Conformal therapy for pancreatic cancer: variation of organ position due to gastrointestinal distention--implications for treatment planning.

Purpose: To quantify nonrespiratory organ motion in the pancreatic region and its effect on clinical target volume.
Materials and Methods: Three-dimensional translations of the geometric centers of the volumes of interest--pancreatic head, body, and tail; left and right kidney; and the superior mesenteric artery--were measured in 20 patients by analyzing three spiral computed tomographic (CT) protocols performed at static exhalation and representing differential gastrointestinal distention. Wilcoxon test for paired differences was applied to determine statistical significance (P <.05). Spearman rank correlation coefficients were calculated between combinations of statistically significant translations. With the assumption that the organ positions were represented by a three-dimensional Gaussian distribution that occurs during treatment, clinical target volume expansions were calculated to account for organ motion and a typical setup error.
Results: Significant translations of the volume of interest were observed. The most mobile parts of the target organs were the pancreatic tail (P =.001) and the superior mesenteric artery (P =.01). Larger variations from the mean in the planning CT protocol in which negative contrast material was used usually resulted in a slightly larger clinical target volume expansion.
Conclusion: Our data may provide a basis for further studies of organ motion and ways of modifying treatment margins.