Case report: MRI and CT imaging features of a melanocytic tumour affecting a cervical vertebra in an adult dog, and review of differential diagnosis for T1W-hyperintense lesions

Elli Elizabeth Michaelidou Adriana Kaczmarska Rodrigo Gutierrez-Quintana Joanna Morris Gawain Hammond† Ana Cloquell

Background

The study focuses on a rare case of a melanocytic tumor affecting the cervical vertebra in a 7-year-old Lhasa Apso dog. This tumor was characterized by its distinctive imaging features on MRI and CT scans, which were critical in its diagnosis and management. The primary interest in this case stems from the rarity of primary melanocytic tumors in the central nervous system (CNS) of dogs, with metastatic instances being more commonly observed. The report aims to document these unique imaging characteristics and contribute to the limited literature on similar canine cases.

Methods

The diagnostic approach included MRI and CT imaging to characterize the lesion's properties and guide treatment planning. MRI used a 1.5-Tesla system, revealing a hyperintense T1-weighted and isointense T2-weighted extradural mass at the left lamina of the C4 vertebra. The CT scans were performed using an 80-slice helical scanner, further detailing the lesion's location and involvement with surrounding structures. Treatment involved surgical debulking followed by radiotherapy and melanoma vaccine administration.

Results

The mass demonstrated T1-weighted hyperintensity and T2-weighted isointensity, common in melanocytic lesions due to melanin’s paramagnetic properties. Histopathological examination confirmed the diagnosis of a malignant melanocytic tumor. Despite initial successful management, the dog eventually exhibited recurrence of symptoms 18 months later, leading to euthanasia. Notably, this case represents a rare instance of a melanocytic tumor at this location, with detailed documentation of the MRI and CT imaging characteristics.

Review of T1 Hyperintense Lesions:

1. Melanocytic Tumors: These include primary melanocytic neoplasms like melanocytoma, melanoma, and melanocytosis. These tumors typically show T1 hyperintensity due to the paramagnetic properties of melanin.

2. Lipid-containing Lesions: These could initially be considered due to the hyperintense nature on T1-weighted images; however, the absence of signal suppression on fat-saturation sequences as observed in this case rules them out.

3. Methemoglobin-containing Lesions: Lesions containing methemoglobin, often due to previous hemorrhage, can also appear hyperintense on T1-weighted images. In this particular case, no hemorrhage was indicated by the imaging characteristics.

4. Proteinaceous Cysts: High protein content can result in T1 hyperintensity. These are typically evaluated further with additional MRI sequences to confirm the presence of proteins.

5. Calcification or Mineralization: Calcified lesions can sometimes appear hyperintense on T1. The use of T2* sequences helps evaluate this possibility by identifying signal voids caused by calcification, which were not present in this case.

6. Inflammatory or Infectious Processes: Certain infections or inflammatory conditions can alter the typical appearance of tissues on MRI scans, potentially leading to T1 hyperintensity.

7. Schwannomas, particularly Melanotic Schwannomas: These are tumors arising from Schwann cells that sometimes contain melanin, which could explain the T1 hyperintensity. The differentiation between melanotic schwannoma and other melanocytic lesions often requires histopathological examination.

8. Other Neoplasms: Various other neoplasms could theoretically mimic this appearance if associated with hemorrhage, high protein content, or melanin.

Limitations

The case report acknowledges that definitive differentiation between a melanotic schwannoma and a metastatic melanoma of unknown primary was not possible based solely on imaging and surgical findings. Also, further investigations were declined after the recurrence of symptoms, limiting the understanding of the tumor's long-term behavior post-treatment.

Conclusions

This report contributes significant insights into the diagnostic imaging characteristics and treatment responses of melanocytic tumors in canine cervical vertebrae. It underscores the importance of comprehensive imaging in diagnosing and managing rare neoplasms and highlights the potential for these tumors to recur despite aggressive treatment. The study also suggests avenues for further research, particularly in improving diagnostic differentiation between similar melanocytic conditions.

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