146. Comparing group based and patient specific cone-beam CT intensity correction methods for VMAT dose recalculations

Abstract

Purpose The aim of this work was to compare two approaches of cone-beam computed tomography (CBCT) image intensity-correction for dose recalculation of volumetric arc radiotherapy (VMAT) plans. Methods and materials CBCT-based dose calculation accuracy was assessed for pelvic, lung, and head and neck (H&N) treatment sites. The dose distributions of 10 patients for each body regions were re-calculated by RayStation (RS) and Eclipse treatment planning systems (TPS). In particular CBCT RS were calibrated by patient specific density assignment method supplied by RS TPS, while CBCT ROI , obtained applying ROI-based lookup tables for each body region were used in Eclipse TPS. Dose Volume Histogram (DVH) statistics and 3D γ -analysis (3%, 3 mm) of the dose maps and dose-volume statistics were determined to compare the CBCT dose distributions with those of the planning CTs in absence of morphological changes. Results The results indicate a comparable performance of both intensity correction techniques in the scope of VMAT. The differences in the investigated DVH parameters with respect to the pCT were mostly below 2% for both methods, however the γ -index analysis, being more sensitive to local dose changes, pointed out slight inaccuracies in the CBCT ROI dose distributions with respect to CBCTRS. In particular γ -index pass-rates were found increased for CBCTRS with respect to CBCTROI in pelvic (98% against 95%) in lung (97% against 93%) and in H&N (99% against 95%). Conclusion Both the CBCT intensity correction approaches are attractive options for CBCT dose recalculation and in vivo dose reconstructions [1] , [2] . No added value was found by using a patient-specific HU table, showing similar results as the group based HU tables in terms of DVH indices while the γ -analysis showed better results for CBCTRS.