Statistical shape model-based planning organ-at-risk volume: application to pancreatic cancer patients

Purpose. To introduce the concept of statistical shape model (SSM)-based planning organ-at-risk volume (sPRV) for pancreatic cancer patients. Methods. A total of 120 pancreatic cancer patients were enrolled in this study. After correcting inter-patient variations in the centroid position of the planning target volume (PTV), four different SSMs were constructed by registering a deformable template model to an individual model for the stomach and duodenum. The sPRV, which focused on the following different components of the inter-patient variations, was then created: Scenario A: shape, rotational angle, volume, and centroid position; Scenario B: shape, rotational angle, and volume; Scenario C: shape and rotational angle; and Scenario D: shape. The conventional PRV (cPRV) was created by adding an isotropic margin R (3-15 mm) to the mean shape model. The corresponding sPRV was created from the SSM until the volume difference between the cPRV and sPRV was less than 1%. Thereafter, we computed the overlapping volume between the PTV and cPRV (OLc) or sPRV (OLs) in each patient. OLs being larger than OLc implies that the local shape variations in the corresponding OAR close to the PTV were large. Therefore, OLs/OLc was calculated in each patient for each R-value, and the median value of OLs/OLc was regarded as a surrogate for plan quality for each R-value. Results. For R = 3 and 5 mm, OLs/OLc exceeded 1 for the stomach and duodenum in all scenarios, with a maximum OLs/OLc of 1.21. This indicates that smaller isotropic margins did not sufficiently account for the local shape changes close to the PTV. Conclusions. Our results indicated that, in contrast to conventional PRV, SSM-based PRVs, which account for local shape changes, would result in better dose sparing for the stomach and duodenum in pancreatic cancer patients.