Kilovoltage and Megavoltage Imaging in Radiotherapy

Radiotherapy treats cancer through irradiating customized radiation beams from different directions, generating a tumoricidal ionization field that conforms to the tumor and spares surrounding normal tissues. The use of X-ray imaging in radiotherapy has greatly improved the accuracy of treatment planning, tumor localization and on-board delivery. X-ray beams at the kilovoltage (kV) energy range encounter more photoelectric interactions, which leads to better image contrasts with sharper bony structures in most of the cases. There are two major advantages of the gantry-mounted orthogonal kV imaging system. First, the orthogonal arrangement of the kV imaging system can effectively avoid the blockage from the gantry in the imaging beamline. Second, the kV and megavoltage (MV) beams are assumed to share the same isocenter. The imaging isocenter matches the treatment isocenter. The MV computed tomography images can be conveniently used for treatment planning and daily dose calculation/accumulation for adaptive radiotherapy, especially for patients with metal implants like hip prosthesis.