A Quantitative Assessment of Lung Tumor Motion under Abdominal Compression using 4D-CT Imaging

Stereotactic Body Radiation Therapy (SBRT) has rapidly emerged as a curative procedure for early stage non-small cell lung cancer (NSCLC). The ablative hypo-fractionated radiation doses used with SBRT requires very small tumor margins, making tumor motion management very important to reduce the risk of long term morbidity. However, in conventional free-breathing situations, lung tumors can reportedly move up to 2.5 cm requiring a large treatment margin. In this clinical study, we analyzed the efficacy of using simple abdominal compression to dampen and suppress tumor motion. Sixty-five patients treated with SBRT between 2009 and 2013 were investigated. Patients were anatomically categorized based on lung lobe location as follows: 17 had lesions appearing in the right upper lobe (RUL), 7 in right middle lobe (RML), and 18 in right lower lobe (RLL), 14 in left upper lobe (LUL) and 9 in left lower lobe (LLL). 4D-CT data sets were acquired using a GE RT16 CT scanner with a 1.25 mm slice width in conjunction with a Varian’s Real-time Position Management (RPM) system. On the GE Advantage 4D workstation, images were binned in 10 phases, T00 being the maximum inspiration phase & T50, the maximum expiration phase. The tumor volume was segmented at its centroid position using the CT-lung window. Tumor displacement was then measured from phase to phase in all three directions superior-inferior, anteriorposterior & medial-lateral. The mean tumor movement in each lobe was as follows: RUL = 3.8 ± 2.0 mm (mean ITV: 9.1 cm3), RML = 4.7 ± 2.8 mm (mean ITV: 9.2 cm3), RLL = 6.6 ± 2.6 mm (mean ITV: 12.2 cm3), LUL = 3.8 ± 2.4 mm (mean ITV: 18.5 cm3), & LLL = 4.7 ± 2.5 mm (mean ITV: 11.9 cm3). These results show tumor motion is anatomic (lobe) location dependent, specifically the magnitude of motion increases as the target nears the diaphragm. Furthermore, we found tumor amplitude to be respiratory cycle dependent as well, generally decreasing for shorter cycle duration. For RUL lesions, a strong correlation between the ranges of tumor movement with respect to respiratory cycle was noticed. Besides, for RML & RLL tumors, a linear correlation between the ratio of GTV50 volume with ipsilateral lung volume vs. tumor motion has been found [R2 > 0.9]. No tumor motion dependence on the tumor size was seen. The present study has demonstrated the usefulness of a simple abdominal compression in the management of NSCLC using radiotherapy.