Evaluation of short-term gastrointestinal motion and its impact on dosimetric parameters in stereotactic body radiation therapy for pancreatic cancer

Abstract

Background: The aim of this study is to quantify the short-term motion of the gastrointestinal tract (GI-tract) and its impact on dosimetric parameters in stereotactic body radiation therapy (SBRT) for pancreatic cancer.

Methods: The analyzed patients were eleven pancreatic cancer patients treated with SBRT or proton beam therapy. To ensure a fair analysis, the simulation SBRT plan was generated on the planning CT in all patients with the dose prescription of 40 Gy in 5 fractions. The GI-tract motion (stomach, duodenum, small and large intestine) was evaluated using three CT images scanned at spontaneous expiration. After fiducial-based rigid image registration, the contours in each CT image were generated and transferred to the planning CT, then the organ motion was evaluated. Planning at risk volumes (PRV) of each GI-tract were generated by adding 5 mm margins, and the volume receiving at least 33 Gy (V₃₃) < 0.5 cm³ was evaluated as the dose constraint.

Results: The median interval between the first and last CT scans was 736 s (interquartile range, IQR:624-986). To compensate for the GI-tract motion based on the planning CT, the necessary median margin was 8.0 mm (IQR: 8.0-10.0) for the duodenum and 14.0 mm (12.0-16.0) for the small intestine. Compared to the planned V₃₃ with the worst case, the median V₃₃ in the PRV of the duodenum significantly increased from 0.20 cm³ (IQR: 0.02-0.26) to 0.33 cm³ (0.10-0.59) at Wilcoxon signed-rank test (p = 0.031).

Conclusion: The short-term motions of the GI-tract lead to high dose differences.

Keywords: 4DCT, four-dimensional computed tomography,; CTV, clinical target volume; FFF, flattening filter-free; GI-tract, gastrointestinal tract; GTV, gross tumor volume; Gastrointestinal tract; IQR, interquartile range; Intra-fractional motion; MV, mega-voltage; PRV, planning at risk volume; PTV, planning target volume; Pancreatic cancer; ROI, region of interest; SBRT; SBRT, stereotactic body radiation therapy; SD, standard deviation; Short-term organ motion; VMAT, volumetric modulated arc therapy.

Fig. 1

(A) The method of target or organ contouring and (B) the method of quantifying the minimum margin to compensate for ROI at the planning CT scan. A (a): Three different CT images (CT_p, CT_CE, and CT_4D) were prepared. (b) The CT_CE and CT_4D images were rigidly registered based on the vertebral bone for the CT_P, then translation-only registration was performed based on the fiducial marker position. (c) Target or organ contours in CT_P (ROI_P) were deformably transferred to the CT_CE or CT_4D, then transferred back to the CT_P after review and modification of deformed contours. Each region of interest (ROI) on CT images are shown as ROI_P, ROI_CE and ROI_4D. (d) Analysis was performed on the CT_p using ROI_P, ROI_CE, and ROI_4D. B (a): ROIs in CT_P, CT_4D, and CT_CE are termed as ROI_P, ROI_CE, and ROI_4D. (b) The sum of all contours is defined as the ROI_all. (c) To evaluate the margin to compensate for ROI_all, margins were determined every 2 mm from the ROI_P to the entire circumference. In this example, the minimum margin is calculated to be 6 mm. ROI: region of interest.

Fig. 2

The GI-tract motion of a case after fiducial-based image registration. The contour of the stomach (blue), duodenum (pink), small intestine (green), and large intestine (purple) are shown on each of the CT images (a-c). The contours of three CT images are simultaneously shown superimposed on the CT_P in panel (d), which indicates variations of GI-tract location. GI-tract: gastrointestinal tract. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Box plot of the necessary margins to compensate for motion in the contoured GI-tracts. Boxes show the interquartile range from 25 to 75 %ile. Median values and outliers are shown as horizontal lines within the box and black circles. GI-tract: gastrointestinal tract.