Post-traumatic neuroma mimicking lymph node metastasis on ¹⁸F-FDG PET/CT in lung cancer: the value of inter-lesional metabolic discordance

Introduction

Fluorine-18 fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) is widely used in oncologic imaging (1). In patients with known malignancy, multiple lesions are often presumed to represent metastatic disease. However, not all lesions necessarily share the same pathological origin, and benign processes may mimic metastases on conventional imaging. In this context, differences in fluorodeoxyglucose (FDG) uptake among lesions may provide an important diagnostic clue (2).

Inter-lesional metabolic heterogeneity may help identify lesions that represent a pathological process distinct from the known malignancy. Here, we report a case of lung adenosquamous carcinoma in which FDG PET/CT revealed a discordant left axillary lesion with low FDG uptake, subsequently identified as a traumatic neuroma. This case emphasizes that careful recognition of inter-lesional metabolic discordance, combined with clinical history, may help avoid misinterpreting benign lesions as metastases.

Case presentation

All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration and its subsequent amendments. Written informed consent was obtained from the patient for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

A 67-year-old man with left upper-lobe lung adenosquamous carcinoma (initial clinical stage cT4N2M1b, stage IVA) underwent ¹⁸F-FDG PET/CT after four cycles of chemotherapy. The images are shown in Figure 1. The maximum-intensity projection image provides an overview of the whole-body FDG distribution (Figure 1A). The primary pulmonary lesion showed persistent hypermetabolism despite interval size reduction, with a maximum standardized uptake value (SUVmax) of 4.01 on PET, CT, and fused PET/CT images (Figure 1B-1D). Mediastinal and bilateral hilar lymph node metastases demonstrated higher FDG avidity, with an SUVmax of 5.59 (Figure 1E-1G), consistent with active metastatic lung cancer. In contrast, a nodular lesion in the left axilla showed markedly lower FDG uptake, with an SUVmax of 1.63, and remained stable in size (Figure 1H-1J). Given the patient’s history of left upper limb amputation, this discordant metabolic pattern raised suspicion for a non-metastatic lesion. Histopathology confirmed a traumatic neuroma, with hematoxylin and eosin staining showing disorganized nerve fiber proliferation (Figure 2A) and immunohistochemistry demonstrating S-100 positivity in proliferating nerve bundles/Schwann cells (Figure 2B).

Figure 1 ¹⁸F-FDG PET/CT and contrast-enhanced MRI findings in a 67-year-old man with left upper-lobe lung adenosquamous carcinoma after four cycles of chemotherapy. (A) Maximum-intensity projection image showing the whole-body distribution of FDG uptake. (B-D) PET, CT, and fused PET/CT images showing persistent hypermetabolism of the primary pulmonary lesion, with an SUVmax of 4.01. (E-G) PET, CT, and fused PET/CT images showing FDG-avid mediastinal and bilateral hilar lymph node metastases, with an SUVmax of 5.59. (H-J) PET, CT, and fused PET/CT images showing a nodular lesion in the left axilla with lower FDG uptake (SUVmax, 1.63) than the primary pulmonary lesion and the mediastinal and bilateral hilar lesions. (K-N) PET/CT and contrast-enhanced MRI images showing a mildly FDG-avid right renal nodule, with an SUVmax of 2.84. The red arrows indicate different lesions. ¹⁸F-FDG, fluorine-18 fluorodeoxyglucose; MRI, magnetic resonance imaging; PET/CT, positron emission tomography/computed tomography; SUVmax, maximum standardized uptake value.

Figure 2 Histopathological and immunohistochemical findings of the left axillary lesion and right renal lesion. (A) Hematoxylin and eosin staining of the left axillary lesion showing disorganized nerve fiber proliferation, consistent with traumatic neuroma (magnification: 20×). (B) Immunohistochemistry demonstrating S-100 positivity in proliferating nerve bundles/Schwann cells. (C) Hematoxylin and eosin staining of the right renal lesion showing features consistent with clear cell renal cell carcinoma. (D) Immunohistochemistry showing CA9 positivity in the renal tumor. CA9, carbonic anhydrase IX.

Additionally, PET/CT also detected a mildly FDG-avid right renal nodule, with an SUVmax of 2.84, which was further evaluated by contrast-enhanced magnetic resonance imaging (MRI) (Figure 1K-1N). The renal lesion was subsequently confirmed as clear cell renal cell carcinoma by histopathology and carbonic anhydrase IX (CA9) positivity (Figure 2C,2D). Overall, the imaging and pathological findings highlight inter-lesional metabolic heterogeneity in the same patient and illustrate the diagnostic value of recognizing discordant FDG uptake patterns, particularly for avoiding misinterpretation of benign lesions as metastases.

Discussion

In oncologic imaging, multiple lesions detected on ¹⁸F-FDG PET/CT in a patient with known malignancy are frequently interpreted as metastatic disease. However, FDG uptake depends on lesion pathology, and metabolic discordance among lesions has been previously reported as a useful clue to alternative diagnoses, including benign conditions that mimic metastases (3,4). Therefore, metabolic heterogeneity should be interpreted carefully in conjunction with lesion morphology and clinical history.

In our patient, the primary lung tumor and mediastinal and hilar lymph node metastases showed consistently high FDG uptake, representing the expected metabolic phenotype of metastatic lung carcinoma. In contrast, the left axillary lesion demonstrated substantially lower FDG uptake despite being a nodular lesion that had been clinically suspected as metastasis. This discordant metabolic pattern, together with the history of upper limb amputation and lesion stability on prior imaging, raised suspicion for a non-metastatic lesion. Traumatic neuroma is a rare, benign, non-neoplastic proliferation of peripheral nerve tissue that develops after nerve injury, surgery, or amputation. Histologically, it consists of disorganized regenerating nerve fascicles embedded in fibrous or collagenized stroma, with S100-positive Schwann cells. Its imaging appearance is nonspecific, and FDG uptake on PET/CT may be variable, occasionally mimicking nodal or soft-tissue recurrence in patients with malignancy. Previous reports have described traumatic neuroma mimicking metastatic disease after cancer surgery, including FDG-avid axillary traumatic neuroma on PET/CT, highlighting the importance of integrating imaging findings with surgical history to avoid misdiagnosis and unnecessary treatment (5,6). Recognition of this lesion as a distinct pathological process was clinically important because it avoided overstaging and prevented inappropriate classification of the axillary lesion as metastatic disease. In addition, PET/CT also detected an incidental mildly FDG-avid renal lesion, which was subsequently confirmed as clear cell renal cell carcinoma. This finding illustrates the additional value of whole-body PET/CT.

Conclusions

Overall, alternative etiologies should be considered when one lesion demonstrates a metabolic pattern different from that of established malignant lesions. Awareness of inter-lesional metabolic heterogeneity, especially when combined with clinical history and prior imaging, can guide targeted biopsy, avoid overstaging, and improve patient management.

Acknowledgments

None.

Footnote

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2026-1-0344/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Helsinki Declaration and its subsequent amendments. Written informed consent was obtained from the patient for publication of this article and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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