Using Source Holder, SeeDOS Code 70022 with the HDR 1000 Plus Brachytherapy Ion Chamber

Confirm wire activity prior to clinical use   Easy and consistent source repositioning   Excellent reproducibility and linearity

We have developed a unique insert for calibration and quality assurance measurements of 500mm iridium wire coil. This insert, called the Source Holder for Iridium Wire Coil, SeeDOS Code 70022, fits into the HDR 1000 Plus Brachytherapy Ion Chamber. It was designed in collaboration with James Green, Head of Radiotherapy Physics, Southend Hospital, Westcliff-On-Sea, United Kingdom. The coil is held in a tube 25mm in diameter. the coil is consistently compressed to 2mm by a piston and positioned at the most sensitive point of the ion chamber. This consistent compression and positioning provides for the good reproducibility of measurements. The design of the HDR 1000 Plus ensures a change in sensitivity of only 0.1% for the first 5mm up or down from this designated measurement point.

A reproducibility test was performed using a single 30mm piece of wire which was repeatedly removed and replaced for ten consecutive measurements. The HDR 1000 Plus Brachytherapy Ion Chamber was used in conjunction with the SeeDOS Code CDX 2000 Charge Digitizing Electrometer. The reproducibility of the electrometer/chamber combination is better than 0.02nC in 1.74nC measurement, i.e. 0.6% (mean = 1.1734, sd = 0.00458).

Linearity of measurements was tested over a range of 124 to 2036MBq. Linearity was found to be <0.1%, with a correlation coefficient of 1.000.

Angular response was tested by placing a 30mm piece of wire in the source holder and then rotating the source holder in 90° increments. The measurements showed no variation, reading 1.74nC in all four locations.

Calibration factors are provided in terms of air-kerma strength, exposure strength, and activity. The factory calibration is accurate to +/-5%.

Two additional tests were performed to determine if the sum of the individual measured values of cut lengths of wire would equal the combined values of the wire. Individual wires were measured in the source holder, and then the wires were collectively measured in the source holder. One set of four wires showed a 0% difference and another set of five wires showed a 0.8% difference.

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