Dual edge apparatus and algorithm for measurement of x‐ray beam spot parameters

Purpose The shape, size, and location of the x‐ray beam spot (where the electron beam strikes the target) in a linac‐based radiation therapy machine are of potential clinical significance. Established techniques to measure the beam spot parameters involve specialized hardware and typically assess size and shape of the beam spot or its position, but not both. A simple apparatus and algorithm for measuring all beam spot parameters simultaneously is proposed here. Methods The apparatus is composed of two partially transmitting edge plates mounted at different vertical positions. The mount for the apparatus slides into the accessory tray of the linac treatment head so that it rotates with the collimator, and it is imaged by the existing electronic portal imaging device (EPID) over multiple collimator angles. A software algorithm takes the acquired images and uses a parallel‐beam CT reconstruction technique to compute beam spot size, shape, and position in one computation. In addition, the wobble of the collimator assembly can be estimated. The overall method was validated with both Monte Carlo simulation and with comparison to in‐house spot camera measurements on a radiation therapy system. Results The algorithm correctly predicted the beam spot parameters used for the Monte Carlo simulation to better than 50 μm accuracy in all cases. Furthermore, results from the dual edge method matched spot camera results with 30 μm accuracy for beam spot size and shape, with 80 μm average accuracy for beam spot position, and better than 200 μm accuracy for collimator assembly wobble. Conclusions We have developed a combination dual edge apparatus and image processing algorithm that, when used on a radiotherapy linac with an EPID, can accurately determine the size and shape of the electron beam spot, its position relative to collimator rotation axis, and the wobble of the collimator assembly.