Direct determination of the absorbed dose to water from I-125 low dose-rate brachytherapy seeds using the new absorbed dose primary standard developed at ENEA-INMRI

作者：Toni M P^*; Pimpinella M; Pinto M; Quini M; Cappadozzi G; Silvestri C; Bottauscio O

来源：Metrologia, 2012, 49(5): S193-S197.

DOI：10.1088/0026-1394/49/5/S193

Low-intensity radioactive sources emitting low-energy photons are used in the clinic for low dose-rate brachytherapy treatments of tumours. The dosimetry of these sources is based on reference air kerma rate measurements. The absorbed dose rate to water at the reference depth d(0) = 1 cm, (D) over dot(w, 1 cm), is then obtained by a conversion procedure with a large relative standard uncertainty of about 5%. This paper describes a primary standard developed at ENEA-INMRI to directly measure (D) over dot(w, 1 cm) due to LDR sources. The standard is based on a large-angle and variable-volume ionization chamber, embedded in a graphite phantom and operating under 'wall-less air chamber' conditions. A set of correction and conversion factors, based on experiments and Monte Carlo simulations, are determined to obtain the value of D-w,D- 1 cm from measurements of increment of ionization current with increasing chamber volume. The relative standard uncertainty on (D) over dot(w, 1 cm) is 2.6%, which is appreciably lower than the current uncertainty. Characteristics of the standard, its associated uncertainty budget, and some experimental results are given for I-125 BEBIG I25.S16.C brachytherapy seeds. Finally, results of the experimental determination of the dose-rate constant Lambda(1 cm), traceable to the D-w,D- 1 cm and the low-energy air kerma ENEA-INMRI standards, are given. The relative standard uncertainty on Lambda(1 cm) is 2.9%, appreciably lower than the typical uncertainty (4.8%) of the values available in the literature.

• 出版日期2012-10