Application of Cone Beam Computed Tomography to Conventional Radiotherapy: Limited Angle of Projections for Lymph Nodes Located above or below the Collarbone

International Journal of Radiology and Radiation Oncology

Research Article

Abstract 
Various techniques are used in image-guided radiotherapy (IGRT). Cone beam computed tomography (CBCT) is widely used in IGRT on linear accelerators. The increased accuracy of IGRT enables reducing planning target volume margins and doses to normal tissues. However, for radiation therapies requiring fractionated irradiation, increased exposure dose attributed to CBCT becomes problematic. We investigated the smallest projection angle required to provide target accuracy for CBCT of lymph nodes located above or below the collarbone in breast cancer patients. Phantom and clinical experiments were conducted on the basis of shifts obtained through gray value-based auto-image registration performed using 360° CBCT. Corrected images obtained from decreasing projection angles in 10° increments were compared with a single 360° image. In the phantom and clinical experiments, the smallest projection angle used as the tolerance level were 80° and 100°, which yielded the same accuracy as the 360° case. When irradiating lymph nodes above or below the collarbone with CBCT, a 100° projection angle was necessary for autoimage registration, whereas a projection angle of 195.8° (180° + fan) was required to produce artifactfree images. This represents 95.8° reductions in the projection angle, resulting in a 48.9% reduction in exposure.



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