Calculation of Cranial and Caudal Margins to Compensate Tumor Motion Change in Lung SBRT Using EPID Cine

Abstract

In radiation therapy for lung cancer (LC), treatment margins are added to compensate respiratory pattern change. The aim of this study was to evaluate tumor motion change with EPID cine at the cranial and caudal sides separately. The subjects were 16 patients with stage I non-small LC treated with SBRT. Simulation processes included respiratory correlated 4-dimensional CT (4DCT) and a Real-Time Position Management system (RPM). 4DCT data were divided into 10 respiratory phases. The coordinates of the geometrical tumor center (TC) along the cranio-caudal (CC) direction was obtained on each bin. During the100% duty cycle (no gating: DC100) and the 50% duty cycle centered on exhalation (gating window 30% - 70%: DC50), the CC coordinates of TC were decided at the most cranial (craTC-4D) and the most caudal positions (cauTC-4D). The average CC coordinate of TC (mean tumor position: MTP-4D) was calculated. Tumor motion on 4DCT was ≥ 5.0 mm for all subjects. In 4 sessions, EPID cine and respiratory motion detected by RPM were consecutively acquired for one treatment beam for every case (64 sets in total) with a common temporal axis. The coordinates of TC along the CC direction were also obtained on every EPID cine frame by in-house software. In DC100 and DC50, the CC coordinates of TC were decided at the most cranial (craTC-cine) and the most caudal positions (cauTC-cine). The average CC coordinates of TC (MTP-cine) was calculated. Patient position was normalized according to MTP-4D and MTP-cine. Margins to compensate difference between craTC-4D and craTC-cine and between cauTC-4D and cauTC-cine were separately calculated using the formula described by Stroom et al. The Table shows differences in CC coordinates of craTC and cauTC between 4DCT and EPID cine and calculated margins to compensate these differences. Positive difference indicates that TC of EPID cine shifted to the caudal side compared with 4DCT. The difference of cauTC was significantly larger than that of craTC in both DC100 (p = 0.005) and DC50 (p = 0.0005). As a consequence, margins to compensate the variations were larger for the caudal side than for the cranial side. We separately assessed variations of tumor motion at the cranial and caudal sides. Patient breathing level is unstable in the inspiratory phase compared with the expiratory phase. This could be the reason of greater variations of TC positions of the caudal side.Poster Viewing Abstract 3595; TableDifferences in CC coordinates of craTC and cauTC between 4DCT and EPID cine and calculated marginsDuty cycle [%]10050craTCcauTCcraTCcauTCDifference: mean (SD) [mm]0.0 (0.1)1.2 (2.4)-0.4 (0.9)1.5 (3.0)Margin [mm]2.45.01.96.5 Open table in a new tab