In-phantom calibration of Selectron-LDR sources

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Cesium-137 (Cs-137) sources are critical components in Selectron-LDR remote afterloading systems for brachytherapy.
• Accurate source strength determination is essential for precise dose delivery in radiation therapy.
• Variations in source strength can lead to significant differences in prescribed doses between medical institutions.

Purpose of the Study:

• To measure the source strength of Cs-137 spherical sources used in Selectron-LDR systems.
• To compare measured source strengths with manufacturer specifications.
• To evaluate the precision and uncertainty of the in-phantom calibration method for source strength determination.

Main Methods:

• Source strength measurements were performed on nine sets of Cs-137 sources from Selectron-LDR systems.
• Mean reference air kerma rate was determined using measurements in a phantom at approximately 5.5 cm from 24-30 sources.
• Source strength distribution within sets was assessed using a well-type ionization chamber.

Main Results:

• Observed discrepancies between measured and manufacturer-specified source strengths ranged from -2.3% to +3.9%.
• Root mean square deviations for the nine source sets varied between 1.0% and 3.2%.
• The in-phantom calibration method demonstrated a statistical uncertainty of 0.3% (1 SD) and a non-statistical uncertainty of approximately 1.2% (1 effective SD).

Conclusions:

• The in-phantom calibration method enhances the precision of Cs-137 source strength measurements.
• Implementing this method can reduce inter-institutional variations in dose delivery.
• Further investigation is required to reduce the non-statistically evaluated component of uncertainty for the in-phantom method.