Portal dosimetry for VMAT using integrated images obtained during treatment. Issue 2 (24th January 2014)

• Record Type: Journal Article
• Authors: Bedford, James L.; Hanson, Ian M.; Hansen, Vibeke Nordmark
• Is Part Of: Medical physics. Volume 41:Issue 2(2014)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2014-01-24
• Subjects: Dose‐volume analysis -- Tissue response -- Conformal radiation treatment -- Dosimetry/exposure assessment -- Therapeutic applications, including brachytherapy -- Ancillary equipment; dosimetry -- ionisation chambers -- lung -- medical image processing -- phantoms -- radiation therapy; VMAT -- arc therapy -- electronic portal imaging device -- verification -- in vivo dosimetry; Radiation therapy -- Biological material, e.g. blood, urine; Haemocytometers -- Measurement of nuclear or x‐radiation -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- Tubes for determining the presence, intensity, density or energy of radiation or particles -- Scintigraphy -- with scintillation detectors -- Ionisation chambers; Medical imaging -- Dosimetry -- Image guided radiation therapy -- Lungs -- Field size -- Ionization chambers -- Multileaf collimators -- Medical treatment planning -- Image detection systems -- Time measurement; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4862515
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: : Portal dosimetry provides an accurate and convenient means of verifying dose delivered to the patient. A simple method for carrying out portal dosimetry for volumetric modulated arc therapy (VMAT) is described, together with phantom measurements demonstrating the validity of the approach. Methods: : Portal images were predicted by projecting dose in the isocentric plane through to the portal image plane, with exponential attenuation and convolution with a double‐Gaussian scatter function. Appropriate parameters for the projection were selected by fitting the calculation model to portal images measured on an iViewGT portal imager (Elekta AB, Stockholm, Sweden) for a variety of phantom thicknesses and field sizes. This model was then used to predict the portal image resulting from each control point of a VMAT arc. Finally, all these control point images were summed to predict the overall integrated portal image for the whole arc. The calculated and measured integrated portal images were compared for three lung and three esophagus plans delivered to a thorax phantom, and three prostate plans delivered to a homogeneous phantom, using a gamma index for 3% and 3 mm. A 0.6 cm 3 ionization chamber was used to verify the planned isocentric dose. The sensitivity of this method to errors in monitor units, field shaping, gantry angle, and phantom position was also evaluated by means of computer simulations. Results: : The calculation model for portal dose prediction was able to accurately compute the portal images due to simple square fields delivered to solid water phantoms. The integrated images of VMAT treatments delivered to phantoms were also correctly predicted by the method. The proportion of the images with a gamma index of less than unity was 93.7% ± 3.0% (1SD) and the difference between isocenter dose calculated by the planning system and measured by the ionization chamber was 0.8% ± 1.0%. The method was highly sensitive to errors in monitor units and field shape, but less sensitive to errors in gantry angle or phantom position. Conclusions: : This method of predicting integrated portal images provides a convenient means of verifying dose delivered using VMAT, with minimal image acquisition and data processing requirements.