Verification of milled two-dimensional photon compensating filters using an electronic portal imaging device

A computer-driven milling machine is being installed at the Mallinckrodt Institute of Radiology to fabricate photon compensating filters for conventional compensation and beam-intensity modulation. Commissioning and quality assurance procedures are being developed for the design, fabrication, and delivery systems prior to using the milled filters in the clinic. The portion of the quality assurance procedures governing the filter fabrication using a computer-driven milling machine includes, in part, a comparison of designed and fabricated filters. Test filters include geometrically regular filters, such as flat surfaces and steps. The verification of these shapes is accomplished using spatial measurements. However, to test the mill's ability to generate complex curved surfaces, filters with more complicated surfaces are generated. These filters do not lend themselves to precise verification using physical measurement of the thickness distribution. A method has been developed to verify the thickness distributions of these complex filters by irradiating the filters with high-energy x rays and comparing the scattered and transmitted fluence to the fluence calculated using the intended filter shape. The fluence measurement is made using a calibrated commercial liquid ionization chamber electronic portal imaging device. The calculated fluence is separated into transmitted primary and scattered fluences and is determined using a convolution of a distributed radiation source kernel with an exponential filter attenuation function. The attenuation coefficient is measured for the filter material (Lipowitz metal) and fit to a second-order polynomial in filter thickness and off-axis distance. The distributed source kernel is measured using a split-field technique and fit to a sum of three two-dimensional Gaussian distributions. The scattered radiation is modeled by the Klein-Nishina cross section. The algorithm is tested by comparing calculated fluences with measured fluences for a series of machined filters: an open field, flat filters of 9.6-, 15.8-, and 31.6-mm thickness, split-field filters, and a pyramid-shaped filter. In each case, the algorithm calculates the fluence to within 3% of the measured values over the entire irradiated field size to within 1.5 cm of the collimated field edges.