Indian Journal or Pure & Applied Physics Vo l. 41 , April 200 3, pp. 253- 25 7 Comparison of calibration procedures of iridium-192 sources used in high dose rate brachytherapy system Alok Kumar *, Sudama Singht & Sushma Agrawal* *Radi ation Oncol ogy, Regio nal Cancer Centre, Indi ra Gandhi In stitute of Medi cal Sciences, Patn a t A N Co ll ege. Patna Rece iv ed 16 Au gust 2002; revised 20 Feb ru ary 200 3; accepted 10 March 2003 Calibrati on or iridium-1 92 hi gh dose rate (HDR ) brachyth erapy sources has been compared by using two independent systems. T wo types or calibrati on systems. we ll -type chamber (SDS), and field dos imeter (0. 6 cc graphite thimbie) with nu cletron calibration jig have been compared. A number of measureme nt s have been perfo rmed to ca li brate the six iri dium- 192 sources according to nati onal /i nte rn ati onal pro tocol during th e clini cal half-li fe of the iridium- 192 so ur ce. All inlluencing factors li ke room-sca tt ering, pos iti onal errors and accur ac y of source pos iti on etc. have been taken care or. bec au se th e accuracy of so urce pos iti on itself within th e applicator can lead to an erro r o r ± 1 .2 % fo r th e radial di stance of I 0 em used in th e ca ii brati on ji g. Taking the 0.6 cc graphite thimble results as reference, perce nt age root mean squ are error (RMS) valu e of 0.33% for well-type chamb er ha s been observed. It has been found th at, both th e ca ii brati on systems are equall y good and can determine accurate and quick source calibration wi th careful measureme nt s. Further, it sho ul d be mand atory to ca li brate th e source, prior to use of th e source for th e treatment of pa tie nt s. [K eywo rd s: lridum- 192 , Braehyth erapy system, Calibratio n] 1 Introduction High d ose rate ( HDR) re mot e after-loading brac hytherap y device is becoming more co mmon world wide. The most c ommon radionuclide u sed in th ese unit s is iridium-192 in the fo rm of a small pe ll e t. Th e most suitable half-life of iridium-192 so urce is 83.825 day s 1 requiring relatively fre quent so urce repl ace ment to maintain short tre atment times. Th e most co mmon supplier of this s ourc e is Mallinckrodt Dia gnostica in the Netherland s. Th e co mpany 's ca libration ce rtific at e prov ided with e ach ne w so urce states an overall un ce rtainty in activity of ±I0%. Th ere for e, it is reco mm ended that , each ti me, when a new HDR source be installed a so urce calibration in the hospital should be ca rried out 2 · 3 . Histori ca ll y, the strength of se aled radi oactive sour ces has been specified, us in g a variety of ph ys ic al quantiti es and units. Radium tub es and nee dl es have traditionally bee n spec ifi ed in te rm s of the actual mass of radium-2 26, which they contain. Wh ereas so urces utiliz in g ra dium substitutes such as, cesium- 1 37 and iridium-1 92 are usually specified in terms of eq uivalent mass of radium when the actua l activ it y co ntained in the so ur ce remams 111 use for radia ti on protec ti on and regulatory purp oses. Ac c ordin gly, the Task Gr oup-41 (Ref. 4) and international guidelin es rec ommended that th ese quantities ar e ab a ndon ed in f avo ur of a single so urce strength sp e cification quantit y (air-ke nn a s trength), which is applicabl e to all photon emttttng brachytherapy s ourc es . Th ey defined the kerma rate to air-in-air at a ref erence distance of I metre 5 Thi s quantity is usually e xpres sed in mG y/h at I m or cGy/h at I m. Th e National Institute of Sc ience and T ec hnology (NIST) does not o ffer c alibr a ti on of ionization chamber with the ga mm a-ray spectrum of iridium-192. Rec omme ndation s of the Radiation ther apy committ ee Ta sk Group No . 43( Re f. 6) has d esc ribed a method to get the ca libration fa ctor fo r iridium-192 poly-energe tic source, which interpolates be tween a ca li bration performed with cobalt- 60 with build-up c ap suffi cient for 1.25 M eV gamma radiation and ortho-volt age X-ray ca librati on with no build-up c ap . Inte rn a ti onal Commis si on of Radiation Units and M eas ur ements (ICRU) reco mmenda ti ons' , w hi ch are also adopted by the American A ssoc ia ti on