PII S0360-3016(01)02818-8 PHYSICS CONTRIBUTION TOWARD AUTOMATED QUALITY ASSURANCE FOR INTENSITY- MODULATED RADIATION THERAPY DANIEL A. LOW,PH.D., JAMES F. DEMPSEY,PH.D., JERRY MARKMAN, D.SC., 1 SASA MUTIC, M.S., ERIC E. KLEIN, M.S., JASON W. SOHN,PH.D., AND JAMES A. PURDY,PH.D. Department of Radiation Oncology, Mallinckrodt Institute of Radiology, St. Louis, MO Purpose: To investigate whether high-quality, relatively inexpensive, document and transparency scanners used as densitometers are sufficiently quantitative for routine quality assurance (QA). Methods and Materials: The scanner we investigated used a linear amplifier, digitizing gray-scale images to 12-bit resolution with a user-selected spatial resolution of 0.170 mm 2 pixels. To reduce Newton’s rings artifacts, the standard glass platen was replaced by glass with an antireflective coating. Conversion of reading to transmission was conducted by permanently placing a calibrated photographic step tablet on the scanner platen. After conversion to light transmission, a zero-phase two-dimensional Wiener filter was used to reduce pixel-to- pixel signal variation. Light-scatter artifacts were removed by deconvolution of a measured light-spread kernel. The light-spread kernel artifacts were significant along the scanner’s detector axis, but were insignificant along the scanning axis. Results: 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. The document scanning system response was compared against a confocal scanning laser densitometer. A series of IMRT dose distribution and dose calibration film sets were scanned using the two scanners, and the measured dose was compared. The maximum mean and standard deviation of the measured dose difference between the document scanner and confocal scanner was 1.48% and 1.06%, respectively. Conclusion: While the document scanners are not as flexible as dedicated film densitometers, these results indicate that, using the intensity and scatter corrections, the system provides accurate and precise measurements up to an optical density of 2.0, sufficient for routine IMRT film QA. For some film types, this requires the reduction in monitor units to limit the dose delivered to the film. The user must be cautious that the delivered IMRT dose is scaled appropriately. This inexpensive and accurate system is being integrated into an automated QA program. © 2002 Elsevier Science Inc. Intensity modulated radiation therapy, Conformal therapy, Densitometry, Quality assurance, Film dosimetry. INTRODUCTION Intensity-modulated radiation therapy (IMRT) is a subset of 3-dimensional conformal radiation therapy (3D-CRT); therefore, the quality assurance (QA) of IMRT will closely follow that of traditional 3D-CRT systems. One significant difference between IMRT and 3D-CRT is the validation of delivered dose. For 3D-CRT treatment planning systems, dose distributions can be validated using system tests, and patient-specific dose validation is typically limited to beam- specific monitor unit (MU) checks and in vivo dose mea- surements. The complexity of IMRT calculation and deliv- ery has made the development of accurate, general, and thorough independent MU calculation checks more diffi- cult. While recent reports have been published describing computational methods for MU verification (1, 2), many institutions continue to rely on direct dose verification mea- surements. Wang et al. (3) and later, Ling et al. (4), de- scribed the use of a polystyrene phantom and measuring the dose for each intensity-modulated beam. In these cases, they measured each beam with the beam central axis normal to the phantom surface and used radiographic film and ioniza- tion chamber measurements at selected points in the beams. Tsai et al. (5) described the use of anthropomorphic phan- toms using thermoluminescent dosimetry (TLD) and radio- graphic film for measuring and validating complete tomo- therapy dose distributions. Verellen et al. (6) used an anthropomorphic phantom with analine dosimeters, TLD, and radiographic film, also to verify tomotherapy dose dis- Reprint requests to: Daniel Low, Ph.D., Department of Radia- tion Oncology, Mallinckrodt Institute of Radiology, 510 South Kingshighway Boulevard, St. Louis, MO 63110. Tel: (314) 362– 2636; Fax: (314) 362–2682; E-mail: low@castor.wustl.edu 1 Present address: Computerized Medical Systems, St. Louis, MO. This work was supported in part by corporate grants from Computerized Medical Systems, NOMOS Corporation, NIH grant R01 CA88409, and by a grant from the American Cancer Society IRG–58 – 01– 42. Received Aug 2, 2001, and in revised form Dec 14, 2001. Accepted for publication Dec 17, 2001. Int. J. Radiation Oncology Biol. Phys., Vol. 53, No. 2, pp. 443– 452, 2002 Copyright © 2002 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/02/$–see front matter 443