Attenuation of intracavitary applicators in 192Ir-HDR brachytherapy.
Unlike the penetrating monoenergetic 662 keV gamma rays emitted by 137Cs LDR sources, the spectrum of 192Ir used in HDR brachytherapy contains low-energy components. Since these are selectively absorbed by the high-atomic number materials of which intracavitary applicators are made, the traditional neglect of applicator attenuation can lead to appreciable dose errors. We investigated the attenuation effects of a uterine applicator, and of a set of commonly used vaginal cylinders. The uterine applicator consists of a stainless steel source guide tube with a wall thickness of 0.5 mm and a density of 8.02 g/cm3, whereas the vaginal cylinders consist of the same stainless steel tube plus concentric polysulfone cylinders with a radius of 1 or 2 cm and a density of 1.40 g/cm3. Monte Carlo simulations were performed to compute dose distributions for a bare 192Ir-HDR source, and for the same source located within the applicators. Relative measurements of applicator attenuation using ion-chambers (0.125 cm3) confirmed the Monte Carlo results within 0.5%. We found that the neglect of the applicator attenuation overestimates the dose along the transverse plane by up to 3.5%. At oblique angles, the longer photon path within applicators worsens the error. We defined attenuation-corrected radial dose and anisotropy functions, and applied them to a treatment having multiple dwell positions inside a vaginal cylinder.
Department of Radiation Oncology, University of Alabama School of Medicine, 1824 6th Avenue South, Birmingham, Alabama 35294, USA. sye@uab.edu
Sung-Joon
SJ
Ye
Ivan A
IA
Brezovich
Sui
S
Shen
Jun
J
Duan
Richard A
RA
Popple
Prem N
PN
Pareek
D000465:Algorithms; D016477:Artifacts; D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods*; D003198:Computer Simulation; D019544:Equipment Failure Analysis / Q000379:methods; D005260:Female; D005833:Genital Neoplasms, Female / Q000532:radiotherapy*; D006801:Humans; D007496:Iridium Radioisotopes / Q000032:analysis* / Q000627:therapeutic use*; D008954:Models, Biological; D009010:Monte Carlo Method; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity
D007496:Iridium Radioisotopes