Session Item

Published organ motion data have been collected from measurements of a limited number of points within the organ, the centroid, or the edge of the organ. We proposed a novel volumetric method for evaluating deformed organ motion using deformable image registration, is called "vector volume histogram (VVH)". In this study, we investigated the inter-fraction reproducibility of the tumor position in expiratory breath-hold (BH) using VVH method.

Subjects comprising 14 patients with lung cancer who were treated with lung stereotactic body radiotherapy (SBRT) under expiratory BH conditions were monitored by the Abches system. The first computed tomography (CT) scan at the expiratory BH was used for treatment planning. The remaining three CT scans were acquired successively without repositioning to simulate inter-BH expiratory BH reproducibility. Gross tumor volume (GTV) was delineated by a physician. The hybrid deformable registration algorithm of RayStation treatment planning system (TPS) was used to deform from the three expiratory BH-CTs to the treatment planning expiratory BH-CT, and calculated the deformation vector field (DVF). DVFs consist of DVFLR (left-right), DVFAP (anterior-posterior), and DVFCC (cranio-caudal). Vectors on each direction are stored in separate arrays. Exported DVF data was converted as the Digital Imaging and Communications in Medicine-Radiation Therapy (DICOM RT). The modified DICOM-RT file was then imported into the RayStation TPS. The VVH calculation approach is a similar concept to that of the dose volume histogram (DVH). To evaluate the inter-fraction BH reproducibility of the tumor positions, we measured the largest motion extent within the organ of LR, AP and CC dimensions and a 3D vector using the VVH method.

The average and standard deviations of the inter-fractional BH reproducibility of the tumor position in the LR, AP, and CC directions, and the 3D vector were 1.7 ± 0.5 mm (range of 0.8–2.7 mm), 2.0 ± 0.7 mm (range of 0.7–4.0 mm), 2.1 ± 0.7 mm (range of 1.0–3.7 mm), and 2.7 ± 0.7 mm (range of 1.5–4.8 mm), respectively. Ten patients exhibited inter-fractional displacements of the lung tumor >3 mm in the 3D vector. No displacement >5 mm was observed in any direction for all patients.

Our study indicated that the inter-fractional BH reproducibility of the tumor position was small for lung-cancer patients, using the Abches system. The VVH method confirmed that the expiratory BH status using the Abches system was sufficiently reproducible.