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

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.IGRT is effective not only in highly accurate treatment but also as a component of conventional RT. Conventional RT may be used to treat lymph nodes above or below the collarbone. For patients with breast cancer and ≥4 auxiliary lymph node metastases [5], additional RT to the lymph nodes above or below the collarbone is recommended as part of a conservative therapeutic strategy. For cases in which <4 auxiliary lymph node metastases are present, the need for additional RT to the lymph nodes above or below the collarbone is determined according to the patient's health details. However, vital organs such as the trachea, esophagus, and spinal cord are adjacent to these lymph nodes, and errors related to accurate delivery of radiation to the lymph nodes in this region can be caused by the positioning of the arm and jaw [6].This study aimed to effectively utilize IGRT for conventional RT of the lymph nodes above or below the collarbone. To deliver highly accurate RT, corrections must be made according to the body position and orientation of internal organs. To obtain information related to organ orientation, an artifact-free image is required and procured using a minimum CBCT projection angle of 180° + fan [7,8]. However, accurate RT on the lymph nodes above or below