We found that SLN nonvisualization occurred in 27.7% of procedures on planar lymphoscintigraphy at 2 h after an intratumoral injection of technetium-99m albumin nanocolloid. This number decreased to 25.1% on late lymphoscintigraphy at 4 h pi. Periareolar reinjection reduced the SLN nonvisualization rate to 9.4% of the total SLN procedures. In addition we showed that the choice of radiotracer does not have an impact on the SLN nonvisualization rate.
To the best of our knowledge, this study is the first to examine the influence of experience of the preparer and administrator of the radiotracer on SLN nonvisualization, and we show that the experience of the preparer or administrator are not associated with SLN nonvisualization. This observation may contribute to the generally accepted view that there are no significant differences in interpretations made by radiology residents and those made by staff radiologist [8–10]. Our results support the notion that SLN detection on lymphoscintigraphy is a very robust technique, that does not depend on the experience of the preparer or administrator of the radiotracer.
We could not find any association between experience of the preparer of the radiotracer and SLN nonvisualization. This was also expected because the preparations of the radiotracers were in full accordance with the recommendation of the “Guideline on current good radiopharmacy practice for the small-scale preparation of radiopharmaceuticals” . The results of this study do not show any association between injected dose or volume of the radiotracer and SLN nonvisualizations. Although the injected volume of tracer solution is a subject of controversy in literature, detection rates of SLN visualizations seem not to be affected by these solution volumes . Tanis et al. showed that a higher amount of radioactivity is associated with less SLN nonvisualizations and recommended a dose of at least 100 MBq of the radiotracer . In our study population, 99.6% of patients received a dose of 100 MBq or more. We could not find a significant association between the used doses and SLN non-visualization.
We found that risk factors for SLN nonvisualization on lymphoscintigraphy at 4 h pi are age ≥ 70 years, BMI ≥ 30 kg/m2, and nonpalpable tumors. These risk factors are in accordance with findings of other studies. Increased age [2–7] and higher BMI [3–6] are well known risk factors for SLN nonvisualization on lymphoscintigraphy. It has been hypothesized that replacement of lymph nodes by fatty tissue decreases the capacity of lymph nodes to retain the radioactive colloid  and that increased fatty tissue in elderly patients causes decreased lymphatic flow in the breasts .
Nonpalpable tumors are less known as a risk factor for SLN nonvisualization . Deeper located tumor are more often labeled as nonpalpable tumors because they are less accessible by palpation. Anatomical studies have shown that the density of lymphatic vessels in the skin is greater compared to breast parenchyma . This difference in density of lymphatic vessels is perhaps the reason why nonpalpable tumors are associated with an increased risk for SLN nonvisualization. Whether tumor location is a risk factor for SLN nonvisualization [3, 6], is still disputable. We and other studies could not find a significant effect of tumor location on SLN nonvisualization [2, 4, 5, 7].
The SLN visualization rate improved to 67.5% of the initial nonvisualized SLN after reinjection. This is comparable with Pouw et al. who found a SLN visualization rate of 62.1% after reinjection . In our study population, the reinjection of the radiotracer increased the visualization rate of the SLN from 74.9–90.6%. This shows that reinjection is an adequate option to improve the rate of SLN visualization for nuclear medicine departments which are flexible enough to apply an additional injection and imaging slots.
The strength of this study is the large number of patients with lymphoscintigraphy data. In addition our analysis included time corrected activity doses of the radiotracer, experience of the preparer and administrator, which was not studied before. However, this study has several limitations that need to be addressed. As patients received a fixed dose of the radiotracer (i.e., no correction for BMI) weight and height measurements were not available in all patients. Despite this limitation the number of the patients in whom weight and height were registered had sufficient statistical power to examine the effect of BMI on SLN nonvisualization. Other limitations were that some characteristics of the tumor (stage, size) and lymph node (exact status, number of positive lymph nodes) were not available. These factors could be confounders, since some studies have indications that these factors are possible associated with SLN nonvisualization [2, 3, 6].