WE-G-BRCD-09: Assessment of Quantitative Differences Between Different Deformable Image Registration Algorithms for Image Guided Radiotherapy of Head and Neck Cancer Patients

PURPOSE

DIR algorithms differ by physical principles employed in their design which in turn determines complexity of the transformation that the algorithm allows. Indiscriminate application of different algorithms without regard for their limitations may lead to significant discrepancies in research results and in clinical procedures. We compare two different algorithms in Head and Neck (H&N) patients to assess what aspects of DIR use are most sensitive to differences between algorithms.

METHODS

H&N patients are enrolled in a protocol which acquires weekly FBCT and daily double CBCT. Each FBCT study is contoured by the same physician. Two DIR algorithms are compared: Small Deformation Inverse-Consistent Linear Elastic (SICLE), and the ITK Diffeomorphic Demons (ITKDD) as implemented in the ITK package. Both algorithms employ significantly different physical principles in their design and consequently impose different restrictions on the complexity of transformation they allow. We compare Jacobian Volume Histograms (JVH), Spatial Discrepancy Volume Histograms (SDVH), BED and physical dose accumulation results and resulting plan evaluation indices.

RESULTS

Analysis of mean Jacobian shows that both algorithms are able to detect changes in structure volumes, though they differ quantitatively from one another and from the ground truth as established by the analysis of changes in contours. Width of Jacobian distributions is very different indicating that Jacobian should not be used as a measure of volume change at a voxel level without independent validation. Analysis of SDVHs shows that dose lookup points implied by both algorithms are separated by 5mm - 10mm over approximately 30% of most volumes. These differences translate into clearly visible though not very significant differences in BED and dose accumulation. We further observe that physical dose accumulation in external beam H&N patients is a good proxy for direct BED accumulation.

CONCLUSIONS

Different DIR algorithms may have to be applied selectively in different areas of treatment planning. Acknowledgments: Supported by NIH Grant P01 CA11602.

Two year longitudinal change and test-retest-precision of knee cartilage morphology in a pilot study for the osteoarthritis initiative

OBJECTIVE

Fast low angle shot (FLASH) and double echo steady state (DESS) magnetic resonance imaging (MRI) acquisitions were recently cross-calibrated for quantification of cartilage morphology at 3T. In this pilot study for the osteoarthritis (OA) initiative we compare their test-retest-precision and sensitivity to longitudinal change.

METHOD

Nine participants with mild to moderate clinical OA were imaged twice each at baseline, year 1 (Y1) and year 2 (Y2). Coronal 1.5mm FLASH and sagittal 0.7mm DESS sequences were acquired; 1.5mm coronal multiplanar reformats (MPR) were obtained from the DESS. Patellar, femoral and tibial cartilage plates were quantified in a paired fashion, with blinding to time point.

RESULTS

In the weight-bearing femorotibial joint, average precision errors across plates were 1.8% for FLASH, 2.6% for DESS, and 3.0% for MPR-DESS. Volume loss at Y1 was not significant; at Y2 the average change across the femorotibial cartilage plates was -1.7% for FLASH, -2.8% for DESS, and -0.3% for MPR-DESS. Volume change in the lateral tibia (-5.5%; P<0.03), and in the medial (-2.9%; P<0.04) and lateral femorotibial compartments (-3.8%; P<0.03) were significant for DESS.

CONCLUSIONS

FLASH, DESS and MPR-DESS all displayed adequate test-retest precision. Although the comparison between protocols is limited by the small number of participants and by the relatively small longitudinal change in cartilage morphology in this pilot study, the data suggest that significant change can be detected with MRI in a small sample of OA subjects over 2 years.