ESTRO 2021 Abstract Book

S1583

ESTRO 2021

Materials and Methods A sample of 20 randomly selected stomachs was arranged in descending order from largest to the smallest size and divided into two groups of 10. The “empty group” is defined as the smaller stomach group (n = 10), while the “full group” is the larger stomach group (n = 10). Planning target volume (PTV) adds a 3 mm margin to the GTV. The prescribed PTV dose was 54 Gy / 6 fx and D 95 coverage of PTV(CTV) (the dose to 95% of the PTV volume). We compared the difference in OAR radiation dose between the two stomach volume groups. Results Gastric dose in the empty group was significantly lower than in the full group (D5 and D10). However, the OAR dose to the left kidney in the empty group was statistically significantly higher than in the full group (V12, V15, and V21). An example of adrenal SBRT in the setting of a large stomach. The intestine is close to the left adrenal ground. CTV coverage is not good due to the intestine.Blue line; adrenal CTV (2 mm margin from GTV). OAR: Liver (pink line), Stomach (purple line), Pancreas (yellow line) and Intestine (light blue line).

Color dose wash list (Dose distribution of the figure. Prescribed dose is 54 Gy / 6 fractions

Conclusion When SBRT was performed on the left adrenal gland, it was found that the smaller the stomach size, the lower the dose to the stomach. The stomach is an abdominal organ that can be artificially resized. Therefore, it is better to perform SBRT on patients with an empty stomach. Furthermore, since it is easy for the empty stomach to have the same stomach shape every time due to fasting, it is better to plan SBRT on an empty stomach than to plan with the full stomach. PO-1859 Proposal of an imaging PTV in stereotactic planning to take into account MRI geometric distortions P. Hinault 1 , P. Hinault 2 , T. Puiseux 3,4,5 , A. Sewonu 3 , I. Gardin 6,7 , P. Gouel 6,7 , H. Souchay 2 , R. Moreno 8,5 , D. Gensanne 6,9 1 University of Rouen Normandie , QuantIF-LITIS EA4108, Rouen, France; 2 General Electric, Healthcare, Buc, France; 3 Spin Up, Spin Up, Strasbourg, France; 4 Institute of Metabolic and Cardiovascular diseases of Toulouse (I2MC-INSERM), UMR1297, Toulouse, France; 5 ALARA Expertise, ALARA Expertise, Strasbourg, France; 6 University of Rouen Normandie, QuantIF-LITIS EA4108, Rouen, France; 7 Henri Becquerel Cancer Center, Nuclear Medicine Department, Rouen, France; 8 Institute of Metabolic and Cardiovascular diseases of Toulouse (I2MC-INSERM) , UMR1297, Toulouse, France; 9 Henri Becquerel Cancer Center, Radiotherapy Department, Rouen, France Purpose or Objective MR images are affected by 2 types of geometric distortions, one related to the system which increases with distance from the isocentre and the other one due to patient-induced magnetic susceptibility effects. Even after MR vendor correction of system-related distortions, both distortions exist. We propose to create an imaging PTV (iPTV) to take into account residual uncertainties. Materials and Methods An iPTV was created as PTV + a margin defined from an evaluation of total distortions on 45 intracranial lesions. All images were acquired on an Optima MR450w GE® (1.5 T) with an axial 3D T1 FSPGR sequence (TE= 4.2 ms/TR= 8.6 ms, FOV=256 mm, matrix 256x256, slice thickness=1 mm, BW=217 Hz/px) and with a 3D distortion correction. Firstly, to establish the systematic iPTV margin, residual system-related distortions were evaluated with a 3D Cartesian phantom (Spin Up, France). It consists of 4600 spherical markers distributed within a 20 cm cylinder filled with water. A 10 mm spacing separates each sphere barycentre, defined as a control point. The distortions were obtained as the position difference between the control points detected from the MR images and the theoretical control points of the phantom, with no CT registration needed. The phantom was scanned with the same sequence than the one used for patients. The phantom center was placed at the isocentre. Then, these differences were applied to patient lesions knowing the radial distance from the lesion to the isocentre. Secondly, the contribution of distortions related to susceptibility effects was assessed with the software MICE Toolkit™ (Norway). Based on these results, the systematic margin was determined and added to PTVs to create iPTVs. To