Towards automated quality assurance for intensity Modulated Radiation Therapy: Film densitometry

Intensity Modulated Radiation Therapy (IMRT) clinical implementation requires manpower-intensive dose distribution measurements and comparisons against calculations. We are developing an automated system to prepare, acquire, analyze, and report the QA results. At the heart of the IMRT QA process is the measurement of multidimensional dose distributions, which in our case will be measured using radiographic film. The availability of high quality, relatively inexpensive, document and transparency scanners has led us to investigate their use as quantitative densitometers. The investigated system uses a linear amplifier, digitizing gray-scale images to 12 bits with 0.169 mm² pixels. Calibration of the reading was conducted using a calibrated photographic step tablet that was placed on the scanner bed for every image. Scanning one-dimensional array densitometers acquire optical density distributions efficiently but suffer from light-scatter artifacts that lead to inaccurate dosimetry measurements, especially in high dose gradient regions. The scanner light-spread function was determined by scanning a narrow open strip placed in the CCD detector and scanning orientations. The light scatter artifact was negligible in the scanning direction, but was significant in the detector direction. Subsequent scans were smoothed using a zero-phase two-dimensional Wiener filter and corrected for the scatter artifact using deconvolution. Pixel-to-pixel noise was better than 2% for optical densities ranging from 0.4 to 2.0 and 0 to 2.7 for the unfiltered and filtered images, respectively. Preliminary results indicate that using the intensity and scatter corrections, the system may provide accurate and precise measurements up to an optical density of 2.0.