ESTRO 2021 Abstract Book

S331

ESTRO 2021

between CBCT and CT were analyzed using the intraclass correlation coefficient (ICC), Dice similarity coefficient (DSC), Pearson correlation and Bland-Altman analysis. A linear regression model was established using the CBCT and CT at 0th fraction and validated by the CBCT and CT at 15th and 25th fraction. Results From October 2019 to September 2020, 56 patients were enrolled. SMA on CBCT demonstrated excellent interobserver reliability. The ICC values were 0.98(0.93; 0.99) and the DSC were 0.92,0.90, 0.97 respectively. SMA on CBCT scan strongly correlated with that on CT (r = 0.97, p<0.001). Bland-Altman analysis revealed that SMA on CBCT was higher than that on CT and 3% of the points were beyond the 95% limits of agreement (figure 1). In part I, we established a linear regression model using SMA on CBCT to predict them on CT, S_CT=0.949*S_CBCT-0.4(r=0.97, p<0.001). In part II, CBCT at the 15th and 25th fraction were used to predict the SMA on CT. The results showed that the predicted SMA using CBCT were similar to the actual value (at 15th r=0.998, p<0.01, at 25th r=0.985, p<0.01). In the second week of radiotherapy, SMA loss of ≥5% was observed in 17 patients (70.8%), while weight loss of ≥5% was noted in only 4 patients (16.6%) (Table 1).

Conclusion CBCT exhibit good reliability and agreement with CT for assessing the size and shape of SMA. Monitoring SMM changes by CBCT is technically feasible in HNC patients during radiotherapy.

PH-0430 The benefits of pre-procedure planning in fiducial marker insertion for CyberKnife treatment

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PH-0431 evaluation of microscopic tumor extension in localized stage non–small-cell lung cancer M. Schmitt 1 , L. Aussenac 2 , J. Seitlinger 3 , V. Lindner 4 , G. Noël 5 , D. Antoni 5 1 Institut de cancérologie Strasbourg Europe , Radiotherapy department , Strasbourg, France; 2 Hautepierre University Hospital, Pathology department , Strasbourg, France; 3 Lorraine Heart and Vessel Institute, Thoracic Surgery Departement, Vandœuvre-lès-Nancy, France; 4 Hautepierre University Hospital, Pathology Department, Strasbourg, France; 5 Institut de cancérologie Strasbourg Europe, Radiotherapy department , Strasbourg, France Purpose or Objective Stereotactic radiotherapy for localized stage Non Small Cell Lung Carcinoma (NSCLC) is an alternative indication for patients who are inoperable or refuse surgery. Local control, specific and overall survival rates are similar to surgery. A study showed that the microscopic tumor extension (ME) of NSCLC varied according to