The residual setup errors of different IGRT alignment procedures for head and neck IMRT and the resulting dosimetric impact

Abstract

Purpose: To assess residual setup errors during head and neck radiation therapy and the resulting consequences for the delivered dose for various patient alignment procedures.

Methods and materials: Megavoltage cone beam computed tomography (MVCBCT) scans from 11 head and neck patients who underwent intensity modulated radiation therapy were used to assess setup errors. Each MVCBCT scan was registered to its reference planning kVCT, with seven different alignment procedures: automatic alignment and manual registration to 6 separate bony landmarks (sphenoid, left/right maxillary sinuses, mandible, cervical 1 [C1]-C2, and C7-thoracic 1 [T1] vertebrae). Shifts in the different alignments were compared with each other to determine whether there were any statistically significant differences. Then, the dose distribution was recalculated on 3 MVCBCT images per patient for every alignment procedure. The resulting dose-volume histograms for targets and organs at risk (OARs) were compared to those from the planning kVCTs.

Results: The registration procedures produced statistically significant global differences in patient alignment and actual dose distribution, calling for a need for standardization of patient positioning. Vertically, the automatic, sphenoid, and maxillary sinuses alignments mainly generated posterior shifts and resulted in mean increases in maximal dose to OARs of >3% of the planned dose. The suggested choice of C1-C2 as a reference landmark appears valid, combining both OAR sparing and target coverage. Assuming this choice, relevant margins to apply around volumes of interest at the time of planning to take into account for the relative mobility of other regions are discussed.

Conclusions: Use of different alignment procedures for treating head and neck patients produced variations in patient setup and dose distribution. With concern for standardizing practice, C1-C2 reference alignment with relevant margins around planning volumes seems to be a valid option.