Faculty Bibliography

Purpose: To investigate the effect of patient positioning in Volumetric Modulated Arc Therapy (VMAT) for Total Body Irradiation (TBI) given the use of multiple isocenters, by simulating offsets in patient positioning and evaluating changes to planned dose distributions.
Method(s): VMAT treatment plans for seven TBI patients treated as part of a prospective stage II clinical trial were evaluated. Plan uncertainties were calculated by introducing 5mm and 10mm translational shifts to the plans' isocenters in the lateral (x), vertical (y), and longitudinal (z) directions. Dose distributions were then re-calculated in the treatment planning system (Eclipse), in order to evaluate dosimetric robustness to one global imaging shift at treatment. Differences in target volume (PTV) coverage and doses to organs at risk were evaluated based on four parameters: lung mean dose, PTV-V100%, PTV-D98%, and kidney mean doses.
Result(s): Lung mean dose increased an average of 8.2cGy, 4.4cGy, and 3.3cGy when shifted 5mm in the x, y, z directions (respectively) across seven patients; 33.2CGy, 18.5cGy, 18.3cGy for 10mm shifts in x, y, z. Target coverage V100% decreased an average of 0.3%, 0.03%, 0.1% for 5mm shifts, and 1.1%, 0.8%, 0.4% for 10mm shifts in x, y, z. D98% decreased 0.9%, 0.3%, 0.3% when shifted 5mm; 3.5%, 2.1%, 1.0% when shifted 10mm in x, y, z. Mean dose to the left kidney increased 6.6cGy, 9.7cGy, 2.8cGy for 5mm, and 28.1cGy, 32.7cGy, 18.0cGy for 10mm shifts in x, y, z. Right kidney mean dose increased 11.9cGy, 8.9cGy, 3.1cGy for 5mm, and 36.5, 30.5, 19.8cGy for 10mm.
Conclusion(s): Though small in relation to total dose, the largest increase in mean lung dose and decrease in coverage was seen with lateral shifts as compared to vertical or longitudinal shifts. These results support the use of an approach with preferential alignment to the chest region (lung-sparing), as long as residual imaging alignment outside the chest is kept below 10mm. Jose Teruel has received honorarium from Varian Medical Systems

The aim of this work was to develop a novel hybrid 3D hyperpolarized (HP) gas tagging MRI (t-MRI) technique and to evaluate it for lung respiratory motion measurement with comparison to deformable image registrations (DIR) methods. Three healthy subjects underwent a hybrid MRI which combines 3D HP gas t-MRI with a low resolution (Low-R, 4.5 mm isotropic voxels) 3D proton MRI (p-MRI), plus a high resolution (High-R, 2.5 mm isotropic voxels) 3D p-MRI, during breath-holds at the end-of-inhalation (EOI) and the end-of-exhalation (EOE). Displacement vector field (DVF) of the lung motion was determined from the t-MRI images by tracking tagging grids and from the High-R p-MRI using three DIR methods (B-spline based method implemented by Velocity, Free Form Deformation by MIM, and B-spline by an open source software Elastix: denoted as A, B, and C, respectively), labeled as tDVF and dDVF, respectively. The tDVF from the HP gas t-MRI was used as ground-truth reference to evaluate performance of the three DIR methods. Differences in both magnitude and angle between the tDVF and dDVFs were analyzed. The mean lung motion of the three subjects was 37.3 mm, 8.9 mm and 12.9 mm, respectively. Relatively large discrepancies were observed between the tDVF and the dDVFs as compared to previously reported DIR errors. The mean  ±  standard deviation (SD) DVF magnitude difference was 8.3  ±  5.6 mm, 9.2  ±  4.5 mm, and 9.3  ±  6.1 mm, and the mean  ±  SD DVF angular difference was 29.1  ±  12.1°, 50.1  ±  28.6°, and 39.0  ±  6.3°, for the DIR Methods A, B, and C, respectively. These preliminary results showed that the hybrid HP gas t-MRI technique revealed different lung motion patterns as compared to the DIR methods. It may provide unique perspectives in developing and evaluating DIR of the lungs. Novelty and Significance We designed a MRI protocol that includes a novel hybrid MRI technique (3D HP gas t-MRI with a low resolution 3D p-MRI) plus a high resolution 3D p-MRI. We tested the novel hybrid MRI technique on three healthy subjects for measuring regional lung respiratory motion with comparison to deformable image registrations (DIR) methods, and observed relatively large discrepancies in lung motion between HP gas t-MRI and DIR methods.

PURPOSE/OBJECTIVE:CT ventilation imaging (CTVI) is being used to achieve functional avoidance lung cancer radiation therapy in three clinical trials (NCT02528942, NCT02308709, NCT02843568). To address the need for common CTVI validation tools, we have built the Ventilation And Medical Pulmonary Image Registration Evaluation (VAMPIRE) Dataset, and present the results of the first VAMPIRE Challenge to compare relative ventilation distributions between different CTVI algorithms and other established ventilation imaging modalities. METHODS: RESULTS: CONCLUSIONS:The VAMPIRE Challenge results demonstrate that the cross-modality correlation between CTVIs and the RefVIs varies not only with the choice of CTVI algorithm but also with the choice of RefVI modality, imaging subject, and the evaluation metric used to compare relative ventilation distributions. This variability may arise from the fact that each of the different CTVI algorithms and RefVI modalities provides a distinct physiologic measurement. Ultimately this variability, coupled with the lack of a "gold standard," highlights the ongoing importance of further validation studies before CTVI can be widely translated from academic centers to the clinic. It is hoped that the information gleaned from the VAMPIRE Challenge can help inform future validation efforts.