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Does the Omission of a CTV for NSCLC SBRT Patients Result in Increased Risk of Distant Metastases?
A Diamant1*, I El Naqa2 , S Faria3 , J Seuntjens4 , (1) McGill University, Montreal, Quebec, (2) University of Michigan, Ann Arbor, MI, (3) McGill University Health Centre, Montreal, Quebec, (4) McGill University, Montreal, QC
Presentations
TU-C1-GePD-T-3 (Tuesday, August 1, 2017) 9:30 AM - 10:00 AM Room: Therapy ePoster Lounge
Purpose: To evaluate whether the omission of a clinical target volume (CTV) has an impact on the risk of distant metastases after stereotactic body radiation therapy (SBRT) in patients with primary stage I non-small cell lung cancer.
Methods: 98 stage I non-small cell lung cancer patients whom received 3D conformal SBRT were included in this study. The primary endpoint considered was distant metastasis (DM). Loco-regional control and radiation pneumonitis were additionally monitored as secondary endpoints. 19/98 patients developed DM after a median follow-up of 27 months. The dose to both an accumulative region and discrete differential shells up to 100 mm outward from the planning tumor volume (PTV) were considered. Four dose metrics were computed and odds ratios with respect to DM risk were estimated. 95% confidence intervals were calculated for all odds ratios. The correlation of each metric with the tumor size was computed to determine whether tumors of varying sizes could be at higher risk.
Results: Every dose metric investigated with respect to DM had an odds ratio below 0.20 and a p-value of less than 0.001 and are thus considered predictive. Notably, the mean dose received by an accumulative spherical shell outside the PTV of thickness 30 mm had an odds ratio with respect to DM of 0.03(0.01,0.23), p-value of ≤0.0001. The correlation coefficient between this dose metric and the PTV volume was found to be 0.64 (0.51, 0.75), p-value of ≤0.0001.
Conclusion: We identified that the dose received by the region directly outside of the PTV has a significant impact on the risk of DM. We hypothesize that inadequate dose coverage from the omission of a CTV results in microscopic disease not receiving a tumoricidal dose. Due to the correlation with PTV volume, caution should be taken particularly while treating small lesions within the lung.
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