Monte Carlo-based optimization of a gamma probe system for sentinel lymph node mapping

Introduction: Sentinel lymph node biopsy (SLNB) is a standard surgical technique to identify sentinel lymph node (SLN) for the staging of early breast cancer. Nowadays, two methods are used for the identification of SLN: blue dye method aiding visually and radioactive dye using gamma detector. A wide range of gamma probe systems with different design and performance are used in intra-operative surgery. The performance of the probes is evaluated by some parameters such as sensitivity, spatial resolution, angular resolution, and shielding efficiency. Methods: In this study, we simulated a gamma probe system, SURGEOGUIDE II based on CsI(Tl) scintillator, a silicon photomultiplier (SiPM), and a tungsten collimator, using the MCNP4C Monte Carlo (MC) method and comparing with experimental measurement. Finally we modeled a series of probe with various crystal material, crystal length, and collimator hole length to evaluate the sensitivity and the spatial resolution in order to propose the optimal configuration. Results:The sensitivity of the system was measured as 2040 cps/MBq in 30 mm distance from the source. The spatial resolution and angular resolution were 43 mm and  at the same distance, respectively. Sensitivity at 30 mm distance from the probe head was the highest for BGO crystal and was the lowest for NaI crystals. The sensitivity and spatial resolution have also been changed by increasing the length of the crystal to a certain amount and then remained constant. Conclusion: The results showed that the best choice for crystal was CdTe and CsI and the best length for CsI crystal in this type of the systems was 10 mm long. Also, based on the specific application, special probe should be designed taking the length of the collimator hole into consideration.