Investigation of the feasibility of selenium-75 as a viable brachytherapy source
OC-0290
Abstract
Investigation of the feasibility of selenium-75 as a viable brachytherapy source
Authors: Jake Reid1,2, Jonathan Kalinowski3,4, John Munro III5, Andrea Armstrong6, Shirin Enger3,4
1Mcgill University, Oncology, Montreal, Canada; 2Jewish General Hosptial, Lady Davis Institute for Medical Research, Montreal, Canada; 3McGill University, Oncology, Montreal, Canada; 4Jewish General Hospital, Lady Davis Institute for Medical Research, Montreal, Canada; 5Montrose Technology, Inc., N/A, Worcester, USA; 6McMaster University, Nuclear Operations & Facilities, Hamilton, Canada
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Purpose or Objective
Selenium-75 (75Se, t1/2 = 118 days, Eγ,avg = 210 keV) is a radioisotope that is widely used in industrial gamma radiography. Its lower photon energy and longer half life compared to Iridium-192 (192Ir, t1/2 = 74 days, Eγ,avg = 380 keV) make it a viable candidate for use as a brachytherapy source. The goal of this study was to investigate the feasibility of using a 75Se source for brachytherapy applications and investigate its shielding properties combined with a novel rectal applicator developed for intensity modulated brachytherapy.
Material and Methods
A 75Se source (Figure 1a) was designed with its active core (0.65 mm diameter, 7 mm length, 3.7 g/cm3 packed density) encapsulated in a titanium (4.5 g/cm3, 0.90 mm outer diameter, 0.25 mm wall thickness) capsule. The length of the active core was chosen such that it can contain 23 Ci of 75Se which gives a dose rate equivalent of 10 Ci 192Ir. The AAPM TG-43U1 brachytherapy dosimetry parameters were calculated for the 75Se using RapidBrachyMCTPS, which is a Monte Carlo based treatment planning system. RapidBrachyMCTPS, was further used to acquire dose distributions in a water phantom. Four different scenarios were simulated where a novel rectal applicator was combined with 3 types of rotating tungsten shields. These shields were developed for intensity modulated brachytherapy and include two rigid shields and a flexible shield. A no shield scenario was also simulated. The results were used to calculate the transmission factors (TF) for the different shield models. The results were compared with simulations performed with a 192Ir source.
Results
The radial dose function and 2D anisotropy function were calculated and plotted in comparison with 192Ir as presented in Figure 1b and 1c. The air kerma strength per unit activity and dose rate constant were calculated for 75Se to be 4.751 +/- 0.005 x 10^-8 U/Bq and 1.116 +/- 0.001 cm^-2 respectively and for 192Ir to be 9.79 +/- 0.01 x 10^-8 U/Bq and 1.110 +/- 0.001 cm^-2 respectively. Dose distributions in a water phantom were calculated and the dose colour maps for all scenarios can be seen in Figure 2a-e. For the rigid shields, 75Se had TF values of 2.7 +/- 0.5 % and 2.3 +/- 0.7 %, and 192Ir had TF values of 15.15 +/- 0.05 % and 13.2 +/- 0.2 %. For the flexible shield, 75Se and 192Ir had TF values of 16.0 +/- 0.5 % and 31.4 +/- 0.1 % respectively. This displayed that 75Se had 5.61 and 5.74 times better attenuation than 192Ir with the rigid shields and 1.96 times better with the flexible shield.
Conclusion
The designed 75Se source was superior with regards to attenuation through tungsten shields due to its lower energy while still being able to produce an equivalent dose rate to 192Ir. These results allow for justification of further analysis of this source for use in conventional brachytherapy and intensity modulated brachytherapy as it is expected to deliver the same absorbed dose to the tumours as 192Ir with similar treatment times while reducing the dose to surrounding organs at risk.