SU‐GG‐T‐482: Effects of Normal Lung Delineation Using PTV Vs. GTV On NTCP and Vx for Lung Cancer Cases Planned with and Without Tissue Inhomogeneity Correction

Abstract

Purpose: There is no clear consensus as to how to define normal lung volume; clinicians subtract either PTV or GTV from total lung volume. This study quantifies the dosimetric and biological differences between these definitions for radiotherapy planning with and without inhomogeneity corrections. Method and Materials: In this study, ten lung cancer patient cases with prescription doses (PD) 50–70Gy (average 62Gy) were evaluated. Dosimetric plans were generated using XIO‐CMS software. Complex (4–6 fields) 3D conformal radiation therapy plans with and without tissue heterogeneity corrections were compared. NTCPs were computed for three scenarios; respiratory pneumonitis (RP) endpoint of SWOG grade ⩾2RP (TD50=30.8Gy, n=0.99, m=0.37; NTCP1), SWOG ⩾1RP (TD50=28Gy, n=0.87, m=0.18; NTCP2), and radiographic & symptomatic pneumonitis (TD50=21.9Gy, n=0.8, m=0.37; NTCP3). Normal lung for NTCP was defined in two ways: [(left lung + right lung) − PTV], and [(left lung + right lung) − GTV]. Volume (Vx) of normal lung receiving 5Gy (V5), 13Gy (V13), 20Gy (V20) and 30Gy (V30) were also investigated. Results: For inhomogeneity non‐corrected plans: average PTV dose coverage was 97% (92.1–101.7%) of PD; with −PTV definition, average V5=41.3% (24.3–70.2%), V13=26.4% (15.9–36.8%), V20=22.2% (13.1–32.4%), V30=18.3% (10.9–27.8%), NTCP1=6.5% (2–16.5%), NTCP2=1.3% (0.01–8.6%), NTCP3=23.3% (6.7–51.2%); and with −GTV definition, average V5=43.2% (25.6–71.3%), V13=28.7% (17.9–39.2%), V20=24.7% (15.1–34.9%), V30=21% (13–30.5%), NTCP1=8.8% (2.6–20.7%), NTCP2=3.5% (0.02–16.3%), NTCP3=31% (9.7–60.8%). For inhomogeneity corrected plans: average PTV dose coverage was 99.6% (93.1–107.6%); with −PTV definition, average V5=43.7% (25.3–74%), V13=27.9% (17–42.7%), V20=23.2% (14.7–33.7%) and V30=19% (11.2–28.5%), NTCP1=7.8% (2.2–19.9%), NTCP2=2.5% (0.01–14.1%), NTCP3=27.9% (7.4–58.2%); and with −GTV definition, average V5=45.5% (26.5–75%), V13=30.2% (19.5–44.5%), V20=25.7% (16.1–36.1%), V30=21.7% (13.6–31.1%), NTCP1=10.7% (2.8–24.9%), NTCP2=6% (0.03–24.9%), NTCP3=36.5% (10.7–67.8%). Discussion and Conclusion: Results revealed that for heterogeneity non‐corrected plans, relative increase (with respect to −PTV definition) of Vxs were 4.7%, 8.9%, 11.3%, 14.7%, and NTCPs were 32.6%, 193.4%, 32.8%; and similar increases were observed for heterogeneity corrected plans.