EP-1884 Commissioning and clinical validation of FRED: Monte Carlo on GPU for proton beam therapy

Purpose or ObjectivePrecise definition of the target volume is one of the crucial factors in the management of Non-Small Cell Lung Cancer (NSCLC) with Stereotatic body radiotherapy (SBRT).It is widely recognized that the motion pattern of lung tumors varies greatly among patients.Therefore tumor motion should be assessed with patient specific image acquisition, to ensure adequate tumor coverage and minimizing dose to the Organs at Risk.The "gold standard" approach for defining an Internal Target Volume (ITV) is to use gross tumor volume (GTV) delineated over several phases in course of one respiratory cycle.It is a time consuming method and different Institutions have adopted several alternative techniques which compress all temporal information into one CT image set, to optimize work flow efficiency.The purpose of this study is to evaluate alternative target segmentation strategies with respect to the gold standard. Material and MethodsTwenty lung cancer SBRT patients, treated on a linac with 4 mm width multileaf-collimator (MLC), were analyzed retrospectively.From the acquisition of a low-pitch helical CT scan (Untag CT) combined with a respiratory monitor system signal, four-dimensional CT (4D-CT) scans were reconstructed for each patient.GTV was delineated based on 4 single respiratory phases and on Maximum Intensity Projection (MaxIP), Minimun Intensity Projection (MinIP), Mean Intensity Projection (MeanIP) CTs and Untag CTs.Comparison was performed on Dice similarity coefficient (DSC).The relative position between the delineated target was evaluated calculating the centroid distance between volumes. ResultsGTVs derived from different MaxIP and MeanIP image sets were at least comparable with the single phase ITV delineation, with DSC values varying from 0.551 to 0.936 for Untag, from 0.331 to 0.877 for MaxIP, from 0.354 to 0.877 for MeanIP.MinIP GTV delineation was less comparable to the 4D-CT ITV with DSC range between 0.07 and 0.8.The differences in relative position of target volume localization were small and in all cases < 3mm.The mean differences ± SD of the centroid distances for ITV and GTV_Untag, GTV_maxIP, GTV_meanIP, GTV_minIP were 0.18±0.16cm, 0.18±0.16cm, 0.23±0.19cm, 0.26±0.19cmrespectively. Conclusion