Evaluation of registration accuracy using short‐arc cone‐beam computed tomography for image‐guided radiotherapy in breast cancer

Abstract Background Image‐guided radiotherapy (IGRT) using cone‐beam computed tomography (CBCT) enables highly accurate imaging registration; however, increased radiation exposure remains a concern. Recently, short‐arc CBCT (SA‐CBCT), which reduces projection data and reconstructs images, has become available. Although previous studies have shown that image quality deteriorates when projection data are reduced, no study has directly evaluated the clinical usefulness of SA‐CBCT or its effect on registration accuracy. Purpose We verified the registration accuracy of SA‐CBCT in breast tangential irradiation using an anthropomorphic phantom and clinical data, and examined its clinical feasibility compared with the commonly used 200° scan. Methods An anthropomorphic phantom was placed on the treatment bed, and CT and CBCT images acquired during treatment planning were aligned. Bed movement was measured for both conventional CBCT and SA‐CBCT images. The difference between the two measurements was quantified as N , defined as the norm of the vector difference between CBCT and SA‐CBCT registration shifts, to evaluate registration accuracy. The acquisition angle was varied to assess the effects of central and total angles. In the clinical study, registration data from 30 consecutive patients who underwent whole‐breast irradiation were analyzed. Data were reconstructed by changing the acquisition angle from 200° to 120°, and N was calculated as in the phantom experiment. Results In the phantom experiment, N was within 0.15 mm (3‐axis) and 0.55 mm (6‐axis) when the acquisition angle was reduced to 120°. In clinical data, the average N was 0.64 mm in 3‐axis registration (84.7% within 1 mm) and 0.85 mm in 6‐axis registration (71.3% within 1 mm). Even with a 120° acquisition angle, the body contour and chest wall edges were sufficiently preserved. Conclusions SA‐CBCT maintained adequate registration accuracy in breast tangential irradiation. These findings suggest its clinical applicability with appropriate acquisition angle settings.