Accuracy of gamma probes in localizing radioactivity: In-vitro assessment and clinical implications

Background: Radioguided surgery (RGS) uses gamma probes to localize radioactive targets. We examined the in-vitro localizing properties of commercially available gamma probes for three common RGS radioisotopes: technetium-99m (Tc-99m), indium-111 (In-111), and iodine-125 (I-125). Methods: The linear and angular localizing properties of five gamma probes were assessed. Radioactive counts in air were obtained at multiple energy threshold settings for each radioisotope. The full width half maximum (FWHM) and angular half maximum (AHM) were calculated to determine localizing acccuracy. Results: The FWHM of the five probes ranged from 1.2 to 3.4 cm for (TC)-T-99m, 1.8 to 2.7 cm for In-111 and 1.6 to 2.8 cm for I-125. AHM of the probes ranged from 20 to 49 degrees for Tc-99m, 14 to 35 degrees for In-111, and 18 to 47 degrees for I-125. The ranking of probe accuracy depended on the radioisotope studied. Absolute count rates varied with energy threshold settings for each probe and radioisotope. Probes required individualized energy threshold settings for optimal performance. Conclusions: The five probes varied in their characteristics for localizing the three radioisotopes. Each probe has particular detection characteristics that may change with different radioisotopes. Differences in radioisotope detection may explain intraoperative variation in localization during lymphatic mapping and sentinel lymph node dissection (LM/SL). Surgeons should be aware of probe performance characteristics before intraoperative use.