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Virtual Bronchoscopy-Guided IMRT Planning for Mapping and Avoiding Radiation Injury to the Airway Tree in Lung SAbR

A Sawant

A Sawant*, A Modiri , R Bland , Y Yan , C Ahn , R Timmerman , University of Texas SouthWestern Medical Center, Dallas, TX


SU-C-BRA-7 (Sunday, July 12, 2015) 1:00 PM - 1:55 PM Room: Ballroom A

Purpose: Post-treatment radiation injury to central and peripheral airways is a potentially important, yet under-investigated determinant of toxicity in lung stereotactic ablative radiotherapy (SAbR). We integrate virtual bronchoscopy technology into the radiotherapy planning process to spatially map and quantify the radiosensitivity of bronchial segments, and propose novel IMRT planning that limits airway dose through non-isotropic intermediate- and low-dose spillage.

Methods: Pre- and ~8.5 months post-SAbR diagnostic-quality CT scans were retrospectively collected from six NSCLC patients (50-60Gy in 3-5 fractions). From each scan, ~5 branching levels of the bronchial tree were segmented using LungPoint, a virtual bronchoscopic navigation system. The pre-SAbR CT and the segmented bronchial tree were imported into the Eclipse treatment planning system and deformably registered to the planning CT. The five-fraction equivalent dose from the clinically-delivered plan was calculated for each segment using the Universal Survival Curve model. The pre- and post-SAbR CTs were used to evaluate radiation-induced segmental collapse. Two of six patients exhibited significant segmental collapse with associated atelectasis and fibrosis, and were re-planned using IMRT.

Results: Multivariate stepwise logistic regression over six patients (81 segments) showed that D0.01cc (minimum point dose within the 0.01cc receiving highest dose) was a significant independent factor associated with collapse (odds-ratio=1.17, p=0.010). The D0.01cc threshold for collapse was 57Gy, above which, collapse rate was 45%. In the two patients exhibiting segmental collapse, 22 out of 32 segments showed D0.01cc >57Gy. IMRT re-planning reduced D0.01cc below 57Gy in 15 of the 22 segments (68%) while simultaneously achieving the original clinical plan objectives for PTV coverage and OAR-sparing.

Conclusion: Our results indicate that the administration of lung SAbR can result in significant injury to bronchial segments, potentially impairing post-SAbR lung function. To our knowledge, this is the first investigation of functional avoidance based on mapping and minimizing dose to individual bronchial segments.

Funding Support, Disclosures, and Conflict of Interest: The presenting author receives research funding from Varian Medical Systems, Elekta, and VisionRT.

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