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Medical Physics / Volume 37 / Issue 6 / RADIATION MEASUREMENT PHYSICS

Med. Phys. 37, 2425 (2010); http://dx.doi.org/10.1118/1.3400043 (10 pages)

A field size specific backscatter correction algorithm for accurate EPID dosimetry

Sean L. Berry

Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027 and Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, New York 10065

Cynthia S. Polvorosa and Cheng-Shie Wuu

Department of Radiation Oncology, Columbia University, New York, New York 10032

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(Received 1 December 2009; accepted 20 March 2010; revised 2 March 2010; published online 5 May 2010)

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Purpose: Portal dose images acquired with an amorphous silicon electronic portal imaging device (EPID) suffer from artifacts related to backscattered radiation. The backscatter signal varies as a function of field size (FS) and location on the EPID. Most current portal dosimetry algorithms fail to account for the FS dependence. The ramifications of this omission are investigated and solutions for correcting the measured dose images for FS specific backscatter are proposed.
Methods: A series of open field dose images were obtained for field sizes ranging from 2×2 to 30×40 cm2. Each image was analyzed to determine the amount of backscatter present. Two methods to account for the relationship between FS and backscatter are offered. These include the use of discrete FS specific correction matrices and the use of a single generalized equation. The efficacy of each approach was tested on the clinical dosimetric images for ten patients, 49 treatment fields. The fields were evaluated to determine whether there was an improvement in the dosimetric result over the commercial vendor’s current algorithm.
Results: It was found that backscatter manifests itself as an asymmetry in the measured signal primarily in the inplane direction. The maximum error is approximately 3.6% for 10×10 and 12.5×12.5 cm2 field sizes. The asymmetry decreased with increasing FS to approximately 0.6% for fields larger than 30×30 cm2. The dosimetric comparison between the measured and predicted dose images was significantly improved (p⪡.001) when a FS specific backscatter correction was applied. The average percentage of points passing a 2%, 2 mm gamma criteria increased from 90.6% to between 96.7% and 97.2% after the proposed methods were employed.
Conclusions: The error observed in a measured portal dose image depends on how much its FS differs from the 30×40 cm2 calibration conditions. The proposed methods for correcting for FS specific backscatter effectively improved the ability of the EPID to perform dosimetric measurements. Correcting for FS specific backscatter is important for accurate EPID dosimetry and can be carried out using the methods presented within this investigation.

© 2010 American Association of Physicists in Medicine

Article Outline

  1. INTRODUCTION
  2. METHOD AND MATERIALS
    1. Quantification of asymmetry due to backscatter
    2. Correction and analysis of clinically delivered fields
  3. RESULTS
  4. DISCUSSION
  5. CONCLUSION
Digital Object Identifier
http://dx.doi.org/10.1118/1.3400043

KEYWORDS and PACS

Keywords

amorphous semiconductors, backscatter, dosimetry, matrix algebra, radiation therapy, silicon, EPID, portal dosimetry, backscatter, field size, dose verification

PACS

PUBLICATION DATA

ISSN

0094-2405 (print)  

Publisher

$abstract.society.value is a Member of CrossRef American Association of Physicists in Medicine

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