Prog Med Phys.  2016 Dec;27(4):203-212. 10.14316/pmp.2016.27.4.203.

Evaluating Correlation between Geometrical Relationship and Dose Difference Caused by Respiratory Motion Using Statistical Analysis

• Shin DS
• Kang SH
• Kim DS
• Kim TH
• Kim KH
• Cho MS
• Noh YY
• Yoon DK
• Suh TS

• KMID: 2376553
• DOI: http://doi.org/10.14316/pmp.2016.27.4.203

Abstract

Dose differences between three-dimensional (3D) and four-dimensional (4D) doses could be varied according to the geometrical relationship between a planning target volume (PTV) and an organ at risk (OAR). The purpose of this study is to evaluate the correlation between the overlap volume histogram (OVH), which quantitatively shows the geometrical relationship between the PTV and OAR, and the dose differences. 4D computed tomography (4DCT) images were acquired for 10 liver cancer patients. Internal target volume-based treatment planning was performed. A 3D dose was calculated on a reference phase (end-exhalation). A 4D dose was accumulated using deformation vector fields between the reference and other phase images of 4DCT from deformable image registration, and dose differences between the 3D and 4D doses were calculated. An OVH between the PTV and selected OAR (duodenum) was calculated and quantified on the basis of specific overlap volumes that corresponded to 10%, 20%, 30%, 40%, and 50% of the OAR volume overlapped with the expanded PTV. Statistical analysis was performed to verify the correlation with the OVH and dose difference for the OAR. The minimum mean dose difference was 0.50 Gy from case 3, and the maximum mean dose difference was 4.96 Gy from case 2. The calculated range of the correlation coefficients between the OVH and dose difference was from −0.720 to −0.712, and the R-square range for regression analysis was from 0.506 to 0.518 (p-value <0.05). However, when the 10% overlap volume was applied in the six cases that had OVH value ≤2, the average percent mean dose differences were 34.80±12.42%. Cases with quantified OVH values of 2 or more had mean dose differences of 29.16±11.36%. In conclusion, no significant statistical correlation was found between the OVH and dose differences. However, it was confirmed that a higher difference between the 3D and 4D doses could occur in cases that have smaller OVH value.

Keyword

4D dose; Overlap volume histogram; Respiratory motion; Geometrical relationship

MeSH Terms

Four-Dimensional Computed Tomography
Humans
Liver Neoplasms

Figure

• 

  Fig. 1. Ssimple calculation process for four-dimensional doses (4D doses). First, deformable image registration was used to calculate the deformation vector fields (DVFs) for each phase image based on the end-exhalation phase image from four-dimensional computed tomography (4DCT). Second, the calculated DVFs were applied to each dose distribution that was calculated from each phase image to obtain the warped dose distributions. Finally, the warped dose distributions were weighted and summed up to calculate the 4D doses.

• 

  Fig. 2. Example of an overlap volume histogram (OVH). (a) three-dimensional view showing the geometrical relationship between the planning target volume (PTV) and the organ at risk (OAR), (b) two-dimensional view on the given PTV and OAR: r1 is the maximum expansion distance where the expanded PTV and OAR did not overlap when PTV was expanded by an arbitrary margin. r2 is the minimum expansion distance where the OAR was perfectly overlapped by the PTV expanded by an arbitrary margin. d is the margin size when the ratio of the overlapped volume (red area) between the expanded PTV and OAR to the volume of OAR was v, (c) OVH of given PTV and OAR.

• 

  Fig. 3. Overlap volume histogram (OVH) for each case. The expansion distance of the horizontal axis represents the size of the margin used when the planning target volume (PTV) was expanded by an arbitrary margin. The vertical axis represents the ratio of the overlapped volume between the organ at risk (OAR) and expanded PTV, which was expanded by an arbitrary margin, to the volume of OAR.

• 

  Fig. 4. Regression line and scatter diagram for the overlap volume histogram (OVH) and the center-of-mass (COM) distance between the planning target volume (PTV) and the organ at risk (OAR), and the mean dose differences between the four-dimensional dose (4D dose) and three-dimensional dose (3D dose) at the OAR. (a) Linear regression line and scatter diagram between the expansion distance and COM distance when the ratio of the OAR volume to the overlapped volume between the OAR and PTV expanded by an arbitrary margin was 50% (OVH50), (b) Linear regression line and the scatter diagram between the mean dose difference and COM distance.

• 

  Fig. 5. Regression line and scatter diagram for the mean dose difference between three-dimensional doses (3D doses) and four-dimensional doses (4D doses); expansion distance according to each volume ratio standard when the ratio of the overlapped volume between the organ at risk (OAR) and the planning target volume (PTV) expanded by an arbitrary margin to the OAR volume ranges from 10% to 50% (OVH10, OVH20, OVH30, OVH40, OVH50). (a) Standard at OVH10, (b) standard at OVH20, (c) standard at OVH30, (d) standard at OVH40, (e) standard at OVH50.

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