An experimental palladium-103 seed (OptiSeedexp) in a biocompatible polymer without a gold marker: characterization of dosimetric parameters including the interseed effect.

Permanent implantation of 125I (iodine) or 103Pd (palladium) sources is a popular treatment option in the management of early stage prostate cancer. New sources are being developed, some of which are being marketed for different clinical applications. A new technique of adjuvant stereotactic permanent seed breast implant, similar to that used in the treatment of prostate cancer, has been proposed by [N. Jansen et al., Int. J. Radiat. Oncol. Biol. Phys. 67, 1052-1058 (2007)] with encouraging results. The presence of artifacts from the metallic seeds, however, can disturb follow-up imaging. The development of plastic seeds has reduced these artifacts. This paper presents a feasibility study of the advantages of palladium-103 seeds, encapsulated with a biocompatible polymer, for future clinical applications, and on the effect of the gold marker on the dosimetric characteristics of such seeds. Experimental palladium seeds, OptiSeedexp, were manufactured by International Brachytherapy (IBt), Seneffe, Belgium, from a biocompatible polymer, including the marker. Apart from the absence of a gold marker, the studied seed has an identical design to the OptiSeed103 [Phys. Med. Biol. 50, 1493-1504 (2005)]; [Appl. Radiat. Isot. 63, 311-321 (2005)]. Polymer encapsulation was preferred by IBt in order to reduce the quantity of radioactive material needed for a given dose rate and to reduce the anisotropy of the radiation field around the seed. In addition, this design is intended to decrease the interseed effects that can occur as a result of the marker and the encapsulation. Dosimetric measurements were performed using LiF thermoluminescent dosimeters (1 mm3) in solid water phantoms (WT1). Measured data were compared to Monte Carlo simulated data in solid water using the MCNP code, version 4C. Updated cross sections [Med. Phys. 30, 701-711 (2003)] were used. As the measured and calculated data were in agreement, Monte Carlo calculations were then performed in liquid water to obtain relevant dosimetric data as required by TG-43U1 recommendations. Comparison of the results with previous studies of OptiSeed103 [Phys. Med. Biol. 50, 1493-1504 (2005)]; [Appl. Radiat. Isot. 63, 311-321 (2005)], and of InterSource103 [Appl. Radiat. Isot. 57, 805-811 (2002)] showed very good agreement for the dose rate constant and for the radial dose function. With respect to the anisotropy function, the relative dose (anisotropy value relative to 90 degrees) from the polymer seed at a distance of 3 cm was close to unity (105%) at 0 degrees, whereas the relative values for the OptiSeed103 with a gold marker and the titanium-encapsulated InterSource103 seed decreased to 70% and 40%, respectively. The interseed effect from one seed was negligible and in the order of calculation uncertainty, making calculation of the dose rate distribution of the studied seeds, according to TG43U1 recommendations, more accurate and closer to reality. This feasibility study shows that due to the low energy of palladium-103, the negligible interseed effect and the reduced artifacts in postimplant medical imaging, this experimental plastic seed would be a good source for breast brachytherapy. This feasibility study was carried out in collaboration with IBt and will be continued with a study of its visibility in different tissues.