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Treasure of the Past IX: Exposure Standardization of Iodine-125 Seeds Used for Brachytherapy

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ABSTRACT

A method for calibrating iodine-125 seeds in terms of exposure has been established. The standard free-air ionization chamber, used for measuring soft x rays, was chosen for the measurements. Arrays of four to six seeds were used to enhance the ionization-current-to-background-current ratio. Seeds from an array were measured individually in a re-entrant chamber. The quotient of the exposure rate for the array by the sum of the ionization currents in the re-entrant chamber is the calibration factor for the re-entrant chamber. Calibration factors were established for three types of iodine-125 seeds. The overall uncertainty for the seed exposure calibrations is less than 6%.

No MeSH data available.


Related in: MedlinePlus

Standard free-air ionization chamber measurements of exposure rate from an array of gold-marker type iodine-125 seeds. The array was rotated through 180° starting and ending with the ends of the seeds facing the FAC diaphragm. The data are normalized to measurements with the plane of the array perpendicular to the FAC axis.
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f2-j65lof: Standard free-air ionization chamber measurements of exposure rate from an array of gold-marker type iodine-125 seeds. The array was rotated through 180° starting and ending with the ends of the seeds facing the FAC diaphragm. The data are normalized to measurements with the plane of the array perpendicular to the FAC axis.

Mentions: The calibration procedure for the iodine-125 seeds is designed to provide data for radiation emitted perpendicular to the seed axis. Since the seeds are implanted in tissue, the variation of exposure with direction from the source is of interest. Measurement of this characteristic was carried out by rotating the source arrays through 180° by means of the device already described. The exposure rate from a source array was found to be nearly a linear function of the sine of the azimuthal angle of the array (at 0 degrees, the source axes are parallel to the axis of the FAC). Normalized exposure data and Ẋn = f(sin θ) are shown in figures 2 and 3. The data were normalized to unity at θ = π/2. The constants for the equations result from a least-squares fit to the data of a first order polynomial in sin θ. A mean exposure rate from a seed can be computed by averaging f(sin θ) over all solid angles so thatX˙¯n=∫0πX˙nsinθdθ∫0πsinθdθ=a+πb4.The values for the constants and for are given in table 20. Higher order polynomials provide equations which fit the data better but the mean value for Ẋn does not change when they are used. The mean exposure rates given in table 20 are for normalized data and are fractions of the exposure rates perpendicular to the long axis of the seeds. The results are in general agreement with the results of other authors [7,8].


Treasure of the Past IX: Exposure Standardization of Iodine-125 Seeds Used for Brachytherapy
Standard free-air ionization chamber measurements of exposure rate from an array of gold-marker type iodine-125 seeds. The array was rotated through 180° starting and ending with the ends of the seeds facing the FAC diaphragm. The data are normalized to measurements with the plane of the array perpendicular to the FAC axis.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4862818&req=5

f2-j65lof: Standard free-air ionization chamber measurements of exposure rate from an array of gold-marker type iodine-125 seeds. The array was rotated through 180° starting and ending with the ends of the seeds facing the FAC diaphragm. The data are normalized to measurements with the plane of the array perpendicular to the FAC axis.
Mentions: The calibration procedure for the iodine-125 seeds is designed to provide data for radiation emitted perpendicular to the seed axis. Since the seeds are implanted in tissue, the variation of exposure with direction from the source is of interest. Measurement of this characteristic was carried out by rotating the source arrays through 180° by means of the device already described. The exposure rate from a source array was found to be nearly a linear function of the sine of the azimuthal angle of the array (at 0 degrees, the source axes are parallel to the axis of the FAC). Normalized exposure data and Ẋn = f(sin θ) are shown in figures 2 and 3. The data were normalized to unity at θ = π/2. The constants for the equations result from a least-squares fit to the data of a first order polynomial in sin θ. A mean exposure rate from a seed can be computed by averaging f(sin θ) over all solid angles so thatX˙¯n=∫0πX˙nsinθdθ∫0πsinθdθ=a+πb4.The values for the constants and for are given in table 20. Higher order polynomials provide equations which fit the data better but the mean value for Ẋn does not change when they are used. The mean exposure rates given in table 20 are for normalized data and are fractions of the exposure rates perpendicular to the long axis of the seeds. The results are in general agreement with the results of other authors [7,8].

View Article: PubMed Central - PubMed

ABSTRACT

A method for calibrating iodine-125 seeds in terms of exposure has been established. The standard free-air ionization chamber, used for measuring soft x rays, was chosen for the measurements. Arrays of four to six seeds were used to enhance the ionization-current-to-background-current ratio. Seeds from an array were measured individually in a re-entrant chamber. The quotient of the exposure rate for the array by the sum of the ionization currents in the re-entrant chamber is the calibration factor for the re-entrant chamber. Calibration factors were established for three types of iodine-125 seeds. The overall uncertainty for the seed exposure calibrations is less than 6%.

No MeSH data available.


Related in: MedlinePlus