Unexpected black-stained lymph nodes: A case report.

1
Introduction
Gastric cancer (GC) is one of the common malignancies with high rates of incidence, mortality, and lymph node metastasis [1]. Currently, surgical treatment is the primary therapeutic approach for GC, and lymph node dissection is a crucial component of radical gastrectomy. Adequate lymphadenectomy is closely linked to the postoperative pathological stage and subsequent treatment, making it key to improving prognosis [2]. However, due to the complex peri-gastric lymphatic system, it is difficult to accurately dissect lymph nodes based solely on physical palpation or clinical experience. To achieve accurate lymph node dissection, researchers introduced the concepts of sentinel lymph nodes and lymph node basins and selected substances with strong lymphatic tropism to mark the lymph nodes [3].
Currently, the commonly used lymph node tracers include indocyanine green, radioactive colloids, blue dyes (methylene blue, isosulfan blue, and patent blue), and nano‑carbon. These tracers are injected around the tumor 24 h before surgery or during surgery to achieve effective lymph node tracing [4]. Although these tracers have the advantages of being simple and cost-effective, they also have certain drawbacks. Therefore, it is necessary to further explore new tracers. Here, we report a case where black-stained lymph nodes were discovered around the tumor following oral administration of bismuth. Postoperative pathology and ICP-MS confirmed that bismuth compounds caused the lymph nodes to turn black. Our case demonstrates that bismuth compounds hold promise as a potential novel lymph node tracer.
The work has been reported in line with the SCARE criteria [5].

2
Presentation of case
A 37-year-old man was admitted to our department, complaining of paroxysmal epigastric distention and pain with belching. The patient described the symptoms had started three months earlier which made him become increasingly concerned. As the symptoms worsened over time, he sought medical help. According to the American Joint Committee on Cancer (AJCC) 8th Edition Tumor-Lymph Node-Metastasis (TNM) system, the patient's preoperative computed tomography (CT) examination indicated that the TNM stage of the tumor was T3N1-2 M0 (Fig. 1). Gastroscopy revealed an ulcer on the posterior wall of the gastric antrum, approximately 0.5 cm × 0.6 cm in size (Fig. 1). A biopsy suggested signet ring cell carcinoma. The patient's laboratory tests showed no abnormalities in various indicators. During preoperative discussion, patient expressed concerns but showed a willingness to proceed with the recommended treatment. The patient then underwent a distal gastrectomy with D2 lymph node dissection. During lymph node dissection, black-stained lymph nodes were found in lymph node stations 1, 7–9, and 12a (Fig. 1). Interestingly, the patient had not used any lymph node tracer preoperatively. We then performed a crude sorting of the lymph nodes to follow up with pathological findings. Postoperative pathology suggested that the histological type of the tumor was low adhesion carcinoma with signet ring cell components, and the TNM stage was T4aN2M0. In total, 23 lymph nodes were detected, including 5 metastatic lymph nodes and 7 black-stained lymph nodes (Table 1). Unfortunately, no metastatic lymph nodes were found among the black-stained lymph nodes. The postoperative pathology indicated the presence of black particles in the black-stained lymph nodes. To clarify the nature of the black particles, the pathologist performed special staining. The results showed that calcium staining and copper staining were negative, while iron staining was weakly positive (Fig. 2). These findings did not clarify the nature of the particles. Subsequently, we informed the patient the pathological results. The patient was quite anxious but expressed a willingness to undergo further treatment. Then we discussed the finding of the black-stained lymph nodes, the patient was very surprised. After obtaining the patient's consent, ICP-MS was performed. The ICP-MS results revealed a high concentration of bismuth in the black-stained lymph nodes (Fig. 2). We then reviewed the patient's medical history. While his past medical history was unremarkable, his medication history indicated that he had self-administered bismuth potassium citrate for 2 months prior to hospitalization. Considering this medication history, the black particles were likely some form of bismuth compound. Then we informed the above results to the patient. Upon learning that the black particles were likely from the bismuth compound, the patient felt relieved that the particles were not cancer-related. The patient attended all scheduled follow-up visits, and expressed high satisfaction with the treatment.

3
Discussion
At present, lymph node tracing technology is widely used in radical gastrectomy for gastric cancer, mainly involving the application of a single tracer or a combination of dual tracers [6]. Commonly used tracers include radioactive colloid, indocyanine green, nano‑carbon, and blue dyes (methylene blue, isosulfan blue, and patent blue). Currently, the gold standard for detecting lymph nodes is a combination of radioactive colloid and blue dye tracing technology. Studies from different centers have shown that, compared to traditional surgery, surgery combined with lymph node tracing technology can effectively increase the number of lymph nodes detected, raise the detection rate of positive lymph nodes, reduce intraoperative blood loss, ensure accurate pathological staging, and even improve patient prognosis without increasing the incidence of complications [[7], [8], [9], [10]]. Li et al. have detailed the progress in the application of these tracers but did not elaborate on the future prospects of lymph node tracers [6]. In our case, the deposition of bismuth compounds in perigastric lymph nodes suggests potential future development directions for lymph node tracers. Currently, the clinical application of tracers is invasive, which greatly increases the physical and psychological burden on patients. Improving tracer application is one of the future development directions. Researchers have begun exploring oral tracers. Matsumoto et al. administered 5-aminolevulinic acid orally to detect lymph nodes [11]. However, due to the poor penetration of blue light, its effectiveness needs further verification. In this case, the patient achieved lymph node staining by taking oral medication, suggesting the possibility of bismuth compounds as oral tracers. Additionally, the compound was black, making it easy to identify without the need for additional equipment. Furthermore, it achieved an effect similar to nano‑carbon, avoiding the risk of interfering with the surgical field of view.
On the other hand, most lymph node tracers rely on their physical properties and lack targeting capabilities. To address this, new nanoprobes have been developed, such as magnetic probes, exosome probes, and arginine-glycine-aspartate integrated indocyanine green probes [12]. During surgery, we found black-stained lymph nodes only around the stomach, suggesting that the compound may have a special mechanism for targeting perigastric lymph nodes. By observing hematoxylin–eosin (HE) stained histological sections of the black-stained lymph nodes, we found that the distribution of particles was similar to that of macrophages. We hypothesize that bismuth potassium citrate adheres to the stomach wall and is engulfed by macrophages, where it reacts with certain sulfides to produce black bismuth sulfide. Later, the macrophages migrate and accumulate in lymph nodes [13]. This provides a new approach for discovering and developing targeted tracers.
Furthermore, improving imaging methods for tracers is another future development direction. Bismuth is a heavy metal, and its compounds have the potential to become contrast agents. It could perhaps be combined with imaging modalities like CT to detect lymph nodes or even for preoperative lymph node staging. Liu once reported a case where an overdose of bismuth potassium citrate resulted in high-density imaging of the colon, suggesting that bismuth compounds may become lymph node contrast agents [14]. Our case suggests the potential for a new type of lymph node tracer, but there is still a long way to go. However, it is important to note that the black-stained lymph nodes in this case were undoubtedly an adverse consequence of long-term bismuth potassium citrate use. This reminds us to avoid overdosing and minimize adverse outcomes in clinical practice. In conclusion, our case provides new possibilities for the future development of lymph node tracers in radical gastrectomy. At the same time, we must be cautious about the adverse effects of overdose in clinical practice.

4
Conclusion
In conclusion, our case highlights a new potential avenue for the development of lymph node tracers in radical gastrectomy. However, it is essential to remain vigilant regarding the possible adverse consequences of overdose in clinical practice.

5
Methods
The work has been reported in line with the SCARE criteria [5].

Author contribution
H.C. and Y.H. manuscript drafting, Data collection, manuscript drafting. Y.H. Study design, data collection, manuscript drafting. M.W. and F.W. Study design, manuscript supervision, manuscript review. L.T. Study design, manuscript supervision.

Consent
Written informed consent was obtained from the patient for publication and any accompanying images. A copy of the written consent is available for review by the Editor-in- Chief of this journal on request.

Ethical approval
This paper was exempt from ethical approval because it does not include any personal information inside it.

Guarantor
Meng Wang.

Research registration number
Not required.

Funding
This work was supported by China Medical Foundation (2023100801).

Declaration of competing interest
The authors declare that there are no conflicts of interest related to this work.