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SU-E-J-40: Comparison of Two Head and Neck Immobilization Device Fabrication and Simulation Protocols: Effects On Setup Deviation and Intrafraction Motion During Treatment.

PURPOSE
To compare two protocols for head and neck immobilization device fabrication and simulation and their effect on daily setup deviation and intrafraction motion over the course of VMAT or static IMRT.


METHODS
Fifteen post-surgical bilateral head and neck patients were immobilized with a custom-fit dental mold attached to a 5-point thermoplastic mask and treated with VMAT or IMRT. After fabrication of the immobilization device, which included 5-10 minutes for mask hardening, the patient either was scanned immediately without any positioning adjustments (N = 10), or got off of the simulation table for approximately 1 minute and was repositioned in treatment position prior to scanning (N = 5). Patients were localized using kV orthogonal imaging matching to overall bony anatomy for 30-35 fractions. Translational setup deviations were determined retrospectively by matching pre- and post-treatment kV orthogonal images to reference DRRs using bony anatomy near isocenter (typically C4) as well as gold fiducial markers embedded in the dental mold. Intrafraction motion was determined based on comparison of pre-treatment kV orthogonal images with post-treatment kV orthogonal images for each fraction. Margins were calculated using the pre- and posttreatment setup deviation data of each patient cohort.


RESULTS
Treatment position repositioning prior to simulation scanning decreased the patient group mean intrafraction motion near the dental mold (P < 0.02) and near isocenter (P < 0.04), but did not statistically significantly change the group mean setup uncertainty.


CONCLUSION
Repositioning of the patient in treatment position prior to simulation reduced intrafraction motion, but did not affect patient setup uncertainty during head and neck treatment. These results indicate that on average repositioning the patient in treatment position decreases patient intrafraction motion over the course of treatment for post-surgical bilateral head and neck patients immobilized with dental molds attached to a thermoplastic mask.