ESTRO 2021 Abstract Book

S1296

ESTRO 2021

PO-1571 Derivation of anisotropic margin expansion using displacement surface map for bladder radiotherapy H.Q. Tan 1 , E.P.P. Pang 1 , C.W.Y. Koh 2 , K.S. Lew 1 , K.W. Ang 1 , T.W.K. Tan 1 , J.K.L. Tuan 1 , J.C.L. Lee 1 , S.Y. Park 1 1 National Cancer Centre Singapore, Division of Radiation Oncology, Singapore, Singapore; 2 National Cancer Centre Singapore, Division Of Radiation Oncology, Singapore, Singapore Purpose or Objective Various strategies such as online selection of plan-of-the-day, emptying of bladder, daily three-dimensional (3D) image guidance generally involve the use of isotropic margins (1-1.5cm) around the clinical target volume for whole bladder irradiation. This case study suggests a novel approach towards the derivation of the required planning margin through a review of the bladder displacement surface map. Materials and Methods Planning CT Images were registered rigidly at the bladder neck to the weekly CBCT images prior to performing the deformable image registration (DIR). The DIR was reviewed by physicians. The deformable vector field from the DIR were extracted along the contour of the bladder, which can be interpreted as the displacement vectors of the bladder in each fraction. A displacement surface map (DSM) was calculated by unwrapping the x, y and z displacement value along the bladder closed surface to form a 2D map. The ant-post, sup-inf and left-right directions were defined on the DSM and the displacement in each direction is obtained by calculating the average along the defined region in the DSM. This method is used to quantify the displacement at the dome region and the variations in the bladder surfaces due to residual bladder filling were presented. Results The schematic of the algorithm is shown in figure 1. The inter fraction variation of the displacement in all six directions are shown in Figure 2. The displacement shows the largest variation in the superior and anterior position consistently in both patients. The average displacement in the bladder dome over all fractions are 2.51mm (0.55 to 7.07 mm) and 1.48 mm (-6.43 to 5.52 mm) respectively in patient 1 and

2.