A variety of processes can occur in the buccal space, such as infections, developmental lesions, and tumoral processes.
Neoplastic lesions in this area origin most commonly from minor salivary glands, such as pleomorphic adenoma, adenoid cystic carcinoma, acinic cell carcinoma, and mucoepidermoid carcinoma.
The most frequent benign glandular tumor is PA, which consists of both mesodermal and glandular tissue . It has smooth, rounded margins and shows low signal intensity on T1w image, high T2 signal intensity, and no diffusion restriction on DWI.
The most common malignant glandular tumor is adenoid cystic carcinoma. Some authors have suggested that lesions with low or intermediate T2-hyperintensity or invading surrounding tissues tend to be more aggressive, whereas lesions characterized by high signal intensity in T2w sequences have lower cellularity and better prognosis .
Other tumors can originate from connective, muscular, neural, and lymphatic tissue, such as rhabdomyosarcomas and neurofibromas ; rhabdomyosarcomas appear as muscle density masses at CT and are hyperintense relative to muscles on T2w MRI often showing bone destruction , while neurofibromas show low intensity on T1w imaging and high intensity on T2w imaging and are often associated with neurofibromatosis. Single neurofibromas characteristically show the target sign with peripheral hyperintensity .
Usually, in diffusion-weighted MRI, the ADC of malignant neoplasms of the head and neck regions is lower than that of benign tumors, and it is an important tool in guiding the diagnostic process .
Metastatic malignant melanoma is an uncommon head and neck neoplasm. The most frequent localizations of melanoma metastases are lymph nodes (73.6%), lungs (71.3%), liver (58.3%), brain (49.1%), bone (48.6%), heart (47.2%), adrenal glands (46.8%), and gastrointestinal tract (43.5%) .
Melanoma localized in the oral cavity and surrounding anatomical regions, such as the buccal fat pad, accounts for 0.2–8% of the total cases of melanomas of the body . However, most studies focus on primary malignant melanoma, while secondary localization of melanoma in this region has been very rarely reported in the literature, with only one study describing a metastatic melanoma in the tongue and, to the best of our knowledge, no reports at all of the secondary melanomas in the buccal space .
Therefore, based on epidemiologic criteria, it is rather hard to diagnose metastatic melanoma in the buccal space in patients with an unknown primary tumor, as in our case.
Yet, MRI may be helpful in the identification of melanoma metastases .
Two main MRI patterns of melanoma can be identified: a melanocytic and amelanocytic pattern .
Lesions showing a melanocytic pattern, which is the more common one, are characterized by signal hyperintensity in T1w and hypointensity in T2w images. This is due to the presence of melanin and blood products inside the lesion, although some authors argue that the T1 hyperintensity is more relevantly associated with the presence of blood products [21,22,23].
Less commonly, malignant melanoma may show an amelanocytic (and therefore a specific) pattern, characterized by hypo- or isointensity in T1w images and hyper- or isointensity in T2w images , which is associated at histopathological examination to lower quantity of melanin inside the lesion .
Macroscopically, metastatic malignant melanoma may present in different ways, ranging from small rapidly growing lesions which may initially go undetected or misrecognized as artifacts, to showing a military pattern .
In our case, MRI showed a single lesion that exhibited diffusion restriction with ADC of 0.9 and high contrast medium uptake, characteristics suggestive of malignancy as it was proposed in the report.
Moreover, the mass was characterized by T1 hyperintensity and T2 hypointensity, a behavior which may be attributed to the melanoma melanocytic pattern; however, it is not specific, since such features may be ascribed to other paraphysiological and pathological processes, including hemorrhages and proteinic fluid collections.
In this case, radiologists supposed that there was a hemorrhagic component inside the lesion, possibly a residue of the FNAB which had been performed 1 month and a half before. Yet, this was a mistake. In fact, only early subacute hemorrhage dating 3–7 days is featured by T1 hyperintensity and T2 hypointensity, whereas chronic hemorrhages dating more than 1 month are rather characterized by both T1 and T2 hypointensity (26).
Overlooking this aspect and ignoring a part of the anamnesis, radiologists assumed that they were dealing with a primitive buccal space neoplasm, namely with a malignant (minor) salivary gland tumor.
On the contrary, this case teaches that T1 hyperintensity and T2 hypointensity, when associated with other malignancy features in buccal space lesion such as low ADC values and contrast enhancement, should always evoke the suspicion of melanoma, provided that no biopsy was performed and no trauma occurred in the 3–7 days before MRI examination.