137Cs dosimetry table for asymmetric source

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Abstract
A common 137Cs brachytherapy source has a 2‐cm physical length and 1.38‐cm active length. The active length is not symmetric with respect to the source center because one source end contains an eyelet. Current dose rate tables assume a symmetric source loading with respect to the source center. A computer program was written to calculate an asymmetric distribution using the manufacturers' source specifications. Corrections were made for attenuation and obliquity through the source materials. Dose rate values in cGy/h for a 137Cs source equivalent to 1 mg of 226Ra are shown as a function of the radius from the source center and angle from the source end containing the eyelet. Dose rate values in the table were confirmed by ferrous sulfate measurements using small volumes. The table values agree with published values at points more distant than a few cm and lying at an angle such that the asymmetry of the source loading has its minimum influence.