Patient-Specific Protocol to Reduce Radiation Therapy Setup Variation Using an Electronic Portal Imaging Device

Abstract

Purpose: Evaluate electronic portal imaging (EPI) as a means to identify and correct field displacement in patients with problematic external beam radiotherapy setups.
Materials and Methods: 14 patients with problematic setups were identified for pretreatment daily EPI beam monitoring as part of a physician-directed therapist intervention protocol. Pretreatment EPIs were used to re-align fields as necessary to bring the setup within the physician-prescribed tolerance level. For comparison, daily EPIs were available for 12 ?control? patients who had no particular setup difficulties and for whom on-line beam re-alignment was not made. Anatomy matching software was used to measure setup variation along medial-lateral, superior-inferior, and anterior-posterior axes.
Results: On-line field re-alignment yielded a significant (p = 0.001) improvement when comparing initial and final setup variations. The mean standard deviation of setup displacement averaged over three axes was reduced from 6.4 mm to 3.1 mm after realignment. The final variation of protocol patients was comparable to that of control patients.
Conclusions: EPI provided effective means to perform on-line beam re-alignment in a group of difficult to position patients. This procedure resulted in a reduction in setup displacement that was statistically significant, clinically relevant, and approached that of a more typical patient group.