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Evaluation of Skull Base Rotation and C-Spine Displacement in Three Head and Neck Immobilization Techniques Using KV Portal Images

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H Wang

H Wang1*, X Wang2 , J Phan3 , L Zhang4 , (1) UT MD Anderson Cancer Center, Houston, TX, (2) UT MD Anderson Cancer Center, Houston, TX, (3) M.D. Anderson Cancer Center, Houston, TX, (4) MD Anderson Cancer Center, Houston, TX


SU-I-GPD-T-74 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Our previous study showed benefit of combination of customized cushion and bite-block with thermoplastic mask for head and neck immobilization. This study is to evaluate the skull rotation and c-spine curvature uncertainties in three immobilization techniques, with and without head cushions, using daily kV portal images.

Methods: Three immobilization techniques are 5-point mask only (M), mask with Klarity cushion and bite-block (M-KB), and mask with foam type proton mold and bite-block (M-PB). Six patients were evaluated for each of the technique. These patients had daily orthogonal kV-imaging including skull base to upper T-spine. For each patient, six daily kV-portal images were compared to planning DRR. A program was developed to compare the spinal column and floor of mandible on AP portal to DRR to evaluate their yaw rotation. On lateral image, the skull pitch rotation were evaluated based on angle change of the line projecting the hard palate to posterior skull base, the mandible displacement is evaluated using a line connecting mandible base to mandible neck, and the c-spine displacement is evaluated using a line connecting C2 to C6.

Results: The systematic errors (random errors) for yaw rotation of the floor of mandible were 1.3° (0.9°), 0.8° (0.8°), 0.9° (0.8°) for M, M-KB and M-PB, respectively. The yaw rotation for spinal column were comparable among three techniques. Pitch rotation for skull base were 1.8° (0.8°), 1.3°(0.8°) and 1.1° (0.8°), mandible pitch rotation were 1.1° (0.9°), 0.8° (0.9°) and 1.0° (0.9°), and angle change for C2-C6 line were 1.2° (1.1°), 0.8° (1.2°), and 1.2° (1.1°) for M, M-KB and M-PB, respectively. Based on the angle errors and isocenter location, target shift can be estimated.

Conclusion: Klarity cushion and proton mold can provide better immobilization and reproducibility of the skull base for head and neck patients.

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