MRI and CT of Nasopharyngeal Carcinoma

Transcription

1 Neuroradiology/Head and Neck Imaging Review Abdel Razek and King Imaging of Nasopharyngeal Carcinoma Neuroradiology/Head and Neck Imaging Review FOCUS ON: Ahmed Abdel Khalek Abdel Razek 1 Ann King 2 Abdel Razek AAK, King A Keywords: cancer, imaging, lymph node, MRI, nasopharynx DOI: /AJR Received March 25, 2011; accepted after revision August 8, This article was presented as educational exhibit at RSNA Department of Diagnostic Radiology, Mansoura University Hospital, Faculty of Medicine, Elghomheryia St, Mansoura DK, Egypt. Address correspondence to A. A. K. Abdel Razek (arazek@mans.edu.eg). 2 Department of Diagnostic Radiology and Interventional Radiology, Chinese University of Hong Kong, Hong Kong, China. AJR 2012; 198: X/12/ American Roentgen Ray Society MRI and CT of Nasopharyngeal Carcinoma OBJECTIVE. This article reviews the MRI and CT of nasopharyngeal carcinoma. Extension of nasopharyngeal tumors, especially into the skull base and the deep facial spaces, is well illustrated on imaging. Assessment of retropharyngeal and cervical lymphadenopathy is important for treatment planning. MRI is commonly used for monitoring patients after therapy. CONCLUSION. Imaging can detect effect of radiation on surrounding structures. The imaging findings that help to differentiate nasopharyngeal carcinoma from simulating lesions are discussed. N asopharyngeal carcinoma (NPC) is a unique disease with clinical behavior, epidemiology, and histopathology that is different from that of squamous cell carcinomas of the head and neck. NPC accounts for 0.25% of all malignancies in the United States and 15 18% of malignancies in southern China. It also accounts for 10 20% of childhood malignancies in Africa. The male to-female ratio is 3:1. It is most common among patients years old, and bimodal age peaks occur in the second and sixth decades of life [1 5]. NPC is caused by the interaction of genetic susceptibility, environmental factors (e.g., exposure to chemical carcinogens), and infection with Epstein-Barr virus. High antibody titers to Epstein-Barr virus antigens are useful diagnostic markers, and there are many tests to detect both IgG and IgA titers. In China, dietary factors for NPC include nitrosamine-rich salted food [2 5]. Patients often present with local symptoms, such as epistaxis and a blocked nose, but may also present with hearing loss, otalgia, headache, or cranial nerve (CN) involvement. However, the nasopharynx is a relatively clinically silent area; therefore, the first presentation may be with cervical nodal or distant metastasis [1 6]. Pathology The World Health Organization classification of NPC recognizes three histologic types. Keratinizing squamous cell carcinoma (type 1) is found more often in nonendemic areas and has the worst prognosis. It is analogous to squamous cell carcinoma elsewhere in the pharynx and is associated with cigarette and alcohol use. Nonkeratinizing carcinoma (type 2) behaves in a fashion similar to type 3. Both types are radiosensitive and have a much better prognosis. Undifferentiated carcinoma (type 3) was previously called B lymphoepithelioma because of the mix of undifferentiated epithelial and nonmalignant T lymphocytes. In North America, around 25% of patients with NPC have type 1, 12% have type 2, and 63% have type 3. The histologic distribution in southern China is 2%, 3%, and 95%, respectively [2 6]. Imaging Techniques MRI The protocol for routine MRI of a nasopharyngeal mass includes unenhanced T1- weighted images to detect skull base involvement and fat planes (in at least an axial and sagittal plane). A T2-weighted fast spin-echo sequence in axial plane is used for the additional assessment of early parapharyngeal tumor spread, paranasal sinus invasion, middle ear effusions, and detection of cervical lymph nodes. Axial and coronal contrast-enhanced T1-weighted images (with and without fat suppression) are used to detect tumor extent, including perineural spread and intracranial extension of the tumor. The slice thickness is 3 5 mm [3 7]. Additional MRI sequences may be used in evaluation of NPC but, at present, are of lim- AJR:198, January

2 Abdel Razek and King ited proven clinical value, although wholebody MRI for metastatic deposits of NPC are promising [8]. Other reported MRI techniques include diffusion-weighted imaging, to aid in differentiating NPC from lymphoma and characterizing of cervical lymphadenopathy [9], and MRI spectroscopy, where choline-to-creatine ratios for the NPC and metastatic nodes are high compared with those for normal neck muscle [10]. CT CT has long been used for staging NPC, especially for the detection of skull base tumor involvement with lytic or sclerotic lesions [6, 7], but it has now largely been replaced by MRI for primary and nodal staging. However, CT is still used for radiotherapy planning and, in some centers, is used together with PET using 18 F-FDG. PET/CT has been shown to be of value in NPC staging, where the main advantage is for the detection of distant metastasis [8]. It is also used for monitoring patients after therapy and detecting NPC recurrence. Detection of NPC MRI is an accurate test for the diagnosis of NPC. MRI depicts subclinical cancers missed at endoscopy and endoscopic biopsy and identifies patients who do not have NPC and who therefore do not need to undergo invasive sampling biopsies [11]. NPCs usually present with intermediate signal intensity, higher than the muscle signal, on T2-weighted images, low signal intensity on T1- weighted images, and enhance to a lesser degree than does normal mucosa. Eighty-two percent of NPCs arise in the posterolateral recess of the pharyngeal wall (Rosenmüller fossa), and 12% arise in the midline. In 6 10% of patients, the nasopharyngeal mucosa appears normal at endoscopy [3 5]. Staging of NPC Staging of NPC according to the seventh edition of the American Joint Committee on Cancer s TNM staging system [12] relies on evaluation of the primary tumor (T category), the draining nodal groups (N category), and evidence or absence of metastatic disease (M category). T Category The T category is determined by the relationship of the primary tumor to adjacent structures [12] (Table 1). The mucosal spread of this tumor shows a preference for superior spread to the skull base, rather than inferior spread to the oropharynx [13]. Tumor often spreads submucosally and through areas of lesser resistance of the pharyngobasilar fascia and into the deep spaces of the neck. Category T1 NPC Tumor confined to the nasopharynx is only found in one fifth of patients [1] (Fig. 1). Mucosal spread of NPC tends to involve the superior portion of the nasopharynx. Deep infiltrating tumors may be found even when the nasopharyngeal component is small [1, 14]. The nasal cavity is commonly involved by NPC. Minimal invasion of tumor to the margin of the choanal orifice is common, whereas more bulky disease extending into the main body of the nasal cavity is encountered less frequently. NPC at the roof may spread centrally along the septum [3, 14]. Inferior superficial extension down to the mucosa of the oropharynx is uncommon. Invasion of the oropharynx rarely occurs as an isolated event and therefore is not usually an early sign of disease [1, 14]. Category T2 NPC Parapharyngeal spread occurs when tumor spreads posterolaterally and usually involves lateral penetration through the levator palatini muscle and pharyngobasilar fascia to involve the tensor palatini muscle and parapharyngeal fat space (Fig. 2). Invasion of the parapharyngeal space is associated with an increased risk of distant metastases and tumor recurrence. It can lead to compression of the eustachian tube with middle ear and mastoid effusion. Further posterolateral spread may also involve the carotid space and encase the carotid artery [15]. TABLE 1: Nasopharyngeal Carcinoma TNM Staging [12] Category T T1 T2 T3 T4 N N1 N2 N3 N4 M M0 M1 Retropharyngeal spread occurs when tumor spreads posteriorly to involve longus capitis muscles and prevertebral space (Fig. 3). This region contains lymphatics and a venous plexus, and so invasion of the prevertebral space is associated with an increased risk of distant metastases. In some patients, this posterior extension is the preferred pattern of tumor spread, with bulky disease continuing down to the foramen magnum and upper cervical spine [16]. Category T3 NPC NPC has a propensity to invade the skull base at diagnosis. The clivus, pterygoid bones, body of the sphenoid, and apices of the petrous temporal bones are most commonly invaded. Axial T1-weighted imaging provides a good overview of the extent of skull base invasion [1, 3]. CT reveals permeative or erosive bone changes of the skull base or spread along foraminal pathways. Also, sclerosis of the pterygoid process with increased attenuation of medullary cavity or thickening of cortical bone may be detected [17] (Fig. 4). Tumor frequently invades the skull base foramina (foramen rotundum, oval, and lacerum and vidian canal) and fissures (pterygomaxillary and petroclival). Tumor extended into the pterygopalatine fossa provides a route of spread to the orbit, infratemporal fossa, nasal cavity, and middle cranial fossa (Fig. 5). Invasion of hypoglossal nerve canal and jugular foramen is less common [1, 18]. Paranasal sinus involvement occurs as a result of direct extension. Maxillary sinus involvement occurs after nasal or infratemporal maxillary wall erosion (6%). Sphenoid sinus extension is common because it Description Primary tumor Tumor confined to nasopharynx, oropharynx, or nasal fossa Tumor extends to parapharyngeal space Tumor invades bony structures of skull base or paranasal sinuses Tumor with intracranial extension or involvement of cranial nerves, masticator space, orbit, or hypopharynx Regional lymph nodes Retropharyngeal lymph node either unilateral or bilateral Unilateral metastasis in lymph nodes, 6 cm in greatest dimension, above supraclavicular fossa Bilateral metastasis in lymph nodes, 6 cm in greatest dimension, above supraclavicular fossa Metastasis in lymph nodes > 6 cm in dimension or in the supraclavicular fossa Distant metastasis No distant metastasis Distant metastasis 12 AJR:198, January 2012

3 Imaging of Nasopharyngeal Carcinoma lies above the roof of the nasopharynx. The ethmoid and sphenoid are less commonly involved. Sinus involvement is recognized by the loss of contiguity of the sinus walls. Intrasinus extension of tumor may be seen. Tumor can be differentiated from reactive mucosal thickening on MRI, where inflammatory mucosal thickening is seen as uniform T2-weighted signal greater than that of tumor, also enhancing to a greater degree than tumor [1, 10]. Category T4 NPC Meningeal involvement appears as nodular enhancement, often along the floor of middle cranial fossa or posterior to the clivus. Direct invasion of the brain is rare. Invasion of cavernous sinus can lead to multiple cranial palsies. NPC may spread into the cavernous sinus from tumor surrounding the horizontal portion of the internal carotid artery, foramen ovale, orbital fissures, or directly through the skull base [1, 6, 10]. The frequency of diagnosed CN palsy in NPC ranges from 8.0% to 12.4%, and the clinical and MRI findings are not always consistent. Nerves are resistant to tumor, and perineural tumor spread is an insidious and often asymptomatic process by which NPC can invade upward and backward through the skull base to the cavernous sinus and middle cranial fossa and invade CN II to VI (upper CN palsy). It may also involve the carotid space, where it may compress or invade CN XII as it exits through the hypoglossal canal, CN IX to XI as they emerge from the Fig year-old woman with nasopharyngeal carcinoma (NPC) localized to nasopharynx (T1). Axial contrast-enhanced T1-weighted image shows small NPC (short arrows) centered in left Rosenmüller fossa (long arrow), which is the most common site for this cancer, and involving posterior wall. Tumor is confined to nasopharynx, and there is small metastatic left retropharyngeal node (curved arrow). jugular foramen (lower CN palsy), and the cervical sympathetic nerves. CN involvement on MRI is seen when there is either enhancement of soft-tissue tumor along the course of the ipsilateral related nerve, replacing the normal structures of the CN on gadolinium-enhanced T1-weighted images; or perineural spread, with enlargement or abnormal enhancement of the nerve, obliteration of the neural fat pads adjacent to the neurovascular foramina, or neuroforaminal enlargement. Maxillary and mandibular nerve involvement is best seen on coronal T1-weighted contrast-enhanced MRI with fat saturation. Hypoglossal nerve involvement may also occur [13, 19] (Fig. 5). Orbital invasion is a marker of extensive disease. Direct orbital invasion is rare, but when present it can invade via the inferior orbital fissure (from tumor in the pterygopalatine fossa), optic canal, and superior orbital fissure. Anatomic masticator space involvement affects the overall survival and local relapsefree survival of patients with NPC. The frequency of masticator space involvement in NPC is 19.7%. Infiltration of the medial and lateral pterygoid muscles, infratemporal fat, and temporalis muscle is found when tumors extend laterally from the parapharyngeal space, pterygoid base, or the pterygomaxillary fissure [4, 20]. Hypopharynx is the most inferior site of tumor invasion included in the staging classification, but it is very rarely involved at diagnosis [1 3]. Fig year-old man with nasopharyngeal carcinoma (NPC) with parapharyngeal extension (T2). Axial contrast T1-weighted image shows NPC (white arrows) with left parapharyngeal extension and involvement of parapharyngeal fat space. Note normal levator palatini muscle (red arrow), tensor palatini muscle (blue arrow), pharyngobasilar fascia (black arrow), and fat space (yellow arrow) on normal right side N Category NPC has a propensity to spread to nodes (Fig. 6) and, in about 75 90% of cases, is found by imaging to have a tendency for bilateral neck spread [21]. Nodal metastases are diagnosed if the shortest nodal axial diameter reaches 5 mm or greater in the lateral retropharyngeal region, 11 mm in the jugulodigastric region, or 10 mm in other nonretropharyngeal nodes of the neck; if there is a group of three or more nodes that are borderline in size; or if the nodes display necrosis or extracapsular spread. Extracapsular spread has also been shown to be an independent prognostic factor [8, 22]. Retropharyngeal Lymph Nodes The diagnosis of enlarged retropharyngeal lymph nodes in patients with NPC can only be made by imaging, and MRI has an advantage over CT in being better able to separate the lateral retropharyngeal nodes from the primary tumor in the adjacent posterolateral nasopharynx. Lateral retropharyngeal nodes are among the most common sites of nodal spread from NPC and have been considered the first echelon of metastatic spread [21] (Fig. 7). However, nodal spread may bypass these nodes and spread to other nodes of the upper neck. Metastatic lateral retropharyngeal nodes can be identified from the skull base to the level of C3. Retropharyngeal node involvement is now classified as category N1, whether unilateral or bilateral [1, 23]. PET/CT Fig year-old man with nasopharyngeal carcinoma with prevertebral extension (T2). Axial T1-weighted contrast-enhanced image shows nasopharyngeal carcinoma (straight arrows) with extensive spread predominantly posteriorly into longus muscles (arrowheads) and clivus (curved arrows). AJR:198, January

4 Abdel Razek and King Fig. 4 Patient with nasopharyngeal carcinoma (NPC) with skull base invasion and pterygoid sclerosis (T3). Axial CT bone window shows large NPC filling nasopharynx and nasal cavity with bony destruction of sphenoid bone, including right pterygoid base, which also shows sclerosis (arrow). Right middle ear effusion is present. reveals increased FDG uptake in metastatic cervical lymph nodes, but MRI appears to be superior to PET/CT for the assessment of retropharyngeal nodal metastasis because of the better discrimination of nodes from the adjacent primary tumor [24]. A Other Cervical Lymph Nodes Metastatic nodes posterior to the jugular vein in the upper neck are the most common sites for nonretropharyngeal nodes [22] and are designated as high internal jugular nodes, although at this site, the internal jugular and spinal accessory nodal chains converge. Nodes then usually spread in an orderly sequence down the neck. Nodes in the submandibular and parotid or periparotid region are far less common at diagnosis. Nodal metastases at supraclavicular fossa increase the incidence of distant metastases [1]. M Category NPC shows a high frequency of distant metastases (5 41%). The most common sites of metastases include bone (20%), lung (13%), and liver (9%). Patients with supraclavicular lymphadenopathy or tumors extension into the parapharyngeal and retropharyngeal space have a significantly higher risk of distant metastases. PET/CT is sensitive to detect bony and soft-tissue metastatic deposits [8]. Whole-body MRI shows a diagnostic capacity similar to that of FDG PET/CT in assessing distant-site status in patients with untreated NPC; in one reported study, the combined interpretation of whole-body MRI and FDG PET/CT showed no significant benefit over either technique alone [24]. Tumor Volume Tumor volume is a significant prognostic factor in the treatment of malignant tumors. However, it is not used presently in staging because technical considerations have prevented tumor volume measurement from being routinely used in a clinical setting and because methods for volume measurement are not standardized. The measurement of tumor volume has always been tedious and often involves tracing the tumor outline. The results are often affected by both intra- and interoperator performance. To overcome this problem, several investigators have developed semiautomated systems to reduce inter- and intraoperator variability. Errors encountered by computer-based techniques are thus likely to be classified as systematic errors and not as resulting from the experience of the operator. Semiautomated tumor volume measurement is now possible for NPC [25, 26]. Pediatric NPC Pediatric NPC is rare and usually poorly differentiated. It has a predilection for adolescents and teenagers. Unfortunately, these tumors tend to be locally advanced by the time they are diagnosed, mainly because the clinical presentation is nonspecific. Gross parapharyngeal space invasion is common, and tumor can also extend to the pterygopalatine fossa. Metastasis to liver and spleen in NPC commonly presents as solitary or multiple solid masses. Lymphoid hyperplasia, which is more common in the younger population, can be differentiated from pediatric NPC by the symmetric configuration and a striped pattern on both T2-weighted and contrast-enhanced images. Also, rhabdomyosarcoma can be differentiated from pediatric NPC by lower peak incidence (3 10 years) and inhomogeneous enhancement with necrotic intratumoral foci [27]. After Treatment The primary treatment for NPC is radiation therapy, but induction chemotherapy with 5-fluorouracil cisplatin is sometimes combined with radiation therapy. NPC is Fig year-old man with nasopharyngeal carcinoma (NPC) with skull base foraminal invasion. A, Coronal T1-weighted contrast-enhanced MRI shows NPC (straight arrows) with skull base invasion at foramen ovale (arrowhead) with invasion into cavernous sinus (curved arrow). B, Coronal T1-weighted contrast-enhanced MRI shows invasion of NPC (straight arrows) into foramen lacerum (arrowheads), where it encases carotid artery and extends into cavernous sinus (curved arrow). C, Axial T1-weighted contrast-enhanced MRI shows NPC invading pterygopalatine fossa (circle), pterygomaxillary fissure (arrow), and vidian canal (arrowhead). B C 14 AJR:198, January 2012

5 Imaging of Nasopharyngeal Carcinoma Fig. 6 Patient with metastatic cervical lymph node (N2). Axial T1-weighted contrast-enhanced MRI shows metastatic node (arrow) posterior to left upper internal jugular vein, which is common site for metastatic node with or without retropharyngeal nodal involvement. treated primarily by a high radiation dose (> 60 Gy), and in conventional (2D) radiotherapy, the nasopharynx and adjacent region are treated by radiation beams from the left and right sides and sometimes also with an anterior radiation beam. The neck lymphatics are usually irradiated by a separate anterior radiation beam. Intensity-modulated radiotherapy offers the opportunity of dose escalation to the tumor without increasing the dose to other organs at risk. These treatments require very accurate delineation of the gross tumor volume [3, 28]. Tumor Recurrence It is advantageous to obtain a scan 3 6 months after radiation therapy to provide a baseline study against which any future imaging can be compared. Regular surveillance imaging is also desirable, but its value has not been proven, especially for patients with early-stage disease in whom the radiotherapy response rates are high. Therefore, follow-up scans are often guided by clinical factors, such as suspicion of tumor recurrence or development of a radiation-induced complication. Any enlarging posttreatment soft-tissue mass or any new deep lesion or intracranial enhancement is concerning for recurrent disease [1, 3]. Differentiating fibrosis from tumor recurrence is difficult on routine CT. PET/ CT often provides an easier method for differentiating tumor recurrence from fibrosis. Fig. 7 Patient with retropharyngeal metastatic cervical lymph node (N1). Axial T1-weighted contrastenhanced MRI shows metastatic node (arrow) in left retropharyngeal region, which is frequently first echelon for nodal spread. Typically, recurrent tumors show uptake of radionuclide tracer, but fibrosis does not. MRI can differentiate mature scar tissue, which shows retraction, low T2 signal, and no contrast enhancement from tumor, which is expansile and of intermediate T2 signal with moderate contrast enhancement on nonfat-saturated images (Fig. 8). However, there may be an overlap between partially treated A tumor and immature scar tissue. MRI shows a trend toward higher accuracy in detecting disease at the primary site than does PET/ CT, although the latter shows a trend toward higher accuracy in detecting nodal disease [28 30]. Nonmalignant Pharyngeal Mass Nonmalignant pharyngeal masses are seen in less than 1% of MRI examinations performed 2 14 years (mean, 8 years) after radiation therapy. It has two patterns. The first is a nasopharyngeal polyp (1 5 cm) that shows mixed heterogeneous T2 signal intensity and marked contrast enhancement (Fig. 9), with the larger polyps having stellate areas of reduced enhancement. The second is a sphenoid sinus mass, which consists of a nonenhancing mass filling a nonexpanded sinus and a heterogeneous-enhancing mass expanding the sinus or nonenhancing rhinoliths in the sphenoid sinus. This appearance in sphenoid sinus, as well as the larger polyps with a stellate appearance, can be similar to that of radiation-induced sarcomas [31]. Trismus With Masticator Space Abnormalities Trismus is most commonly due to abnormality of masticator muscles as a result of the effects of radiation and rarely is secondary to damage of the mandibular nerve. It may be due to osteoradionecrosis of the mandibular ramus and temporomandibular joint Fig. 8 Patient with nasopharyngeal carcinoma (NPC) recurrence. A, Image obtained before treatment shows NPC involving nasopharyngeal mucosa, centered in right Rosenmüller fossa (straight arrow) with deep posterior extension into longus muscles (curved arrow). B, Image obtained 3 months after treatment shows that mucosal component of tumor has resolved (straight arrow) leaving behind mild symmetric posttreatment mucosal thickening in nasopharynx. Deep component is small residual mass (curved arrow), which is nonspecific and could represent early scar tissue or residual cancer. B AJR:198, January

6 Abdel Razek and King Fig year-old man with nonmalignant pharyngeal mass. Axial T1-weighted contrastenhanced MRI shows small markedly enhancing inflammatory polyp (arrow) arising from posterior wall of nasopharynx. or abnormality in the perimasticator tissues as a result of radiation fibrosis or inflammation spreading from sinusitis. One half of patients have no significant abnormality on MRI [4, 32] (Fig. 10). Temporal Lobe Injury Temporal lobe injury occurs in 3% of patients of NPC with a latent period of years. Depending on the radiation field, it may be bilateral or unilateral. It can involve the gray and white matter simultaneously or the gray matter alone; however, isolated white matter lesions are rare. Temporal lobe injury resulting from radiation is not always an irreversible and progressive process but is one that can regress or resolve at MRI. In the evolution of radiation injury, white matter lesions are seen first and are followed by contrast-enhanced lesions, which have an increasing tendency to become necrotic with increasing size. Cysts are the least frequent manifestation and arise in the late stages (Fig. 11). MRI spectroscopy in early A Fig. 10 Patient with changes to pterygoid muscle after radiation therapy. Axial T2-weighted MRI shows increased T2 signal in pterygoid muscles (arrows) mainly involving left side. delayed phase of injury shows reduced N-acetyl aspartate and creatine levels and increased choline levels as a result of demyelination. The late delayed phase of radiation injury shows the decrease of N-acetyl aspartate, choline, and creatine levels [33]. Osteoradionecrosis Osteoradionecrosis may occur 1 year after irradiation. It is believed to be secondary to osteoblastic destruction with subsequent vascular damage. The skull base, cervical spine, and the mandible are commonly affected. Imaging findings include areas of osteolysis and mixed sclerosis (Fig. 12) within the irradiation portal. Fragmentation and sloughing of necrotic bone may also be found. There is surrounding inflammatory soft-tissue mass that may mimic tumor recurrence or osteomyelitis [34]. B Fig year-old man with radiation-induced injury to temporal lobe. Coronal T2-weighted MRI shows bilateral radiation-induced injury to white matter in temporal lobes (arrows). Radiation-Induced Tumors Radiation-induced tumors arise 5 10 years after irradiation of NPC in % of cases. Sarcomas and squamous cell carcinomas arise in the high-dose field zone and involve sites around the maxillary region, such as the palate, maxillary sinus, alveolar process, and nasal cavity. Squamous cell carcinomas also arise in the low-dose field, may occur many years after radiotherapy, and may involve peripheral sites such as the temporal bone. The presence of a heterogeneous tumor or rapidly growing large destructive mass that displays different signal intensity from NPC should suggest the possibility of a radiation-induced sarcoma. The presence of calcification or ossification points strongly to a diagnosis of radiation-induced sarcoma [2, 35]. Differentiation of Npc From Simulating Lesions Lymphoma The nasopharynx is one of the most common sites of extranodal non-hodgkin lymphoma in the head and neck region. It usually occurs in the sixth decade of life and is associated Fig year-old man with osteoradionecrosis. A, Axial CT scan bone window shows osteoradionecrosis in skull base with sclerosis and osteolysis. B, Sagittal CT scan bone window shows osteoradionecrosis in anterior arch of C1 (long arrow) and tip of dens (short arrow). 16 AJR:198, January 2012

7 Imaging of Nasopharyngeal Carcinoma with gastrointestinal tract lymphoma in up to 10% of patients at either the time of diagnosis or relapse. Lymphoma is frequently located in the midline, unlike NPC, which often arises laterally. Bone invasion is not common even in large tumors, and as with NPC, nodes are frequent but these may involve sites such as the submandibular and parotid nodes, which are less frequently involved at presentation in patients with NPC. Also, lymphoma has a lower apparent diffusion coefficient value than does NPC because of its higher cellularity [6 8]. Adenoid Cystic Carcinoma Adenoid cystic carcinoma usually affects patients during middle age and there is no reported sex predilection. Unlike patients with NPC, patients with adenoid cystic carcinomas rarely present with cervical lymphadenopathy. This tumor has a greater propensity for perineural spread than does NPC. The tumor exhibits higher apparent diffusion coefficient value on diffusion-weighted MRI because of its cystic component [6, 7]. Extramedullary Plasmacytoma Extramedullary plasmacytoma is a rare malignant soft-tissue tumor, but 80% of these tumors occur in the head and neck with the nasopharynx being a common site. It is most commonly seen in the sixth and seventh decades and has an 80% male preponderance. The tumor transgresses into a multiple myeloma in 20 30% of cases. The lesion may present as a submucosal homogeneous and enhancing polypoid nasopharyngeal mass several centimeters in diameter, with or without bone destruction [6]. Pleomorphic Adenoma Pleomorphic adenoma occurs in the pharyngeal mucosal space, arising from minor salivary gland tissue. When associated bone changes are present, benign-appearing bone remodelling is the typical pattern. However, slowly progressive bone destruction with an aggressive appearance can be observed [36]. Tuberculosis Nasopharyngeal tuberculosis is rare and is thought to result from direct infection of the upper respiratory tract. It mimics NPC, especially in Asian patients. It has two patterns. The first pattern is a discrete polypoid mass in the adenoids, and the second pattern is a more diffuse soft-tissue thickening of one or two of the walls of the nasopharynx. Extension outside the confines of the nasopharynx is not usually a major feature [37]. Pseudotumor Fibrosing inflammatory pseudotumor is a nonspecific inflammatory process of uncertain cause that rarely involves the nasopharynx. MRI findings that help to differentiate pseudotumors from NPC are ill-defined less likely contour bulging features, with local infiltration, hypointensity on T2-weighted images, relatively weak enhancement, no significant regional lymphadenopathy, and good response to steroid therapy [38]. Amyloidosis On CT, amyloidosis appears as a well-defined submucosal homogeneous calcified mass without bone destruction with or without lymphadenopathy. The lesion exhibits minimal enhancement. On MRI, the submucosal location, distinctive hypointensity on T2-weighted imaging, and early enhancement on dynamic contrast-enhanced MRI helps to differentiate amyloidosis from NPC [39]. Conclusion In conclusion, MRI is essential for detection of early NPC, staging of the primary tumor, and evaluation of associated retropharyngeal and cervical lymphadenopathy. It has been used for monitoring patients after therapy to detect tumor recurrence and radiationassociated changes in the soft tissue and bone. Imaging is valuable for the differentiation of NPC from other simulating lesions. References 1. King A, Bhatia KS. Magnetic resonance imaging staging of nasopharyngeal carcinoma in the head and neck. World J Radiol 2010; 2: Chong VF, Ong CK. Nasopharyngeal carcinoma. 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