Dosimetric consequences of asymmetric positioning of active source in ¹³⁷Cs and ²²⁶Ra intracavitary tubes

Recently Cesium-137 tubes have been acquired to replace our old stock of Radium-226 for intracavitary applications. Their construction is such that an active length of 15 mm is positioned asymmetrically within the physical length, with a spacing of 4 mm between eyelet end of tube and active source length and a spacing of 1.5 mm at the opposite end. Evaluation of standard radium tubes reveals asymmetry also, the extent of which depends on source strength. The difference in physical structure of these sources leads to dosimetry variations of about 20% depending upon the arrangement of the multiple sources in a tandem. Our thermoluminiscent dosimetry (TLD) measurements of dose distributions around the Delclos type mini-ovoids and Fletcher-Suit applicators using these ¹³⁷Cs tubes indicate possibly significant dosage mis-estimates. However, by carefully orientating the sources or by correcting apparent source locations on localization films, these errors may be minimized. The design considerations of a new applicator and our dosimetric measurements on such a system are discussed.