner slice size requires a longer study time, which may degrade the reproducibility of the respiratory wave form.

Here, we performed volumetric cine CT scan of lung tumor under free breathing using the 256MSCT and evaluated tumor displacement quantitatively, this scan method provided same respiratory cycle in all slices.

Section snippets

256-Multi-slice CT-scanner (256MSCT)

The second model of the 256MSCT was based on the design of the first [12], [13], which used a wide-area cylindrical 2D detector incorporating present CT technology mounted on the gantry frame of a 16-slice CT [14] (Aquilion, Toshiba Medical Systems). The 256MSCT has 912 (transverse) × 256 (cranio-caudal) elements, each approximately 0.5 mm × 0.5 mm at the center of rotation. The 128 mm total beam width allows the continuous use of several collimation sets. SI coverage is 128 mm per rotation. Rotation

Characteristics of tumor motion

Volumetric cine imaging of the lung satisfactorily obtained continuous movement of the tumor in the sagittal section (Fig. 4, patient no. 8). Observation was facilitated by superimposition of the sagittal image at PE on images at each respiratory phase. Motion artifacts due to breathing are frozen by a temporal resolution of 250 ms, allowing the tumor shape to be evaluated accurately. Moreover, the thin slice thickness and short total acquisition time (approximately 6 s) helped determine target

Discussion

We evaluated the characteristics of tumor motion under free breathing and estimated internal margin accurately using the 256MSCT. Conventional 4DCT and RGCT scan methods need to patch together different CT slices from different respiratory cycles at consecutive couch positions, however, the 256MSCT provided same respiratory cycle in all slices. Most tumors showed a hysteresis-like movement, but those in contact with the lung wall moved in correspondence with the wall. These results are useful

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