A protocol for the reduction of systematic patient setup errors with minimal portal imaging workload

Purpose: To evaluate a new off-line patient setup correction protocol that minimises the required number of portal images and perform a comparison wit h currently applied protocols. Methods and Materials: We compared two types of off-line protocols: (a) the widely applied shrinking action level (SAL) protocol, in which the setup e rror, averaged over the measured treatment fractions, is compared with a th reshold that decreases with the number of measurements, to decide if a corr ection is necessary; and (b) a new "no-action-level" (NAL) protocol, which simply calculates the mean setup error over a filed number of fractions, an d always corrects for it. The performance of the protocols was evaluated by applying them to (a) a database of measured setup errors from 600 prostate patients (with, on average, 10 imaged fractions/patient) and (b) Monte Car lo-generated setup error distributions for various values of the population systematic and random errors. Results: The NAL protocol achieved a significantly higher accuracy than the SAI. protocol for a similar workload in terms of image acquisition and ana lysis, as well as in setup corrections. The SAL protocol required approxima tely three times more images than the NAL protocol to obtain the same reduc tion of systematic errors. Application of the NAL protocol to measured setu p errors confirmed its efficacy in systematic error reduction in a real pat ient population. Conclusion: The NAL protocol performed much more efficiently than the SAL p rotocol for both actually measured and simulated setup data. The resulting decrease in required portal images not only reduces workload, but also dose to healthy tissue, if dedicated large fields are required for portal imagi ng (double exposure). (C) 2001 Elsevier Science Inc.