ESTRO 37 Abstract book

S1105

ESTRO 37

Purpose or Objective Uncertainties in deep inspiration breath hold (DIBH) radiotherapy (RT) for locally advanced non-small cell lung cancer (NSCLC) have previously been evaluated in a pilot study of 17 patients, extensively imaged in DIBH, but treated in free breathing [Ref XXX]. Based on these uncertainties adequate planning target volume (PTV) margins were established prior to initiation of a larger prospective trial, treating patients in DIBH. The purpose of the current work is to validate the findings of the previously published pilot study using patient data from our prospective DIBH trial. Material and Methods Patients were included from 2015-2017 in a prospective DIBH trial at a single institution. Respiratory coaching with visual guidance was used to ensure comfortable voluntary DIBHs for RT imaging and delivery. Patients underwent 3 consecutive CT scans in 3 consecutive visually guided DIBHs as part of imaging for RT planning. Second and third DIBH CTs were rigidly registered onto the first DIBH CT, based either on primary tumour (T), mediastinal lymph nodes (N) or carina (C). The following inter-DIBH uncertainties were evaluated: position of the T, N and C, differential motion between T & N and uncertainty of using C as a surrogate for N during daily CBCT image guidance. PTV margins were calculated based on [van Herk 2004]. Clinical parameters, DIBH related uncertainties and PTV margins from DIBH trial were compared to the pilot study. Non-parametric statistics were applied. Results DIBH CT scan sets from 61 included patients were available. Two patients were not deemed compliant for RT in DIBH (tumour position deviations between consecutive DIBHs >1cm) and were excluded from the analysis. Final analysis was performed on 59 patients. Median lung volume increase in DIBH was 54% in the DIBH trial, which did not differ significantly from the 60% increase in the pilot study. Median age in both studies was 67 years. GTV size, gender, performance status, smoking status and clinical disease stage did not differ significantly between the studies (see Table 1). The only clinical parameter differing significantly between the studies was forced expired volume in 1 second (FEV1%), which was higher in the DIBH trial group (81% vs 64%, p=0.04). 3D inter-DIBH uncertainties in DIBH trail were small: 1.9±1.6 mm (mean ± SD) for T and 1.7±1.4mm for N. Intra-fractional 3D differential motion between T and N was 0.4±1.6 mm. Table 2 presents details for cardinal axes and comparison to the pilot study data. Geometrical uncertainties, evaluated in both studies, did not differ significantly; however the resulting PTV margins differed (Table 2). The PTV based on DIBH trial was reduced by ~1mm in two directions and increased by1.5mm in left- right for N and by ~1mm in one or two directions for T.

Conclusion PTV margins for DIBH RT for locally advanced NSCLC changed ~1mm after analysing a larger cohort of patients. This validation study highlights the necessity of designing PTV margins in large and representative patient cohorts. EP-2021 Simulation system for evaluating the tracking accuracy toward RTRT using kV imaging with MV scatter T. Shiinoki 1 , T. Uehara 2 , H. Hanazawa 1 , S. Kajiki 3 , Y. Mishina 4 , K. Shibuya 1 1 Yamaguchi University, Department of Radiation Oncology, Ube, Japan 2 Yamaguchi University Hospital, Department of Radiological technology, Ube, Japan 3 Shimadzu Corporation, Research & Development Department- Medical Systems Division, Kyoto, Japan 4 Shimadzu Corporation, Global Marketing Department- Medical Systems Division, Kyoto, Japan Purpose or Objective We have archived respiratory-gated radiotherapy using the real-time tumor-tracking radiotherapy (RTRT) system; SyncTraX developed by Shimadzu for lung and liver cancer. SyncTraX consists of two color image intensifiers and x-ray tubes. Using color fluoroscopic images acquired along two directions, the 3D coordinates of a fiducial marker close to a tumor is calculated. The kV imaging of SyncTraX with MV beam delivery on linac will be able to allow for RTRT during a treatment delivery with high- dose rate. However, tracking accuracy must be evaluated