SU-E-J-221: MVCT-Detector Data Sinograms for In Vivo Quality Assurance of Helical TomoTherapy Treatments of Left Sided Breast Cancer Patients: Effect of Soft-Tissue Deformations

Abstract

Purpose: Objectives are to compute delivered doses in megavoltage CT (MVCT) datasets using the MVCT‐detector‐data sinograms (MVCT‐DDS), collected during breast‐cancer treatments, and to study the impact of soft‐tissue‐deformations (SD) on dose‐volume‐histograms (DVHs) of the PTVs and organs at risk (OARs). Methods: Tomotherapy Delivery Verification (DV) software was used to reconstruct delivered fluence using pretreatment MVCTs and MVCT‐DDS for 4 patients (20 fractions) treated in our clinic. Delivered fluence was back projected onto the MVCT datasets to compute dose distributions to the patients anatomy at the time of delivery. A commercial deformation algorithm was used to deform planning CT‐datasets to match the pre‐treatment anatomy. The same deformation matrix was applied to plan structures (PS) for true volumes. Both image fusions and deformed structures (DS) were reviewed by a radiation oncologist and a medical physicist. DS were manually edited when necessary. Only fractions with the entire treatment volume contained in the MVCT scans were used. CT‐datasets, planned and reconstructed‐doses (RD), PS and DS were exported to CERR for computing dose‐volume‐indices (DVI). DVI of the planned and reconstructed doses, of both non‐deformed (RD‐ND) and deformed (RD‐D) structures, were averaged and compared. Dose deformations were ignored in this study. Results: PTV D9 5 was 49.1±0.6Gy, 48.0±0.8Gy, 45.8±1.0Gy for the plan, RD‐ND and RD‐D volumes. PTV dose heterogeneity (D5–D9 5) increased by 3.0Gy for deformed targets. Mean dose (MD) of both lungs reduced slightly for both non‐deformed and deformed volumes, however, they were not significant. Heart MD, V2 5 and V3 5 were also non‐significantly reduced. The Sign tests revealed that absolute relative differences between planned and delivered dose distributions were significantly greater than 5% (p<0.05) for D9 5 and D5–D9 5 and heart V2 5 . Conclusion: This study showed that its feasible to reconstruct delivered fraction‐doses in pretreatment MVCT datasets utilizing MVCT‐DDS. Soft‐tissue deformations are important for accurate dose volume evaluations. Research is partly funded by Tomotherapy Inc now called Accuray Inc