Breast patient setup error assessment: comparison of electronic portal image devices and cone-beam computed tomography matching results.

PURPOSE: To quantify the differences in setup errors measured with the cone-beam computed tomography (CBCT) and electronic portal image devices (EPID) in breast cancer patients. METHODS AND MATERIALS: Repeat CBCT scan were acquired for routine offline setup verification in 20 breast cancer patients. During the CBCT imaging fractions, EPID images of the treatment beams were recorded. Registrations of the bony anatomy for CBCT to planning CT and EPID to digitally reconstructed-radiographs (DRRs) were compared. In addition, similar measurements of an anthropomorphic thorax phantom were acquired. Bland-Altman and linear regression analysis were performed for clinical and phantom registrations. Systematic and random setup errors were quantified for CBCT and EPID-driven correction protocols in the EPID coordinate system (U, V), with V parallel to the cranial-caudal axis and U perpendicular to V and the central beam axis.
RESULTS: Bland-Altman analysis of clinical EPID and CBCT registrations yielded 4 to 6-mm limits of agreement, indicating that both methods were not compatible. The EPID-based setup errors were smaller than the CBCT-based setup errors. Phantom measurements showed that CBCT accurately measures setup error whereas EPID underestimates setup errors in the cranial-caudal direction. In the clinical measurements, the residual bony anatomy setup errors after offline CBCT-based corrections were Σ(U) = 1.4 mm, Σ(V) = 1.7 mm, and σ(U) = 2.6 mm, σ(V) = 3.1 mm. Residual setup errors of EPID driven corrections corrected for underestimation were estimated at Σ(U) = 2.2mm, Σ(V) = 3.3 mm, and σ(U) = 2.9 mm, σ(V) = 2.9 mm.
CONCLUSION: EPID registration underestimated the actual bony anatomy setup error in breast cancer patients by 20% to 50%. Using CBCT decreased setup uncertainties significantly.