2256 - A Cylindrical Slit Approach to Study the Focal Spot of Photon Beams from a Medical Linear Accelerator

G. X. Ding¹, and K. L. Homann²; ¹Department of Radiation Oncology, Vanderbilt University School of Medicine, Nashville, TN, ²Vanderbilt University, Nashville, TN

Purpose/Objective(s): Accelerator focal spot size and shape can significantly impact delivered dose output and beam profiles. Knowledge of the photon source information can improve the beam modeling in treatment planning systems and lead to more accurate patient dose calculations, especially for small fields used in stereotactic radiosurgery. Several methods have been developed for studying spot size. The most common method is to use a slit to measure a series of parallel lines which are then used to reconstruct spatial distribution of the focal spot based on CT reconstruction principles. This study explores a new approach using multiple co-axial cylindrical slits to measure spatial distribution of the focal spot directly without needing to apply CT reconstruction. Materials/

Methods: The Monte Carlo EGSnrc system was used to investigate 6 MV-15 MV beams from medical linear accelerators. We used simulations to optimize the design of the co-axial cylindrical experimental apparatus and to quantify anticipated experimental results. The cylindrical apparatus consisted of multiple co-axial cylindrical 0.05 mm thick air gap slits sandwiched between heavy metal layers. The length of the of cylinder was 40 cm. The apparatus was designed to allow photons that are only parallel to beam central axis to pass through the slits. Photons that passed through the slits were analyzed to obtain the spatial distribution of the focal spot.
Results: The intensity of the photon fluence scored at the bottom of the cylinder apparatus is directly correlated to the focal spot intensity distribution with only a linear correction proportional to the cylindrical radial distance from the beam central axis. The thickness of air slit and metal layers can be optimized by balancing the trade-off between the signal efficiency and resolution of the source intensity distribution to be determined. Based on the analysis of photon fluence, the shape and distribution of the source focal spot obtained from our simulations reproduce that of beams with known spatial distribution of the focal spot of a 6 MV beam. Although the result of the Monte Carlo study is based on the scored photon fluence that pass the multiple co-axial slits, these photons can be used to calculate dose to a film in a phantom which can be experimentally validated.
Conclusion: Using Monte Carlo simulation techniques, we investigated a novel approach of using a multiple cylindrical slit apparatus to measure the intensity distributions of focal spots from megavoltage photon beams. Our results indicate this new approach is feasible. The method is able to obtain source intensity distributions directly from measurement without the need of performing additional CT reconstruction. Monte Carlo simulation techniques can be used to optimize the design of the experimental apparatus to avoid multiple fabrication of the apparatus through a trial-and-error approach.