诊断

Review

Skull-base Invasion of Nasopharyngeal Carcinoma

Yizhuo Li, Peihong Wu, Chuanmiao Xie

Sun Yat-sen University Cancer Center, Guangzhou, Guangdong, China

Corresponding author: Yizhuo Li; Email: liyizhuo68@126.com

 

 

Citation: Li YZ, Wu PH, Xie CM. Skull-base Invasion of Nasopharyngeal Carcinoma. J Nasopharyng Carcinoma, 2014, 1(3): e3. doi:10.15383/jnpc.3.

Competing interests: The authors have declared that no competing interests exist.

Conflict of interest: None.

Copyright:http://journalofnasopharyngealcarcinoma.org/Resource/image/20140307/20140307234733_0340.png2014 By the Editorial Department of Journal of Nasopharyngeal Carcinoma. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

 

 

Abstract: The nasopharynx is situated just below the base of the skull. Nasopharyngeal carcinoma (NPC) has a great tendency to invade adjacent regions fairly early. Due to its deep and hidden anatomical location, the infiltrating ability of the tumour, and the non-specific nature of the symptoms, it is not diagnosed until it reaches an advanced stage. So, NPC may present with base of the skull erosion and/or cranial nerve involvement. Magnetic resonance imaging (MRI) could provide useful information about the position and quality of NPC because of its ideal soft tissue contrast and multi-planar imaging capacity. With gadolinium contrast and fat suppression sequences, it has greater sensitivity over clinical examination in detecting the base of the skull with bone erosion and/or perineural invasion. MRI is the choice of imaging modality for evaluating tumour extension. The aims of this article were to review the imaging studies of skull-base invasion and its impact on tumour staging and prognosis.

Key words: nasopharyngeal carcinoma, magnetic resonance imaging (MRI), imaging diagnosis, tumour stage

 

 

 

Malignant tumours in the nasopharynx often spread intracranial, and the skull-base and the nerve channels are the main spread pathway. Skull-base was composed by sphenoid bone, portions of the occipital and temporal bone. The intracranial surface of the skull-base was divided in to three portions: anterior cranial fossa, middle and posterior cranial fossa.

 

Anatomy

The clivus and basisphenoid bone make up the posterior wall and the roof of the nasopharynx (1). The sphenoid bone is the foundation of the central cranial base, which is made up of a body, greater and lesser wings and pterygoid processes (2,3). The anterosuperior surface of the sphenoid body forms a part of the floor of the anterior cranial fossa. The sella turcica lies just above the sphenoid sinus, bound anteriorly and posteriorly by the clinoid processes. Laterally and inferiorly, the greater wings of the sphenoid form the floor of the middle cranial fossa. The posterior edge of the lesser wing of the sphenoid delimits the aspect of the middle cranial fossa. Posteroinferiorly, the anterior aspect of the basiocciput articulates with the posterior part of the body of the

 

sphenoid, forming the clivus. The clivus, the anterior tip of the petrous apex and the basisphenoid bone form the foramen lacerum. Above the foramen lacerum and lateral to the clivus is the cavernous sinus. The abducens nerve (VI) passes through the cavernous sinus; the oculomotor nerve (III), the trochlear nerve (IV) and the first division of the trigeminal nerve are found in the lateral wall of the cavernous sinus. The optic nerve (II) lies medial to the cavernous sinus.

Several important foramina traverse the sphenoid bone. The optic canals are marginated by the roots of the lesser wings of the sphenoid. The optic nerves and ophthalmic artery go throughout it. The superior optic fissure lies between the lesser and the greater wings and transmits cranial nerves (CN) III, IV VI, V1 and ophthalmic vein. The inferior orbital fissure allows communication between the inferior orbit and the pterygopalatine fossa. The pterygopalatine fossa is a medial depression of the ptergomaxillary fissure, which lies between the ptergoid process and the maxillary sinus. It is in free communication with the infratemporal fossa laterally. The infratemporal fossa lies lateral to the paranasopharyngeal space. It is bound by the posterior wall of the maxillary antrum anteriorly, and the deep head of the temporalis muscles and the zygomatic arch laterally.

Foramen rotundum, foramen ovale and foramen spinosum perforate the greater wing of the sphenoid bone from anteromedial to posterolateral. Foramen rotundum is separated from the superior orbital fissure by a thin bar of bone. Foramen rotundum passes through the space between the back wall of the maxillary sinus and the body of the pterygiod, and transmits the second division of CN V (V2). The largest of the foramina, foramen ovale, perforate the sphenoid bone at the level of the junction of the body and the greater wing. The orifice of the foramen ovale is surrounded by fat, within which the mandibular branch of the trigeminal nerve (V3) is transmitted. Posterolateral to the foramen ovale, the foramen spinosum provides a pathway for the middle meningeal artery.(figure1,2).

 

 

 

 

Figure 1. Photograph of the normal intracranial surface of skull-base from the downward view showing bone landmarks and foramina. The right half was distinguished by different colours. From anterior to posterior are frontal bone (blue), sphenoid (yellow), temporal bone (orange) and occipital bone (cyan). The left half showing Foramen rotundum, foramen ovale and foramen spinosum perforate the greater wing of the sphenoid bone from anteromedial to posterolateral. The clivus, the anterior tip of the petrous apex and the basisphenoid bone form the foramen lacerum

GW=greater wing of sphenoid; LW=lesser wing of sphenoid; PA=petrous apex; OB=Occipital bone; FR=foramen rotundum; FO=foramen ovale; FS=foramen spinosum; FL=foramen lacerum.

 

 

Figure 2. CT findings axial bone window of the skull-base showing several foramina and canals. foramina and canals are showing relative low density.(PF=pterygopalatine fossa; PP=pterygoid process; BB=basisphenoid bone; PA=petrous apex; FO=foramen ovale; FS=foramen spinosum; FL=foramen lacerum; C=clivus; CC=carotid canal).

 

 

The bony landmark of the exocranial surface of the skull-base was identified in relation to the roof of the parapharyngeal space (PPS). The anterior limit of the PPS roof lies at the apex of the scaphoid fossa. The free border of the medial pterygoid plate splits superiorly to form the scaphoid fossa. The spine of the sphenoid bone forms the lateral limit of the roof of the PPS. The exocranial surface of the petrous temporal bone makes up the major portion of the bone of the roof of the PPS. The posterolateral border of the roof of the PPS is the styloid process and the posterior border is the most posterior aspect of the posterior margin of the jugular foramen.

 

Imaging

Clinical examination and endoscopic examination of the nasopharynx can provide valuable information on mucosal lesion and local tumour extension. However, it cannot determine deep tumour extension, such as skull-base erosion and intracranial spread. Cross-sectional imaging has resolved this problem easily. The primary role of imaging is accurate tumour mapping and display tumour extension to the skull-base and the deep fascial

spaces. Up to now, there are two scanning instruments used in imaging: Computed Tomography (CT) and MRI.

 

CT scanning

CT scanning areas include the nasopharynx and neck. Axial scans were obtained at 5-mm intervals with the infraorbitomeatal(OM) line parallel to the gantry(5). The nasopharynx scans were obtained from the suprasellar cistern to the inferior edge of C2 vertebra. The neck scans were obtained from the inferior edge of the C2 to the sternal end of the clavicle. The coronal scans were performed with the hard palate perpendicular to the gantry (6) and the region from the posterior ethmtoid sinus to the dorsal jugular foramen. After the axial and coronal scans were obtained before injection of the contrast material, an intravenous contrast injection was used to enhance the difference between the tumour and other tissues, and for better delineation of the carotid artery and jugular vein. The coronal scan was not used routinely (5).

CT diagnosis

The CT findings of skull-base erosion were classified into three types (7): A, bone destruction, CT scans show the bone erosion ranged from intensive bone destruction, through varying degrees of intracranial invasion with CN involvement, to subtle bone defect without CN involvement (5,6); B, bone sclerosis, CT scans show the density in the invaded region of the skull-base is higher than that of the normal area; and C, both of them simultaneously, both the bone defect and abnormal high density area were present.

 

MR Imaging

The region from the suprasellar cistern to the inferior margin of the sternal end of the clavicle was examined with a combined head and neck coil. T1-weighted images in the axial, coronal and sagittal planes (repetition time [TR] ms/echo time [TE] ms, 500-600/10-20) and T2-weighted images in the axial plane (4000-6000/95-110) were obtained before injection of contrast material. After intravenous gadopentetate dimeglumine (Magnevist, Schering, Berlin, Germany) at a dose of 0.1 mmol per kilogram of body weight, T1-weighted axial and sagittal sequences, and T1-weighted fat saturation axial and coronal images were acquired sequentially using parameters similar to those used without gadopentetate dimeglumine. The slice thickness was 5-mm with a 1-mm intersection overlap for all the sequences. Bone involvement on MRI is diagnosed when the following could be seen: a defect in the low signal intensity of the cortex and marrow being replaced by tumour with contrast enhancement in bone (8) (Figure 3).

 

image2  (a)            image3  (b)

Figure 3. NPC in a 51 year-old man. Pretreatment sagittal (a) T1W images without fat saturation show the erosion of the clivus (arrow). Contrast-enhanced sagittal (b) T1W images show the erosion clivus (arrow) with moderate enhancement.

Values comparison between CT and MRI in NPC

Several studies (9-12) demonstrated that MRI could provide useful information about the position and quality of skull-base because of its ideal soft tissue contrast and multi-planar imaging capacity. With gadolinium contrast and fat suppression sequences, it has greater sensitivity over clinical examination and CT scanning in detecting the base of the skull. MRI can also avoid the bony artefacts of the skull-base which are very serious in CT scanning. So MRI is the choice of imaging modality for evaluating local tumour extension and perineural spread in NPC patients.

 

Patterns of skull-base erosion

Previous studies (8, 13, 14) reported that skull-base erosion may be seen in 25–35% of cases. With improvement in imaging techniques, these percentages will increase. In recent data (15), the incidence of skull-base invasion was 65.5%, which is higher than previously reported. This may be because some studies were based on CT scanning, while others were based on MRI without the use of fat saturation techniques. Li YZ(15) and Liang SB (16) classified the anatomic sites surrounding the nasopharynx into three risk grades: high group (≥35%), medium group (≥5%-35%) and low group (<5%). High risk grade included the base of the sphenoid bone, pterygoid process, clivus and petrous apex. The base of the sphenoid bone is the most common erosion site, followed by the pterygoid process, clivus and petrous apex. These anatomic sites lack any barrier and are only a short distance from the original tumour. The medium group included the greater wing of the sphenoid, lateral pterygoid plate, foramen lacerum and cavernous sinus. These sites are relatively at a longer distance from the original tumour compared to the high group. The low group include foramen ovale, occipital condyle, etc. These areas are more distant from the original tumour than the medium group. These data demonstrate that tumours spread gradually with decreasing movement from proximal sites to more distant sites surrounding the nasopharynx. According to the anatomic site, the pathways of skull-base involvement were classified into five routines (7,15): anterior spread, superior spread, super-lateral spread, super-anterior spread and super-posterior spread.

 

Relationship between skull-base invasion and tumour staging

In the UICC/AJCC staging system (7th Edition, 2010) (17), bony structure invasion are classified as T3 lesions. The result is mainly in terms of survival rate. However, Li YZ (15) et al. proposed another opinion: the MR images showed that the extent of bone invasion ranged from minimal confined sphenoid base disease through varying degrees to more extensive bone erosion with or without CN involvement. Most cases in the low group had T4 lesions (65/74). This may be because the low group was combined completely with other T4 structure involvements (such as intracranial extension and/or cranial nerve involvement). Cases in the high and medium groups did not reveal any intracranial extension or CN involvement on MRI. The majority of medium group cases also had T4 lesions (154/250). Only in the high group were T3 cases the majority (190/255). This result indicates the skull-base invasion in different groups probably impacts the T-staging and prognosis in NPC patients.

 

Impact of skull-base invasion on prognosis

NPC is a tumour that is sensitive to radiation therapy. However, due to its unique anatomic site, the dose-limiting organs surrounding the nasopharynx are also damaged during radiation therapy, which affects the quality of life of the survivors (18). Patients may suffer from a variety of late complications. These sequelae include auditory problems (19), oral complications, such as dry mouth (20, 21), dysphagia (22), CN palsies, memory loss (23), and cognitive (24) and neuropsychological dysfunction (25). So we suggest that the quality of life should become a factor of prognosis evaluation in cancer patients.

 

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