ABSTRACT
PURPOSE: To compare the geometric alignments of soft-tissue implanted markers to the traditional bony-based alignments in head-and-neck cancers, on the basis of daily image guidance. Dosimetric impact of the two alignment techniques on target coverage is presented. METHODS AND MATERIALS: A total of 330 retrospective alignments (5 patients) were performed on daily megavoltage computed tomography (MVCT) image sets using both alignment techniques. Intermarker distances were tracked for all fractions to assess marker interfractional stability. Using a deformable image registration algorithm, target cumulative doses were calculated according to generated shifts on daily MVCT image sets. Target D95 was used as a dosimetric endpoint to evaluate each alignment technique. RESULTS: Intermarker distances overall were stable, with a standard deviation of <1.5 mm for all fractions and no observed temporal trends. Differences in shift magnitudes between both alignment techniques were found to be statistically significant, with a maximum observed difference of 8 mm in a given direction. Evaluation of technique-specific dose coverage based on D95 of target clinical target volume and planning target volume shows small differences (within +/-5%) compared with the kilovoltage CT plan. CONCLUSION: The use of daily MVCT imaging demonstrates that implanted markers in oral tongue and soft-palate cancers are stable localization surrogates. Alignments based on implanted markers generate shifts comparable overall to the traditional bony-based alignment, with no observed systematic difference in magnitude or direction. The cumulative dosimetric impact on target clinical target volume and planning target volume coverage was found to be similar, despite large observed differences in daily alignment shifts between the two techniques.
