Optimization of Parallel-Hole Collimators for Intraoperative Localization of Iodine-125 Seeds

In the treatment of breast cancer, breast-conserving surgery (BCS) may result in incomplete excision of the tumor. In order to improve the outcome of this surgery, iodine-125 seeds may be inserted in the tumor under mammographic or ultrasonic guidance prior to surgery. These seeds provide intraoperative information about the location of the tumor. A system of two gamma camera heads and two parallel-hole collimators was recently proposed to record projection images from which the location of a single radioactive seed can be derived. In the present study, computer simulations were used to evaluate the influence of two standard and 95 non-standard collimators on the accuracy and precision with which this location can be derived. Simulation results indicate that this location can be derived with accuracies ± precisions of 5.2 mm ± 1.8 mm and 5.6 mm ± 2.1 mm if the system is equipped with standard collimators. Simulation results also indicate that this location can be derived with an accuracy ± precision of 4.9 mm ± 1.7 mm if the system is equipped with non-standard collimators (each of these collimators had a resolution of 5 cm at a collimator-source distance of 75 cm). These results were observed when the signal from the radioactive seed with an activity of 7.4 MBq was recorded at a collimator-source distance of 75 cm with an imaging time of 2.5 s (this seed was positioned within a 6.5 cm thick block phantom). In conclusion, these results indicate that the accuracy with which the 3D location of a single radioactive seed can be established during BCS by this gamma camera system cannot be significantly improved if non-standard collimators are used instead of standard collimators.