MVCT-Detector Data Sinograms for In Vivo Quality Assurance of Helical TomoTherapy Treatments of Left Sided Breast Cancer Patients: Effect of Soft-Tissue Deformations
V Rodriguez, A Garsa, I Zoberi, E Klein, S Goddu*, Washington University, St. Louis, MOSU-E-J-221 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
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).
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.
PTV D₉₅ 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-D₉₅) 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, V₂₅ and V₃₅ 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 D₉₅ and D₅-D₉₅ and heart V₂₅.
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.
Funding Support, Disclosures, and Conflict of Interest: Research is partly funded by Tomotherapy Inc now called Accuray Inc