Contours (per RTOG Lung Atlas and RTOG 0813):

GTV: Gross tumor on lung window. Fuse PET to assist.

ITV: Includes movement on 4DCT

• Be sure to review all phases of breathing cycle (ie on 4DCT). Do NOT only contour on MIP.

CTV? No true CTV as there is no microscopic disease to treat with lung SBRT

PTV: iGTV + 5 mm

• Central = within 2 cm of proximal bronchial tree (PBT) and/or abutting mediastinal pleura
• Ultra-central = abutting the proximal bronchial tree

• In the US, SBRT is defined for billing purposes as ≤ 5 fractions, but more fractions may be appropriate for central/ultra-central tumors.
• In RTOG 0813, all arms above 10 Gy x 5 (10.5-12) experienced dose-limiting toxicities including Grade 5 toxicities.
• BED ≥ 100 Gy is associated with significantly better local control (Onishi et al. J Thorac Oncol 2007)
• Unsafe:
• 20-22 Gy x 3 found to be unsafe (Timmerman et al. JCO 2006)
• Can consider 18 Gy x 3 if prescribing to ITV with zero PTV margin (Shaverdian et al. BJR 2016)
• Central: 10 Gy x 5, 12-12.5 x 4 (Chang et al. IJROBP 2008)
• Ultra-central: 7.5 Gy x 8, 10 Gy x 6-7 (Chang et al. IJROBP 2014)

• Plan should be normalized such that 100% corresponds to center of mass of PTV (typically isocenter)
• Prescription isodose line should be chosen so that:
• 95% of PTV is covered by Rx dose
• 99% of PTV receives 90% of Rx dose
• High Dose Spillage:
• Any dose > 105% of Rx should occur within PTV
• D105% outside PTV < 15% of PTV volume
• Conformality ratio: V100%/VPTV ideally < 1.2, but < 1.5 acceptable.
• Not required in small tumors < 2.5 cm axial dimension
• Low Dose Spillage:
• Maximum dose should be encompassed by PTV + 2 cm
• R50%: Ratio of V50%/PTV volume ideally < 5 (PTV volume dependent)