1. Anatomy of Paranasal Sinuses 

Fig 1: Anatomy of the Paranasal Sinuses : Coronal Section

Fig 2: Anatomy of the Paranasal Sinuses : Sagittal Section

2. Simplified Classification of Malignant Sinonasal Tumors 

Fig 3: A simplified classification framework of malignant sinonasal tumors with respective subtypes

3. Imaging Characteristics of Malignant Sinonasal Tumors 

3.1. Epithelial Tumors

3.1.a. Squamous Cell Carcinoma 

Squamous cell carcinoma (SCC) is the most prevalent type of sinonasal carcinoma, generally arising from the maxillary sinus and nasal cavity [7]. Smaller lesions are typically homogeneous, while larger ones show heterogeneity with necrosis and hemorrhage. Their imaging characteristics include: [8]

• Intratumoral necrosis
• Aggressive bony destruction of adjacent sinus walls 
• Invasion into the contralateral sinonasal area, orbit, infratemporal fossa, or skull base in advanced stages
• T1WI: Isointensity 
• T2WI: Slight hyperintensity
• Contrast enhanced T1WI: Moderate enhancement

Fig 4: Contrast enhanced CT images of a 57 year old male who presented with long standing right sided nasal congestion, show a large, enhancing, isodense lesion involving the right maxillary (red arrow in image A), ethmoid (red arrow in image B) and frontal sinus (red arrow in image C) with a few hypo-enhancing areas within causing destruction of the maxillary antrum along with breech into the right orbit and lamina papyracea (red arrowhead in image B) and soft tissue involvement of the face. It was histopathologically confirmed to be papillary squamous cell carcinoma.

3.1.b. Adenocarcinoma 

Adenocarcinomas constitute approximately 20% of malignant neoplasms affecting the sinonasal tract, with a predeliction for the ethmoid sinuses [9,10]. They usually have comparable features to SCC, appearing as solid, heterogeneous masses with areas of necrosis and irregular margins. However, certain differences include:

• Extension into skull base and frontal lobes
• Areas of calcification in mucin producing subtypes
• Contrast enhanced T1WI: Gradual enhancement in mucinous types
• T2WI: Hyperintensity in mucin producing subtypes, and iso to hypointensity in non-mucin producing subtypes

3.1.c. Salivary Gland Type Carcinoma 

Salivary gland tumors typically present as soft tissue masses originating in the palate and extend into the nasal cavity and paranasal sinuses [11]. Among these, adenoid cystic carcinoma is the most common salivary gland-type malignancy affecting the sinonasal region. Key imaging features of adenoid cystic carcinoma include [12,13]:

• Large irregular mass with bone destruction and necrotic areas
• Low FDG uptake due to low metabolic activity
• Enlargement of bony neural foramina in cases of perineural spread
• Intermediate T1 signal with variable T2 signal, depending on cellularity

Other rare salivary gland tumors like sinonasal mucoepidermoid carcinomas predominantly affect the nasal cavity and maxillary antrum, while pleomorphic adenomas most often originate from the nasal septum and typically present with a spherical morphology [11]. 

Fig 5: Contrast enhanced CT images of a 40 year old male who presented with epistaxis and headache demonstrate a heterogeneously enhancing mass lesion in the left sphenoid sinus, with bone destruction in the lateral and inferior aspects (red arrows in A-C). There is soft tissue extension into the masticator space, base of pterygoid plates, and pterygoid muscles, with superolateral abutment of the temporal lobe. Focal bony erosions are also visualised in the carotid canal, ethmoidal cells, and left pterygomaxillary fissure, along with minimal soft tissue involvement in the infratemporal fossa. It was histopathologically proven to be a clear cell salivary gland type tumor.

3.1.d. Sinonasal Undifferentiated Carcinoma 

Sinonasal Undifferentiated Carcinoma (SNUC) is a rare and aggressive tumor, commonly originating in the superior nasal cavity and ethmoid sinus. These frequently involve multiple paranasal sinuses, causing extensive destruction and invasion of orbital bones, cranial cavity, or the nasopharynx. SNUC has a poor prognosis, with high rates of nodal involvement, distant metastases, and a 5-year survival rate of less than 20% [12,14]. Their imaging characteristics include [15,16]:

• Regions of central necrosis
• Obstruction of adjacent sinuses
• Destruction and invasion of adjacent structures including anterior cranial fossa, adjacent paranasal sinuses, and orbits
• T1WI: Isointense
• T2WI: Iso to hyperintense
• Heterogeneous enhancement on gadolinium-enhanced images

Fig 6: Contrast enhanced CT images of a 42-year-old female who presented with epistaxis and chronic headache, demonstrate a large, heterogeneously enhancing, ill-defined soft tissue mass in the nasopharynx, obliterating the bilateral fossae of Rosenmüller, torus tubarius, and ostium pharyngeum (red arrows in A-C). The mass causes narrowing of the nasopharyngeal airway, permeative destruction of the clivus, and erosion of the floor of the sphenoid sinus. It extends posteriorly into the prevertebral space, inferiorly to the anterior arch of the atlas, and laterally into the parapharyngeal and carotid spaces bilaterally. Anterolaterally, it encases and erodes the medial pterygoid plates. Widening of the left Vidian canal with soft tissue extension is also visualised. It was histopathology confirmed to be sinonasal undifferentiated carcinoma.

3.2 Soft Tissue Tumors 

3.2.a. Fibrosarcoma

An extremely rare tumor, typically found in the maxillary sinus, distinguished by a high rate of local recurrence but a low likelihood of distant metastasis. It is occasionally mistaken for a papilloma on imaging [17,18]. Imaging features include [19]:

• Solitary lobulated or irregular mass
• Bony destruction
• Homogeneous on CT
• T2WI: Mildly hypointense
• Heterogeneous delayed contrast enhancement pattern

3.2.b. Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is a rare and aggressive soft tissue malignancy, predominantly affecting children and adolescents. Distant metastases, commonly to the lungs, bone, and liver, significantly worsen prognosis. Rare instances of spread to the breast, pancreas, and peritoneal cavity have also been reported [20-22]. 

• Ill-defined margins with adjacent bony destruction and extension into surrounding spaces
• Contrast enhanced CT: Isodense or slightly hypodense mass with homogeneous enhancement
• T1WI: Isointense relative to muscle
• T2WI: Hyperintense relative to muscle
• Contrast enhanced T1WI: Moderate homogenous enhancement 

3.2.c. Synovial Sarcoma 

Synovial sarcoma is a high grade malignancy primarily found in the extremities. In sinonasal tumors, features such as calcification, bony changes, and hemorrhage, collectively known as the triple sign or cobblestone appearance on CT/MRI, should raise suspicion for this diagnosis [23].

Fig 7: Contrast enhanced MR images of a 47 year old male who presented with chronic headache show an irregular mixed- intensity heterogenously enhancing lesion involving the anterior nasal cavity, anterior ethmoid air cells and right frontal sinus(red arrows in A & B), with erosion into left frontal extra dural space and extra conal space. Erosion through the anterior wall of the left frontal sinus (red arrow in D) is also visualised, resulting in a subcutaneous lesion (arrowhead in C). It was histopathologically proven to be sinonasal synovial carcinoma.

3.3 Tumors of Bone and Cartilage 

3.3.a. Chondrosarcoma 

Chondrosarcoma is a slow-growing, painless tumor with cartilage formation with a tendency for multiple recurrences, progressive spread, and a poor prognosis [24]. Certain imaging features include [25,26]:

• Soft tissue mass with stippled and amorphous calcifications
• CT: Bone destruction and irregular contrast enhancement, increasing with tumor grade
• T1WI: Lower signal intensity of the chondroid matrix compared to bone matrix
• T2WI: Hyperintense chondroid tissue and hypointense calcified areas
• Post contrast images: Distinct curvilinear septal enhancement of fibrovascular tissue and non-ossified cartilage 

Fig 8: Contrast enhanced CT images of a 32 year old female who presented with epistaxis and decreased vision show a minimally enhancing hypodense lesion involving both nasal cavities, with extensive destruction of the nasal septum, ethmoid bone and bilateral turbinates (red arrows in A-C). Axial bone window image (D) shows erosion into adjacent structures and multiple course calcifications within the lesion (red arrowhead in D). It was histopathologically confirmed to be low-grade chondrosarcoma.

3.3.b. Osteosarcoma 

Osteosarcoma is an aggressive neoplasm characterized by bone destruction and marked heterogeneity on CT and MRI, with intense enhancement. A significant predisposing factor is prior radiation therapy for retinoblastoma [25]. Imaging reveals [8]: 

• Sunburst effect, cortical breakthrough, and alveolar margin disruption
• Calcifications of the osteoid or osteoid-Iike substance within the tumor
• Cortical erosion, osteoblastic sclerosis, and matrix mineralization
• T1WI: Low to intermediate signal intensity 
• T2WI: High signal intensity

3.4 Neuroectodermal tumors 

Ewing sarcoma and Primitive Neuroectodermal Tumors (PNET) are aggressive small, blue, round cell tumors, rare in the sinonasal region. These tumors primarily affect children and young adults, often with a history of prior radiation therapy. Imaging reveals an expansile mass with bone destruction. Periosteal reactions are observed, with the onion-skin pattern being more common than sunburst. [11]

Fig 9: Contrast enhanced CT images of a 43 year old female who presented with right sided epistaxis, reveal a homogenously enhancing soft tissue lesion in the right nasal cavity involving the right superior and middle turbinates & right uncinate process with no evidence of calcifications (red arrows in A-C). It was histopathologically proven to be Ewing sarcoma.

3.5 New and Emerging Sinonasal Tumors 

Numerous emerging sinonasal tumor entities have been identified, including NMCs, biphenotypic sinonasal sarcomas, HPV-related multiphenotypic carcinomas, and SMARCB1-deficient carcinomas. While some exhibit distinct histopathological features, others remain provisional diagnoses due to their undefined characteristics. They often share locally destructive imaging features, though certain findings, such as calcifications in SMARCB1-deficient carcinomas or cystic and calcified components in chondromesenchymal hamartomas, may occasionally be observed. However, pathognomonic imaging characteristics are generally lacking. [26,27]

4. Diagnostic Approach to Sinonasal Malignant Tumors

4.1. Tumor Characterization

Sinonasal tumors predominantly originate in the maxillary sinuses, followed by the nasal cavity and ethmoid sinuses. Frontal and sphenoid sinus involvement is rare. MRI is superior for delineating tumor boundaries and tissue components, showing:

• Intermediate T2 signals
• Hypo-isointensity on T1
• Low ADC values
• Heterogeneous contrast enhancement

Definitive diagnosis, however, requires histology.

4.2. Osseous Involvement

Sinonasal malignancies often invade intracranial structures through bone. CT excels in detecting bony changes. Three main patterns are observed:

• Erosion (SCC)
• Remodeling (inflammatory or benign tumors)
• New bone formation (chondrosarcoma, osteosarcoma)

4.3. Dural Invasion

MRI is preferred for assessing skull base and dural invasion. Signs of dural invasion include:

• Dural thickening >5 mm
• Nodularity
• Pial enhancement

Brain edema or enhancement may suggest parenchymal invasion.

4.4. Orbital Involvement

Pre-surgical imaging evaluates orbital invasion via direct extension or spread along anatomical pathways like the nasolacrimal duct or infraorbital nerve. CT and MRI features that needs to be assessed are:

• Bony wall integrity
• Orbital fat stranding
• Extraocular muscle abnormalities

4.5. Perineural Spread

This is common in adenoid cystic carcinoma and SCC. Tumors often spread via the pterygopalatine fossa to adjacent regions, affecting V2 and V3 nerves. Perineural spead is best identified on MRI, which shows:

• Nerve enlargement
• Nerve enhancement
• Cranial foramen involvement

4.6. Lymph Node Involvement

Lymphadenopathy occurs in about 15% of sinonasal malignancies. Metastases typically involve level Ib and retropharyngeal nodes, identified by:

• Nodal size >1 cm
• Nodal necrosis
• Low ADC values