Characterization of linear accelerator X-ray source size using a laminated beam-spot camera

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

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.