ESTRO 2020 Abstract book

S333 ESTRO 2020

recalculate can help identify patients that would benefit from ART substantially, thereby enabling efficient ART application. Anatomical changes that did lead to ART are difference in shoulder position, and contour changes in combination with a critical OAR directly adjacent to the target. The contour changes and tumor shifts typically observed after positioning on CBCT did not deteriorate the delivered dose to the extent that ART was considered necessary in the vast majority of cases. PD-0554 CTV-to-PTV margin assessment for esophageal cancer radiotherapy based on accumulated dose analysis M. Boekhoff 1 , I. Defize 1 , A. Kotte 1 , N. Takahashi 1 , J. Lagendijk 1 , S. Mook 1 , G. Meijer 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective In modern-day radiotherapy for esophageal cancer, it is common practice to rigidly align the bony anatomy on cone-beam CT before dose delivery. To account for residual set-up errors, a wide variety of CTV-to-PTV margins are currently applied in daily practice. Aim of this work is to assess the appropriate CTV-to-PTV margin in all three main directions based on analysis of the accumulated dose in an in-silico emulation of a 5-fraction treatment in a cohort of 20 esophageal cancer patients. Material and Methods Twenty esophageal cancer patients underwent six T2- weighted transversal and sagittal MRI scans (1 prior to treatment and 5 during neo-adjuvant chemoradiation (23x1.8 Gy) at weekly intervals). The clinical target volume was accurately delineated on all scans by an expert. Follow-up scans were registered to the first (reference) scan with the Elastix toolbox. The registration consisted of a two-step process: First, a rigid translation based on bony anatomy in the proximity of the tumor was performed to mimic an online CBCT setup strategy. Secondly, in order to adequately register the tumor, vertebrae and the image in general, a multi-metric registration was performed. This consisted of a kappa statistic metric on the tumor mask, a rigidity penalty metric on the vertebrae mask and a mutual information metric on all grey values in the image, respectively. An online bone-match IGRT treatment of 5 fractions with 0- mm CTV-to-PTV VMAT plans was emulated for each patient. First, the planned dose was projected on each follow-up MRI using the bony anatomy registration. Next, the deformation vector field, obtained from image registration, was used to warp the dose of each fraction back to the reference scan to mimic the accumulated dose of a plan with a 0-mm CTV-to-PTV margin (Fig 1). Subsequently, for every voxel of the CTV that was not covered by the 95% level of the prescription dose, the shortest vector to the 95% prescription dose surface was calculated. Vectors analysis was done to assess the margins in all directions which would have achieved sufficient target coverage. Results To assure full dose coverage to the CTV for all patients while maintaining high conformality, anisotropic margins are required (Fig 2). Smallest CTV-to-PTV margins sufficed in the anterior-posterior direction (6 mm) mainly due to the restricted freedom of movement in this direction due the vertebrae. Margins of 8 mm in the left-right direction were required to provide adequate dose coverage the CTV. Largest margins were demanded in the cranial-caudal direction. Here a 10-mm margin was necessary to absorb

all interfraction deformation in this direction often related to day-to-day differences in stomach filling. Conclusion This 3-dimensional volumetric dose-assessment analysis revealed that in a population of 20 esophageal cancer patients anisotropic CTV-to-PTV margins (6mm A-P, 8mm L-R and 10mm Cr-Ca) are needed to provide a minimum (accumulated) dose of 95% of the prescription dose to the CTV for all patients.

PD-0555 Dose-Dependent Splenic Volume Changes during Adrenal Adaptive Stereotactic Ablative Radiotherapy H. Chen 1 , O. Bohoudi 1 , J. Van Sornsen de Koste 1 , F. Spoelstra 1 , B. Slotman 1 , M. Palacios 1 , S. Senan 1 1 Amsterdam UMC Location VUmc, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective The spleen has important hematological and immunological functions. Structural splenic atrophy can manifest years after uniform or low dose gradient splenic