ESTRO 2021 Abstract Book

S1588

ESTRO 2021

Figure 1 and gamma passing rate in the junction region was found as 86.23%. Additionally, the calculated and measured dose profiles in anterior-posterior direction agreed well and there were no cold or hot spots. Additionally, γ mean (0.63) and γ 1% (2.47) were in the tolerance limits.

Conclusion EPID based in-vivo dosimetry system confirmed the safety of i3DCRT CSI delivery with no cold or hot spot point in the junction region for simulated treatment plan in anthropomorphic Alderson Rando phantom.

PO-1864 Dosimetric impact of the segmentation through HU threshold in head and neck treatments S. Pena Vaquero 1 , L. Cardoso Rubio 2 , L. Gómez Heras 2 , Á. del Castillo Belmonte 1 , A.E. Hurtado Romero 1 , M.M. Agulla Otero 1 , C. Andrés Rodríguez 1 , D. Miguel Pérez 1 , I. Conles Picos 1 , D. Alonso Hernández 1 , R. Torres Cabrera 1 1 Hospital Clínico Universitario, Servicio de Radiofísica y Protección Radiológica, Valladolid, Spain; 2 Hospital Clínico Universitario, Servicio de Oncología Radioterápica, Valladolid, Spain Purpose or Objective Currently, the guidelines for correct contouring in radiotherapy do not include the external contour. Planning systems include tools for their self-segmentation, although its operation may go unnoticed. In the case of patients with head and neck (H&N) pathology, the external boundaries are diffuse, due to the use of thermoplastics for their immobilization, generating uncertainties about whether the created structure is the adequate one or not. The aim of this work is to see how the choice of a lower Hounsfield Unit (HU) threshold in the segmentation of the body in H&N treatments affects the applied dosimetry. Materials and Methods Six patients with head and neck pathology treated with volumetric modulated arc therapy were selected retrospectively. The total number of planning target volumes (PTV) was 16, distributed in the six patients with three prescribed dose levels: 70, 60 and 54 Gy. The treatment planning system used was Eclipse v15.6 and the algorithm AcurosXB 15.6.04. For each patient, the structure set is duplicated and then modified by varying the external contour through the application of a lower threshold of -950 HU in the structure excluding the support of the immobilizer. The absorbed dose is calculated using the same plan for both sets of structures, the one containing a lower threshold and the automatically generated one. The minimum absorbed dose and the dose received by the 98% of the volume (D98%) are analyzed. Results Figure 1 shows the ratio of the minimum absorbed dose to the prescribed one for each PTV reflecting an average of 72.87 ± 16.03 % for the automatic contour and 86.26 ± 3.84 % for the modified ones.