Comparison of measured and calculated dose rates in water near I-125 and Ir-192 seeds

Journal Article · Mon Jul 01 04:00:00 EDT 1991 · Medical Physics; (United States)

DOI:https://doi.org/10.1118/1.596631· OSTI ID:5308008

Williamson, J F ^[1]

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1. Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110 (US)

Recent theoretical and experimental work indicates that currently accepted {sup 125}I dosimetry data may overestimate dose in water at 1 cm by 10%--24%. Among the most comprehensive measurements are those of the NCI-sponsored brachytherapy contract participants. Absolute dose rates in water calculated by the Monte Carlo method have been compared with the NCI dose measurements about {sup 125}I and {sup 192}Ir seeds embedded in solid-water phantoms. The photon transport code allows realistic geometric simulation of the complex internal seed structure, the National Institute of Standards and Technology air-kerma strength standardization geometry, and the dose measurement setup. When the appropriate measurement medium and geometry are assumed, agreement between theory and measurement is excellent, within 3% at 1 cm and averaging 3% at larger distances. However, the data do not support the water equivalence of solid water at {sup 125}I energies indicating that solid-water measurements underestimate {sup 125}I specific dose-rate constants in water by 4.3%. Because of its higher ratio of absorption to scatter, {sup 125}I dose distributions measured in solid water are less penetrating (by 35% at 10 cm) than those measured in liquid water. For model 6711, model 6702, and steel-clad {sup 192}Ir seeds, Monte Carlo calculations yielded specific dose-rate constants (assuming liquid water medium) of 0.877, 0.932, and 1.122 cGy cm{sup 2} h{sup {minus}1} per unit air-kerma strength, respectively. For {sup 125}I, currently accepted values are 18% and 11% larger for the two seed models.

• OSTI ID: 5308008
• Journal Information: Medical Physics; (United States), Journal Name: Medical Physics; (United States) Vol. 18:4; ISSN MPHYA; ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
655003* -- Medical Physics-- Dosimetry
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
BETA-PLUS DECAY RADIOISOTOPES
DAYS LIVING RADIOISOTOPES
DOSE RATES
DOSIMETRY
ELECTRON CAPTURE RADIOISOTOPES
GAMMA DOSIMETRY
HEAVY NUCLEI
HYDROGEN COMPOUNDS
INTERMEDIATE MASS NUCLEI
INTERNAL CONVERSION RADIOISOTOPES
IODINE 125
IODINE ISOTOPES
IRIDIUM 192
IRIDIUM ISOTOPES
ISOMERIC TRANSITION
ISOTOPES
MOCKUP
MONTE CARLO METHOD
NUCLEI
ODD-EVEN NUCLEI
OXYGEN COMPOUNDS
PHANTOMS
RADIOISOTOPES
STRUCTURAL MODELS
WATER