OC-0167: Time-evolution of the regional dose-response of lung following radiotherapy for NSCLC
Radiotherapy and Oncology
Purpose/Objective: Stereotactic body radiation therapy (SBRT) has emerged as an effective treatment option for hepatic metastases. We evaluated the intrafractional movement of liver metastases during SBRT using helical Tomotherapy. Materials and Methods: From april 2011 onwards we treated twelve patients with hepatic metastases with SBRT. A dose of 60 Gy in 3-6 fractions was administered. Before the start of RT, we implanted three gold markers around the metastasis for setup and for movement
... and for movement visualization due to respiration. We used the Body Pro-Lok tm System with an abdominal compression plate combined with a vacuum cushion for immobilization. The compression plate was placed right under the xyphoid to reduce liver movement. A respiratory-correlated 4DCT was made to determine the maximum extreme positions of the markers during respiration. In general a radial margin of 6 mm and a cranial-caudal (CC) margin of 10 mm from GTV to PTV was used. The difference between end-inhalation (EI) and end-exhalation (EE) was used to adapt these margins. The tumor was delineated on a average CT registered with a contrast CT and a MRI. Before the delivery of each fraction, a pre-treatment MVCT was performed and matched with the planning CT. Since this MVCT is a slow scan, respiratory movements result in a blurred representation of the markers. We verified that the center of this blurred signal corresponds with the center of mass of the marker. After each treatment, all patients underwent a post-treatment MVCT. The gold markers shifts between the pre-and post-treatment MVCT were determined to quantify the intrafractional movements. Results: We evaluated 42 MVCT's from twelve patients. The median intrafractional marker movement for all patients was 0.6 mm (range, 0 -2.9 mm), 1.3 mm (range, 0.1-5.8 mm) and 0.7 mm (range, 0 -4.1 mm) in lateral, CC and AP direction, respectively. In lateral direction there was one patient and in AP direction two patients who had shifts larger than 2mm. In CC direction six patients had a shift below 2mm. One patient had a CC movement of more than 4mm in each fraction. There was no correlation between the differences in intrafractional movements of the sequential fractions. All shifts where within the administered margins Conclusions: Using Tomotherapy the median intrafractional movements in most patients was limited. In two cases a larger movement was observed. In future studies we will analyze whether the administered margins can be reduced in individual cases based on this evaluation.