Filtered versus unfiltered technetium sulfur colloid in lymphatic mapping: A significant variable in a pig model

Background: Lymphatic mapping with sentinel node biopsy is becoming a standard diagnostic test for melanoma and is being extensively investigated for use with other soft tissue tumors. Both filtered and unfiltered technetium sulfur colloid (Tc 99) have been used for preoperative lymphoscintigraphy, as well as intraoperative lymphatic mapping, and it is not clear if one is preferable over the other. The purpose of this study was to compare these two preparations to determine whether the form of Tc 99 used affects the results of lymphatic mapping. Methods: Mock skin sites were placed on each extremity of 12 domestic pigs totaling 48 skin sites. Twenty- four of the lesions were injected with unfiltered Tc 99; the remaining 24 were injected with Tc 99 passed over a 0.2-μm filter. Both preparations of Tc 99 were mixed with 1 mL of isosulfan blue before injection. Sentinel node dissection was performed using a gamma probe, with counts recorded over a 10- second period and timed to begin 5 minutes after injection. Results: Sentinel nodes were identified in all 48 lymph node basins draining the mock sites and characterized as hot (10x background), blue, or both. Significantly more sentinel nodes were found in the filtered (105 total, X̄ = 4.4/basin), than in the unfiltered group (total 53, X̄ = 2.2/basin, P <.0001). The filtered group had both a higher number of nodes that were hot (35 vs. 6) and more nodes that were hot and blue (69 vs. 43). In addition, hot secondary level lymph nodes (iliac and deep cervical) were found in 11 of 24 of the basins (46%) in the filtered group compared to 1 of 24 (4%) in the unfiltered group (P <.003). There was no significant difference in injection site or residual basin counts between the two groups, but in vivo counts over the sentinel node sites were significantly lower in the unfiltered group (X̄ = 2670 ± 1829 vs. X̄ = 6027 ± 4333; P = .003). Conclusion: Use of filtered Tc 99 results in more sentinel nodes (both hot/blue and hot non-blue) and a higher proportion of secondary lymph nodes. These findings indicate that the Tc 99 preparation used is a significant variable in the results of lymphatic mapping. It is critical that future clinical studies document which preparation of Tc 99 was used. Only large clinical trials will be able to determine whether the additional nodes found with filtered Tc 99 increase the sensitivity of the technique or merely increase the number of nodes that must be removed unnecessarily.