Sinonasal melanoma is a rare and aggressive malignant neoplasm arising from melanocytes in the nasal cavity and paranasal sinus mucosa. It constitutes only 1% of all melanomas and approximately 3-5% of sinonasal malignancies. It most commonly occurs in the nasal cavity (septum and inferior turbinate), with paranasal sinus primary localization being rarer. Due to melanin production, it shows characteristic signal features on MRI: hyperintense on T1, hypointense on T2 — resulting from paramagnetic melanin granules shortening T1 and T2 relaxation times. The amelanotic variant (10-30%) may not show these classic signal features and poses diagnostic challenges. Clinically, it follows an aggressive course: local recurrence rate 60-80%, distant metastasis rate 40-50%, and 5-year survival 20-35%. Treatment involves wide surgical resection + adjuvant radiotherapy; immunotherapy (ipilimumab, nivolumab, pembrolizumab) is increasingly used in metastatic disease.
Age Range
50-80
Peak Age
65
Gender
Equal
Prevalence
Sinonasal melanoma arises from melanocytes (neural crest-derived pigment-producing cells) present in the nasal and sinus mucosa. These melanocytes embryologically migrate from the neural crest and settle in the sinonasal mucosa, normally present in small numbers at the basement membrane level. NRAS mutations (20-30%), KIT mutations (15-25%), and BRAF mutations (much lower than cutaneous melanoma — <5%) play roles in malignant transformation; the relatively high rate of KIT mutation molecularly distinguishes mucosal melanomas from cutaneous melanomas and creates a treatment option with c-kit inhibitors (imatinib). Melanin synthesis occurs through conversion of tyrosine to DOPA and then to melanin polymer via the tyrosinase enzyme; melanin granules (melanosomes) contain high concentrations of free radicals and paramagnetic metal ions (Fe³⁺, Cu²⁺) — this paramagnetic property shortens T1 and T2 relaxation times, creating T1 hyperintensity and T2 hypointensity on MRI. In amelanotic melanoma, melanin synthesis is minimal, causing the loss of these classic signal features and the tumor shows non-specific signal. The aggressive biological behavior of sinonasal melanoma results from high mitotic activity, early lymphatic and hematogenous metastasis capacity, and tendency to invade surrounding tissues.
A nasal cavity mass showing hyperintense signal on non-contrast T1 and hypointense signal on T2 reflects the paramagnetic effect of melanin content and is considered pathognomonic for sinonasal melanoma. This dual signal signature is the strongest MRI finding in differentiating from all other tumor types in the sinonasal region. Amelanotic melanoma (10-30%) does not show this signal characteristic.
On non-contrast T1-weighted sequences, melanotic melanoma characteristically shows hyperintense or intermediate-hyperintense signal. This T1 shortening results from paramagnetic ions (Fe³⁺, Cu²⁺, Mn²⁺) and stable free radicals in melanin granules. The degree of T1 hyperintensity is directly proportional to melanin content: tumors with dense melanin show markedly hyperintense signal, those with less melanin show mildly hyperintense or intermediate signal. This signal feature is a highly specific finding for sinonasal melanoma and is not seen in other sinonasal tumors (except hemorrhage). In amelanotic melanoma (10-30%), T1 hyperintensity is not seen due to minimal melanin content and the tumor may be confused with SCC or lymphoma. Because intrinsic T1 hyperintensity may be confused with enhancement on contrast-enhanced T1 sequences, a non-fat-suppressed non-contrast T1 sequence must be obtained.
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On non-contrast T1-weighted sequences, markedly hyperintense signal is noted in the lesion, consistent with melanin content and strongly suggesting the diagnosis of melanoma.
On T2-weighted sequences, melanotic melanoma characteristically shows hypointense signal. This T2 shortening results from the paramagnetic effect of melanin granules and together with T1 hyperintensity creates the 'dual signal signature' (T1 bright + T2 dark) of melanoma. The degree of T2 hypointensity is proportional to melanin concentration. In amelanotic melanoma, T2 hypointensity is not seen and the tumor may show intermediate or slightly hyperintense T2 signal — this may be confused with SCC, inverted papilloma, or lymphoma. Hemorrhage areas within the tumor (met-hemoglobin — subacute hemorrhage) may also show T1 hyperintense and T2 variable signal, which can be confused with melanin signal; gradient-echo (GRE) sequence helps in this differentiation by detecting hemosiderin deposition. Intratumoral necrosis or cystic degeneration areas may be hyperintense on T2, creating heterogeneous appearance against the overall T2 hypointense background.
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On T2-weighted sequences, markedly hypointense signal is noted in the lesion, which when evaluated together with the hyperintensity on non-contrast T1 is consistent with paramagnetic effect from melanin content.
On CT, sinonasal melanoma appears as a polypoid or lobulated soft tissue density (35-55 HU) mass in the nasal cavity. The mass typically originates from the nasal septum or inferior turbinate. Bone destruction is common and may be aggressive — erosion/destruction may be seen in the septum, turbinate bone, lamina papyracea, and medial wall of the maxillary sinus. CT density is not specific and cannot be distinguished from SCC or other malignancies; melanin does not create a density difference on CT. Moderate to marked heterogeneous enhancement is seen on contrast-enhanced CT. Intratumoral hemorrhage may appear as high-density foci (50-70 HU) on CT. CT is used for bone anatomy assessment and preliminary evaluation of intracranial/orbital extension but cannot replace MRI in tissue characterization.
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A polypoid mass of soft tissue density originating from the septum/inferior turbinate is noted in the left/right nasal cavity with destruction of surrounding bony structures.
On DWI, sinonasal melanoma shows marked diffusion restriction (ADC typically 0.6-0.9 × 10⁻³ mm²/s). These low ADC values reflect the high cellularity of the tumor. The hyperintense appearance of melanoma on DWI can be affected by both diffusion restriction and the T2 shortening effect of melanin. The ADC map differentiates true diffusion restriction from T2 effects. DWI helps differentiate melanoma from surrounding normal tissue, obstructed secretions, and postoperative changes. Amelanotic melanoma also shows similar ADC values because ADC depends on cellularity rather than melanin content — therefore DWI is particularly valuable in diagnosing amelanotic melanoma.
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Marked diffusion restriction is noted in the lesion on DWI with low signal on the ADC map (ADC: approximately ... × 10⁻³ mm²/s); this finding is consistent with high cellularity.
On fat-suppressed contrast-enhanced T1 sequences, sinonasal melanoma shows heterogeneous solid enhancement. Enhancement evaluation can be complicated by intrinsic T1 hyperintensity in melanotic melanoma: since the hyperintense signal on non-contrast T1 can be confused with enhancement, pre-contrast and post-contrast T1 comparison is mandatory. Subtraction images (post-contrast T1 − pre-contrast T1) help differentiate true enhancement from intrinsic T1 hyperintensity. Contrast-enhanced MRI evaluates the true boundaries of the tumor, differentiation from peritumoral edema, dural/orbital invasion, and lymph node metastasis.
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On fat-suppressed contrast-enhanced T1 sequences, the mass shows heterogeneous solid enhancement with true enhancement confirmed on subtraction images independent of intrinsic T1 hyperintensity.
Criteria
Form showing significant melanin production (70-90%). Macroscopically pigmented (dark brown/black).
Distinct Features
Classic MRI dual signal signature: T1 hyperintense + T2 hypointense. Diagnosis usually strongly supported by MRI.
Criteria
Variant showing minimal melanin production (10-30%). Macroscopically light-colored (pink/white).
Distinct Features
Classic dual signal signature absent on MRI — T1 hypointense/intermediate, T2 intermediate. May be confused with SCC or lymphoma. Biopsy (S-100, HMB-45, Melan-A immunohistochemistry) is mandatory for diagnosis.
Criteria
Rare variant characterized by prominent fibrous stroma. Usually amelanotic. High tendency for perineural invasion.
Distinct Features
Solid mass mixed with T2 hypointense fibrous stroma on MRI. Perineural spread may show pattern similar to ACC. Less aggressive course — local recurrence common, distant metastasis later.
Distinguishing Feature
SCC does not show melanin-related T1 hyperintensity (T1 hypointense/intermediate), shows intermediate signal on T2 (different from melanoma's T2 hypointensity), more commonly originates from maxillary sinus (melanoma more commonly nasal cavity septum/turbinate origin), and perineural spread is less prominent.
Distinguishing Feature
ONB originates from cribriform plate (melanoma from septum/turbinate), shows dumbbell configuration and peritumoral cyst (absent in melanoma), does not show T1 hyperintensity, and demonstrates homogeneous intense enhancement.
Distinguishing Feature
Lymphoma shows very low ADC (<0.6-0.7 — lower than melanoma), demonstrates homogeneous enhancement, does not show T1 hyperintensity, exhibits 'bone replacement' pattern, and usually shows bilateral/diffuse presentation.
Urgency
emergentManagement
surgicalBiopsy
NeededFollow-up
İlk 2 yıl her 3 ayda bir, 3-5 yıl arası 6 ayda bir MRG + PET-BT ile takip. Lokal rekürrens oranı %60-80 olduğundan yakın takip zorunludur.Sinonasal melanoma is an aggressive malignancy with wide surgical resection as the primary approach in treatment; however, wide-margin resection is difficult due to anatomic constraints. Adjuvant radiotherapy (proton or IMRT) improves local control rate. In metastatic disease, immunotherapy (anti-PD-1: nivolumab/pembrolizumab, anti-CTLA-4: ipilimumab) has expanded treatment options although response rates are lower than cutaneous melanoma. In KIT mutation-positive cases, c-kit inhibitors (imatinib) can be used as targeted therapy. Prognosis is poor: 5-year survival 20-35%, median survival 2-3 years. The amelanotic variant can cause diagnostic delay and biopsy is mandatory for diagnosis (S-100, HMB-45, Melan-A immunohistochemistry).
Sinonasal melanoma is rare but extremely aggressive. On MRI, melanin content provides characteristic signal features (T1 hyperintense, T2 hypointense). Amelanotic forms may not show these features. Treatment involves surgery combined with radiotherapy/immunotherapy. Five-year survival is below 30%.