CyberKnife SABR for Lung Tumors

All the patients were treated with CyberKnife® G4 (Accuray, Sunnyvale, CA, USA). For respiratory management, patients were administered SABR using either the fiducial-less, direct tumor tracking system (XSight Lung Tracking System®, Accuray, Sunnyvale, CA, USA) or a tracking system involving skeletal structures (XSight Spine Tracking System®, Accuray, Sunnyvale, CA, USA) without implanting fiducials.
Radiotherapy was performed as per our previous report [12 (link)]. Four-dimensional computed tomography (CT), with a slice thickness of 1 mm, was performed. Primary lesions were delineated as GTV in the lung window CT setting. On the CT images, the solid tumor components (GTV core) of the GTV (window level, –200 Hounsfield units; window width, 1 Hounsfield unit), were contoured (Fig. 1). The internal target volume (ITV) was calculated from the GTVs during each respiratory phase. Finally, PTV was set as the ITV, plus a 3–8-mm safety margin. The PTV margin depends on the tracking system used, and when we use XSight Spine Tracking System®, the distance from the vertebrae.Fig. 1

CT images of contoured GTV (A) and GTV core (the solid tumor components of the GTV) B. GTV was delineated in the lung window CT setting, while GTV core were contoured (window level [WL], –200 Hounsfield units; window width [WW], 1 Hounsfield unit). GTV, gross tumor volume

In cases with GTV cores, a dose prescription was defined as 99% of the GTV core. Overall, 93 peripheral lung tumors were prescribed a dose of 52 Gy in 4 fractions. Additionally, the centrally located lung tumors received 60 Gy (13 tumors) in 10 fractions, 70 Gy (1 tumor) in 10 fractions, 60 Gy (1 tumor) in 8 fractions, and 56 Gy (1 tumor) in 8 fractions. In cases where the tumor did not have a solid component and had a ground-glass shadow, the dose prescription was defined as 95% of the PTV, based on which 3 tumors were prescribed 42 Gy in 4 fractions. This translated to a biologically effective dose (BED) 10 of 86.1 Gy, which is approximately equivalent to the prescription dose of 48 Gy to the isocenter. These values were reported by the Japan Clinical Oncology Group (JCOG) 0403 trial for primary lung cancer [13 (link)]. We used the Monte Carlo (Multiplan®, Accuray, Sunnyvale, CA, USA) dose calculation algorithm to determine the final doses.