Verification and correction of setup deviations in tangential breast irradiation using EPID: gain versus workload

When a deviation in the treatment setup is identified, when or how should it be corrected? With the aim of improving the precision and reducing the systematic errors while maintaining the workload to a minimum, a study was performed to define the proper set of actions for the correction of tangential breast setup deviations during the course of the treatment.

Clinical data were taken from a prospective study of more than 2200 on-line Electronic Portal Images (EPI) from 20 patients treated with tangential irradiation following a partial mastectomy for an early stage (I and II) breast cancer. The values of the central lung distance were then entered in the verification/correction procedure and modified to correct only the portion of the deviation most likely attributed to systematic errors according to the maximum likelihood. The verification/ correction procedure uses an action level equal to FML x alpha/square root of N, where alpha is proportional to the standard deviation (alpha = n sigma) and N is the number of consecutive fractions delivered after the start of the treatment. FML is the fraction of the deviation due to the systematic errors estimated from the maximum likelihood of the two distributions. In addition to n, one needs to specify Nmax, the maximum number of consecutive measurements without correction, to apply the procedure. The combination of these two parameters C(n,Nmax) will determine the reduction of systematic errors (gain) and the number of measurements and corrections (workload) associated to the procedure.

The effects of three combinations, C(1,1), C(2,2), and C(3,4) were studied. Also, the analysis of the results after application of the procedure with and without the factor of maximum likelihood made individually for each patient demonstrates the importance of the FML.

The verification/correction procedure with the inclusion of the FML can effectively improve the accuracy when applied to clinical data. With the specific workload related to measurements and corrections performed at our institution, we have found that in the case of the tangential breast treatment, the optimum combination uses an action level equal to 2 sigma and a maximum of two consecutive measurements without correction.