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Comparative Study
. 2003 Winter;4(1):40-50.
doi: 10.1120/jacmp.v4i1.2540.

Patient specific quality assurance for the delivery of intensity modulated radiotherapy

Nzhde Agazaryan  1 , Timothy D Solberg, John J DeMarco
Affiliations
Comparative Study

Patient specific quality assurance for the delivery of intensity modulated radiotherapy

Nzhde Agazaryan et al. J Appl Clin Med Phys. 2003 Winter.

Abstract

A patient specific quality assurance program has been developed to facilitate the clinical implementation of intensity modulated radiotherapy (IMRT) delivered using a micro-multileaf collimator. The methodology includes several dosimetric tasks that are performed prior to the treatment of each patient. Film dosimetry is performed for each individual field and for the multifield composite plan. Individual field measurements are performed at a depth of 5 cm in a water equivalent slab phantom; export of dose calculations from the treatment planning system is similarly specified. For the composite distribution, parameters from the patient plan are applied to an IMRT phantom, and film is exposed in an axial orientation. Distributions are compared with the aid of software developed for the specific tasks. The measured and calculated dose distributions can be superimposed and positioned graphically using move, rotate, and mirror tools, as well as by specifying isocenter coordinates and using fiducial marks. Horizontal and vertical profiles are available for analysis. Dose difference, distance-to-agreement, and gamma index, the minimum scaled multidimensional distance between a measurement and a calculation point determined in combined dose and physical distance space, are calculated along a specified isodose line and displayed. gamma provides an excellent measure of disagreement between measurement and calculation for complex intensity distributions. We specify 3% dose difference and 3 mm distance as our scaling acceptability criteria. Absolute dosimetry for each composite plan is performed using an ionization chamber. To date, excellent agreement between measurements and calculations has been observed.

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Figures

Figure 1
Figure 1
(Color) (Left) Schematic diagram of the experimental setups of Radiology Support Devices (RSD) Dry Water™ Slab phantom used for single field IMRT measurements. (Right) A picture of a single field IMRT measurement setup.
Figure 2
Figure 2
(Color) The MED‐TEC IMRT phantom made of Virtual Water™ during the absolute dose measurement.
Figure 3
Figure 3
(Color) The treatment planning system has an option of mapping the complete patient treatment onto any phantom. A seven field prostate IMRT plan is shown mapped onto the MED‐TEC IMRT phantom.
Figure 4
Figure 4
(Color) The 20%, 50%, and 80% isodose lines of calculation and measurement are shown. Measured data is in the form of color wash, and calculated data is presented in terms of solid lines. The γ index distribution is superimposed on the calculated and measured dose maps. For the regions where the γ index is larger than unity, the program outputs a map of γ index with different intensities of green corresponding to different magnitudes of γ. Values of 3% and 3 mm have been used for dose difference and distance tolerances respectively. (Left) An example of a QA analysis with significant difference between measurement and calculation. (Right) An example of a QA analysis with no points on the map with γ index larger than unity.
Figure 5
Figure 5
(Color) (Top) Horizontal and (bottom) vertical profiles of measured and calculated dose distribution along with the γ index. The example shown is from the data presented in right.
Figure 6
Figure 6
DTA values along the specified (80%) isodose line of the calculation data. The average DTA is less than 0.5 mm and the maximum DTA is less than 2.0 min.
Figure 7
Figure 7
(Color) Film reading along the specified (80%) isodose line of the calculation data.
Figure 8
Figure 8
(Color) Gamma index values along the specified (80%) isodose line of the calculation data.
Figure 9
Figure 9
(Color) 20%, 50%, and 80% isodose lines of calculation and measurement of a composite seven field IMRT plan. Measured data is in the form of color wash, and calculated data is in terms of solid lines. The γ index distribution is superimposed on the map using different intensities of green for different magnitudes of gamma.
Figure 10
Figure 10
(Color) The histogram of the measured and calculated absolute dose differences for all the patients since establishing the QA protocol.
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