International Journal of Radiation Oncology*Biology*Physics
Physics ContributionA Four-Dimensional Computed Tomography Analysis of Multiorgan Abdominal Motion
Introduction
Precise knowledge and control of the dose distribution within the patient is essential in accurate radiotherapy. For moving targets, this begins with detailed knowledge of organ motion. In the abdomen, the primary source of organ motion is respiration. Knowledge of patient-specific motion is needed when designing a treatment plan (e.g., aperture, beam angles, gating). Motion can change the radiologic depth from entrance surface to the target; its effect on photon therapy dose of moving lung tumors is small (1), whereas the potential effect on charged particle beam can be large 2, 3. Because critical organs adjacent to liver and pancreas tumors may be overirradiated by beam overshoot (particularly with proton or other charged particle beams), understanding respiratory-induced motion of abdominal organs may improve treatment planning of tumors within these organs.
Abdominal organ motion studies have been previously reported, using different imaging modalities, breathing protocols, and analysis metrics. One of the seminal papers on organ motion (before the availability of four-dimensional computed tomography [4DCT] and four-dimensional magnetic resonance imaging [4DMRI]) was written by Langen and Jones (4). More recently, 4D imaging modalities have been used in addition to helical CT, MRI, fluoroscopy, and ultrasound 2, 5, 6, 7, 8, 9, 10, 11, 12, 13. Organ motion has been reported in terms of center of mass (COM) motion, edge (bounding box) motion, and points of interest motion (fiducials). Organ motion data have been acquired under a variety of breathing conditions, including free breathing, breath hold, and deep inspiration breath hold. Data have been acquired from one breath cycle to ∼15 cycles. The various approaches affect both the values reported and the interpretation of their comparison.
In this analysis, we present our study of abdominal multiorgan motion of pancreatic and liver tumor patients. 4DCT data are used for the analysis, and the focus is primarily on the COM and bounding box motions. We compare our results with studies previously reported.
Section snippets
Patients studied
Eleven patients with liver cancer and 7 with pancreatic cancer treated with proton beam therapy were sequentially selected for this retrospective study. The image data were analyzed under a protocol approved by the institutional review board to retrospectively analyze anonymized 4DCT data. The patients included 11 men and 7 women (liver: 7 men and 4 women; pancreas: 4 men and 3 women). Two of the liver patients had two lesions each. After initial analysis, 2 patients were excluded because of
Abdominal tumor motion
The COM and bounding box plots for a CTV of a representative liver tumor patient are shown in Fig. 1. Between exhale and inhale, the CTV COM moves inferiorly by ∼10 mm and anteriorly by ∼4 mm during inhalation. In this patient, we also observed a left–right COM motion of ∼4 mm.
The bounding box analysis provided additional information. In this patient, the superior and inferior bounding box planes of the CTV moved in unison, with essentially the same amplitude. Bounding box motion along the AP
Discussion
Understanding multiorgan motion facilitates personalized planning treatment. Characterization of patient-specific motion can be used to design anisotropic margins for appropriate geometric coverage. If motion (and the radiologic pathlength variations of each beam ray) is minimal, motion mitigation is not indicated. Analysis of organ motion and its dosimetric coverage over multiple patients may lead to a threshold value, below which such motions can be accepted. Values above the threshold would
Summary
We report on the abdominal motion of 16 patients who underwent 4DCT under quiet breathing for treatment of abdominal tumors. The mean COM motions were below 10 mm for both pancreas and liver lesions. Specific aspects of motion are highly dependent on tumor location and structural boundaries, tumor location, and patient breathing characteristics. The multiple organs move synchronously. A gating window around exhale (T50) significantly reduces the range of motion by almost a factor of 10.
Acknowledgment
The authors thank Brian Napolitano for assistance in volume calculations and Paul Burkard for assistance in figure preparation.
References (20)
- et al.
Effects of respiration-induced density variations on dose distributions in radiotherapy of lung cancer
Int J Radiat Oncol Biol Phys
(2009) - et al.
Four-dimensional measurement of intrafractional respiratory motion of pancreatic tumors using a 256 multi-slice CT scanner
Radiother Oncol
(2009) - et al.
Impact of intrafractional bowel gas movement on carbon ion beam dose distribution in pancreatic radiotherapy
Int J Radiat Oncol Biol Phys
(2009) - et al.
Organ motion and its management
Int J Radiat Oncol Biol Phys
(2001) - et al.
Four-dimensional computed tomography-based treatment planning for intensity-modulated radiation therapy and proton therapy for distal esophageal cancer
Int J Radiat Oncol Biol Phys
(2008) - et al.
Quantification of respiration-induced abdominal tumor motion and its impact on IMRT dose distributions
Int J Radiat Oncol Biol Phys
(2004) - et al.
Target volume definition for upper abdominal irradiation using CT scans obtained during inhale and exhale phases
Int J Radiat Oncol Biol Phys
(2000) - et al.
Respiration-induced movement of the upper abdominal organs: A pitfall for the three-dimensional conformal radiation treatment of pancreatic cancer
Radiother Oncol
(2003) - et al.
Interfraction and respiratory organ motion during conformal radiotherapy in gastric cancer
Int J Radiat Oncol Biol Phys
(2010) - et al.
Daily targeting of intrahepatic tumors for radiotherapy
Int J Radiat Oncol Biol Phys
(2002)
Cited by (0)
Supported by Award Number P01CA021239 from the National Cancer Institute.
The content of this article is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.
Conflict of interest: none.