Purpose: In this study, local control for 93 prior lung SBRT patients was evaluated in regard to dosimetric uncertainties related to the lung heterogeneity correction.
Methods: Under a retrospective IRB protocol, 93 patients who received SBRT for non-small-cell lung cancer from 2007-2014 with treatment plans originally calculated with AAA (n=86) or PBC (n=7) were retrospectively recalculated using Acuros. A subset (n=22) was also recalculated with Monte Carlo for comparison. Dosimetric uncertainty was quantified for PTV mean dose, D95%[Gy], D1%[Gy], mean lung dose, lung V12Gy[cc], biologically effective dose (BED), and tumor control probability (TCP). Correlation with local control was analyzed.
Results: 8 of 93 patients experienced local failures within four years. For local failures and controls, BED from Acuros was 107.2±14.8Gy and 127.7±27.0Gy, respectively, for PTV mean dose (p=0.03) and 119.2±10.8Gy and 151.8±31.5Gy, respectively, for D1%[Gy] (p=0.005). After recalculation with Acuros, the original BED and TCP decreased by 5.2±6.2% and 4.2±4.9%, respectively, with 10% of all plans having their PTV mean dose decreasing by >12.2% and D95%[Gy] by >26.7%. For one particular failure, BED decreased from 109.9Gy to 72.5Gy and TCP from 0.55 to 0.26. Plan parameters that were correlated with dosimetric uncertainties in a multivariate analysis included beam energy (ΔD95%[Gy] of -5.8%, -7.2%, and -11.5% for 6X, 10X, and 15X, respectively), lung volume (ΔD95%[Gy] = -2.9% per 1000cc), PTV mean density (ΔD95%[Gy] = 1.9% per 100HU), and PTV size (ΔD95%[Gy] = -0.82% per 25cc). Consistent with previous studies, Acuros had better agreement with Monte Carlo (ΔD95%[Gy] = -1.7%±1.4%) than did AAA (ΔD95%[Gy] = -9.7%±7.8%).
Conclusion: For lung SBRT, dosimetric uncertainty from the heterogeneity correction can have a substantial effect on BED, potentially leading to increased probability of local failure. Future work will include expanding the patient cohort for further statistical analysis of local control.
Funding Support, Disclosures, and Conflict of Interest: This work was partially funded by a grant from Radialogica. Dr. Justus Adamson reports ownership of Clearsight RT LLC, which is unrelated to this study.