Experimental determination of the TG-43 dosimetric characteristics of EchoSeed™ model 6733 125 I brachytherapy source A. S. Meigooni, a) Sharifeh A. Dini, Keith Sowards, Joshua L. Hayes, and Awni Al-Otoom Department of Radiation Medicine, University of Kentucky Medical Center, Lexington, Kentucky 40536 Received 5 November 2001; accepted for publication 20 February 2002; published 3 May 2002 Recently an improved design of a 125 I brachytherapy source has been introduced for interstitial seed implants, particularly for prostate seed implants. This design improves the in situ ultrasound visu- alization of the source compared to the conventional seed. In this project, the TG-43 recommended dosimetric characteristics of the new brachytherapy source have been experimentally determined in Solid Water™ phantom material. The measured dosimetric characteristics of the new source have been compared with data reported in the literature for other source designs. The measured dose rate constant, , in Solid Water was multiplied by 1.05 to extract the dose rate constant in water. The dose rate constant of the new source in water was found to be 0.998% cGyh -1 U -1 . The radial dose function was measured at distances between 0.5 and 10 cm using LiF TLDs in Solid Water™ phantom. The anisotropy function, F ( r ,  ), was measured at distances of 2, 3, 5, and 7 cm. © 2002 American Association of Physicists in Medicine. DOI: 10.1118/1.1470210 Key words: dosimetry, brachytherapy, 125 I, thermoluminescent dosimeter, TLD, TG-43 I. INTRODUCTION Transrectal ultrasound TRUS guided 125 I and 103 Pd seed implants are commonly performed in the United States for the treatment of prostate cancer. It has been proven that the efficacy of this treatment modality is similar to that of radical prostatectomy and it is effective for all Gleason grades of the prostate carcinoma when combined with external beam therapy. 1–7 The number of TRUS-guided implants in the treatment of prostate cancer has greatly increased within the last few years. Accurate intraoperative seed localization plays a critical role in the outcome of this treatment modal- ity. However, the reduced ultrasound signatures of conven- tional seed designs can result in seeds failing to be visual- ized. Recently an innovative design of 125 I brachytherapy source, EchoSeed™ model 6733, has been made commer- cially available by Amersham Amersham plc, Amersham Place, Little Chalfont, Buckinghamshire, HP7 9NA, United Kingdom. This design enhances the visualization of the seeds for interstitial prostate implants. The external surface of this source has several circular grooves, which improves the ultrasound signature of the source over a wider range of angular orientation. As recommended by American Association of Physicists in Medicine 8 the radiation properties of the new sources must be determined using experimental and/or Monte Carlo simu- lations, before its use in clinical applications. The main objective of this project was to determine the dosimetric characteristics of the new 125 I brachytherapy source. These determinations were performed using experi- mental methods following the AAPM recommendations in the TG-43 protocol 9 and the AAPM recommendation on source calibration. 10 These determinations are performed with the latest NIST i.e., after October 8, 2000 calibration standard of the 125 I source. A complete dosimetric evaluation of the new source is presented here. II. MATERIALS AND METHOD A. EchoSeed 125 I Figure 1 shows a schematic diagram of the EchoSeed 125 I source manufactured by Amersham. The source has a physi- cal length of 4.5 mm and an outer diameter of 0.8 mm. This source was manufactured by placing a 3.0 mm long and 0.5 mm diameter of silver wire covered with 125 I inside a 0.05- mm-thick titanum tube with six circular grooves as shown in Fig. 1. The two ends of the cylindrical tube are welded. Model 6733 seeds are available in the activity range of 0.2–5 mCi. B. Dosimetry technique Characteristics of the EchoSeed 125 I source were deter- mined experimentally following the AAPM recommenda- tions published in the TG-43 report. 9 Following this proto- col, the absorbed dose rate distribution around a sealed brachytherapy source can be determined using the following formalism: D ˙  r ,   =  . S K G r ,   G 1 cm,  /2 g  r  F  r ,   , 1 where  is the dose rate constant, G ( r ,  ) is the geometry function, g ( r ) is the radial dose function, F ( r ,  ) is the an- isotropy function. The above-mentioned quantities are de- fined and discussed in detail in TG-43. 9 In the following sections, methods of determination of these parameters are described briefly. 939 939 Med. Phys. 29 „6…, June 2002 0094-2405Õ2002Õ29„6…Õ939Õ4Õ$19.00 © 2002 Am. Assoc. Phys. Med.