Dosimetric study of a new polymer encapsulated palladium-103 seed

The use of low-energy photon emitters for brachytherapy applications, as in the treatment of prostate or ocular tumours, has increased significantly over the last few years. Several new seed models utilizing Pd-103 and I-125 have recently been introduced. Following the TG43U1 recommendations of the AAPM (American Association of Physicists in Medicine) (Rivard et al 2004 Med. Phys. 31 633), dose distributions around these low-energy photon emitters are characterized by the dose rate constant, the radial dose function and the anisotropy function in water. These functions and constants can be measured for each new seed in a solid phantom (i.e. solid water such as WT1) using high spatial resolution detectors such as very small thermoluminescent detectors. These experimental results in solid water must then be converted into liquid water by using Monte Carlo simulations. This paper presents the dosimetric parameters of a new palladium seed, OptiSeed(TM) (produced by International Brachytherapy (IBt), Seneffe, Belgium), made with a biocompatible polymeric shell and with a design that differs from the hollow titanium encapsulated seed, InterSource(103), produced by the same company. A polymer encapsulation was chosen by the company IBt in order to reduce the quantity of radioactive material needed for a given dose rate, and to improve the symmetry of the radiation field around the seed. The necessary experimental data were obtained by measurements with LiF thermoluminescent dosimeters (1 mm(3)) in a solid water phantom (WT1) and then converted to values in liquid water using Monte Carlo calculations (MCNP-4C). Comparison of the results with a previous study by Reniers et al (2002 Appl. Radiat. Isot. 57 805) shows very good agreement for the dose rate constant and for the radial dose function. In addition, the results also indicate an improvement in isotropy compared to a conventional titanium encapsulated seed. The relative dose (anisotropy value relative to 90 degrees) from the seed at a distance of 3 cm is close to 70% at 0 degrees whereas that for the titanium encapsulated InterSource(103) seed is close to 40%. This paper also presents some new Monte Carlo calculations relating to shadowing produced by the seeds in an array implanted for a prostate cancer treatment. Recently, Mobit and Badragan (2004 Phys. Med. Biol. 49 317 1) reported shadowing resulting in a 10% decrease in dose from titanium encapsulated 1251 seed. We used Monte Carlo simulations (MCNP-4C) to evaluate shadowing for the InterSource(103) titanium encapsulated seed and the OptiSeed(TM) polymer encapsulated seed. For a specific geometry specified, dose decreases of 13% and 7% were found for the InterSource(103) titanium encapsulated and the OptiSeed(TM) polymer encapsulated seed, respectively.