British Journal of Radiology 75 (2002),345-350 © 2002 The British Institute of Radiology
The use of stereotactic navigation guidance in minimally invasive transnasal nasopharyngectomy: a comparison with the conventional open transfacial approach
E W H To, FRCS
1
E H Y Yuen, FRCR
2
W M Tsang, FRACDS
4
E C H Lai, MBChB
1
G K C Wong, AFRCS
3
D T F Sun, FRCS
3
D T M Chan, FRCS
3
J M K Lam, FRCS
3
A Ahuja, FRCR
2 and
W S Poon, FRCS
3
1 Division of Head and Neck—Plastic and Reconstructive Surgery, Department of Surgery, 2 Department of Diagnostic Radiology and Organ Imaging and 3 Division of Neurosurgery, Department of Surgery, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong and 4 Oral Maxillofacial Surgery and Dental Unit, Prince of Wales Hospital, Shatin, Hong Kong
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Abstract
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The purpose of this paper is to study the efficacy of applying stereotactic navigation guidance to nasopharyngectomy via a minimally invasive transnasal approach as compared with the conventional open transfacial approaches. The nasopharynx is the centre of the anterior skull base, which is remote from the surface of the facial skeleton. It is well known that there are several surgical approaches for access to resect tumours from the nasopharynx. However, the open techniques have been associated with much morbidity and only provide access to, and identification of, the ipsilateral internal carotid artery that forms the lateral boundary and resection limit of the nasopharynx. The coupling of stereotactic navigation guidance and a minimally invasive transnasal approach for nasopharyngectomy allows the surgeon to identify and protect the internal carotid artery bilaterally at the nasopharynx. This technique reduces operating time and morbidity to a minimum and yet is oncologically sound for resecting nasopharyngeal lesions. We compare 15 patients who underwent the stereotactic navigation guidance approach with 20 patients who received a conventional open transfacial approach.
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Introduction
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The use of stereotaxis with a three-dimensional (3D) Cartesian coordination system for spatial localization was first developed in animal models by Horsley et al [1]. The first human stereotactic apparatus was invented by Spiegel et al in 1947 [2]. Since then, rapid advances in neuroscience and CT have made the development of stereotactic surgery possible in almost every field where precise spatial localization is required.
Nasopharyngectomy has been well established as a surgical treatment for recurrent and residual disease of nasopharyngeal carcinoma (NPC) [3–10]. The various surgical approaches described for nasopharyngectomy include trans oro-palatal [3, 4], maxillary swing [5, 11], intra-oral Le Fort I osteostomy [12] and mandibular swing [6]. A new technique [7] with transnasal approach has been developed in our institution by employing a mid-face degloving intra-oral incision [9, 13, 14]. Previous complications such as facial scar, trismus, ectropion, damage to the lingual nerve or infra-orbital nerve, palatal dehiscence, nasal regurgitation, malocclusion, excessive blood loss and prolonged operating time and hospital stay could be avoided or reduced. Therefore the stereotactic navigation-guided transnasal approach has reduced the morbidity of the procedure to a minimum and improved the quality of life with the same survival [7, 15]. However, the limitation of this transnasal mid-face degloving technique [9] is the restricted access in identifying the internal carotid arteries that are located bilaterally in the paranasopharyngeal space. Stereotactic navigation guidance (SNG) [7] was applied successfully to locate and protect the internal carotid arteries and facilitate dissection and clearance of the tumour in the nasopharynx. This report compares the advantages of the image guidance transnasal technique [7] with the conventional open transfacial approaches [3–6].
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Material and methods
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Hospital records of 35 patients with recurrent NPC between March 1997 and June 2001 were studied. All patients were operated on by the first author under general anaesthesia and through a tracheostomy tube. The technique of frameless SNG was performed on 15 patients using Vector Vision 2 (Brain LAB, Heimstetten, Germany) equipment (Figure 1
). A standard mid-face degloving incision [9, 13, 14] is used to expose the nasal cavity and maxilla. Bilateral medial maxillectomy is then performed. The posterior nasal septum, bilateral inferior turbinate and middle turbinate are excised to expose the entire nasopharynx, including the paranasopharyngeal space (Figure 2). The internal carotid arteries are then located by the navigation probe before dissection of the tumour (Figures 3–7). The other 20 patients received open transfacial approaches.
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Figure 1. The stereotactic navigation guidance system (Vector Vision 2; Brain Lab, Heimstetten, Germany) in theatre.
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Figure 2. Retracted upper lip and bilateral medial maxillectomy allowing a minimally invasive transnasal approach to the nasopharynx.
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Figure 3. Computer generated three-dimensional image showing identification of the left internal carotid artery with the navigation probe.
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Figure 4. Axial CT image showing identification of the left internal carotid artery with the navigation probe.
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Figure 5. Coronal CT image showing identification of the left internal carotid artery with the navigation probe.
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Figure 6. Contrast enhanced axial CT showing recurrence of nasopharyngeal carcinoma in the right nasopharynx (arrow) invading posterolaterally.
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Figure 7. Arial T1 weighted MRI image 3 monthspost-operative of nasopharyngectomy. Nasopharyngectomy was performed via the transnasal approach.
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Imaging protocol in stereotaxy
CT stereotactic localization is performed using a GE HiSpeed Advantage RP Scanner System (General Electric, Milwaukee, WI). Six temporary fiducial CT markers (containers filled with contrast medium) are placed over the patient's forehead at specific sites for stereotaxy. The head is placed in a neutral position with respect to the craniocaudal axis [10, 11]. Scanning parameters include zero gantry tilt, 120 kV, 170 mA, 3 mm slice thickness with 1:1 pitch, 512 x 512 matrix size and 48 cm x 48 cm scan field of view. The examination extends from the cranial fossae down to the base of the symphesis menti, including all the fiducial markers, the entire nasopharynx and the distal common carotid arteries, the carotid bifurcations and the internal carotid arteries up to the foramen lacerum. Intravenous contrast medium is given as a bolus of 90 ml of Ultravist 240 (Schering, Germany) by power injector at a rate of 2 ml sec-1. A typical study comprises 50–60 slices and can be completed within 1 min. The entire examination lasts about 5 min. The patient must keep their head still and avoid swallowing to reduce artefacts and registration errors. After CT scanning, the raw data are transferred to a designated SNG workstation and the images fused to give a set of stereotactically matched images. T1 weighted MRI with contrast medium (no gantry tilt) might also be carried out. Four patients in the transnasal group received both CT and MRI, however, the benefit obtained from MRI was not clinically obvious and therefore not applied to other patients to avoid prolonged imaging time and for better distribution of resources.
The stereotactic navigation technique
The SNG system consists of two major components. The first is a computer workstation for the capture of neuro-images, image fusion and subsequent surgical planning. The second is a pair of digital infrared cameras and a number of reflective reference markers attached to various instruments. The angle between the pair of infrared cameras is calibrated. The cameras are positioned to overlook the operating field. The workstation calculates the 3D position of the reflective markers based on the camera angle and location of the markers on the camera images. A set of reference markers is attached rigidly to the skull via a skull clamp during the operation (Figure 1
). Coregistration of the navigation system takes about 15 min. A wireless navigation probe with reflective markers is used to register the location of the same set of CT markers in the operating room.
The error between the true position of the fiducial markers and the position as depicted on the computer screen is the fiducial registration error (FRE). This is defined as the root mean square distance between corresponding fiducial points after registration. FRE, using the set up described, is typically between 0.8 mm and 1.2 mm. Registration of a set of fiducial markers allows the computer to determine a virtual 3D coordinate system that is used to match imaging data to the position of the patient's head on the operating table and to trace the position of the navigation probe in relation to neuro images during the operation (Figures 3–5).
During application of the navigation probe the target registration error (TRE) [16], which is the error in the position of an anatomical target, must be kept in mind. Therefore it is important for the surgeon to point at a known surface anatomical structure with the navigation probe and to visually correlate this with the computer screen to reaffirm that the whole system is functioning accurately and pointing at the correct anatomy before the probe is applied to the target anatomy and resecting the lesions.
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Results
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29 male and 6 female patients who received nasopharyngectomy for their recurrent NPC at Prince of Wales Hospital, The Chinese University of Hong Kong between March 1997 and June 2001 were studied. The patients' ages ranged from 26 years to 69 years, with a mean age of 55.5years. Four surgical approaches were applied, namely transnasal, maxillary swing, mandibular swing and combined maxillary–mandibular swing. 15 transnasal approach operations were carried out using SNG, and 20 patients received an open transfacial approach without SNG.
There was no peri-operative mortality in the transnasal group and two cases of post-operative meningitis, resulting in death, in the open transfacial group. In one patient, the left internal carotid artery was damaged during dissection (open maxillary swing approach). The artery was ligated and the patient recovered uneventfully. Other parameters such as operating time, blood loss, hospital stay, ectropion, damage to nerves, palatal dehiscence, trismus, velopharyngeal incompetence and nasal regurgitation are compared and illustrated in Table 1.
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Table 1. Comparison of operating time, blood loss, hospital stay and incidence of other morbidities associated with different surgical approaches to nasopharyngectomy
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Discussion
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The nasopharynx is situated at the centre of the anterior skull base. The anatomical relations of the nasopharanynx include the clivus posteriorly, the posterior nasal cavity anteriorly and the carotid spaces laterally. Superiorly lies the sphenoid sinus, and the inferior boundary is defined by the level of the soft palate. Access to the nasopharynx for tumour resection has remained a challenge to surgeons owing to its deep anatomical position and its proximity to major structures such as the internal carotid arteries.
NPC is a common malignancy in Hong Kong [17]. The treatment of primary disease is with radiotherapy, whereas surgery followed by post-operative intracavity brachytherapy has proven to be one of the choices in treating locally recurrent NPC so as to avoid the complications of high dose re-irradiation to the nasopharynx [3–9] and nearby structures.
Transfacial approaches (Figure 7) such as maxillary swing [5, 11] or mandibular swing [6] have been well described. However, these approaches involve major dissection, including splitting the face, upper or lower jaw, palate or floor of mouth, with transection of the infra-orbital or lingual nerves. All these tissues will have been heavily irradiated during primary treatment and carry a risk of radionecrosis if surgical trauma is inflicted for access to the nasopharynx. Wei et al [5] and King et al [8] reported an approximate 30% of palatal dehiscence in their series of nasopharyngectomy carried out via open transfacial approaches. King et al [8] also elaborated other morbidities associated with the open technique. In order to avoid inflicting surgical trauma to the previously irradiated tissue, a transnasal technique has been described [7, 9]. The advantages of the transnasal technique are that it can completely avoid a facial scar and avoid splitting the facial skeleton and oral cavity, thus avoiding the risk of unhealing wounds or formation of surgical scar. Any formation of surgical scar, as caused by open techniques, in the soft palate or floor of mouth would impair their functional movements thus causing post-operative dysphagia, nasal regurgitation and a hypernasal speech. Any unhealed wounds in the palate will form a fistula or iatrogenic cleft palate that would cause the same side effects.
The essence of the transnasal technique is therefore to work above the palate and not interrupt its anatomical and functional integrity. The intra-oral mid-face degloving incision avoids a facial incision or scar. However, the transnasal approach [9] has a major disadvantage in that it does not allow the surgeon to locate directly the internal carotid artery as is possible in the open face technique.
The application of SNG in locating and protecting the internal carotid artery bilaterally has been successfully performed on 15 patients in this study. SNG has been employed in neuro-otological surgery [18–20]; its use in nasopharyngectomy for recurrent NPC has enabled us to dissect tumours safely from the internal carotid artery bilaterally, whereas the open transfacial approach would allow only the ipsilateral internal carotid artery to be identified. The operating time, blood loss, hospital stay and other morbidities have been much reduced as compared with open transfacial approaches (Table 1).
Other important aspects of SNG are the initial high capital cost and the need for careful calibration to minimize the TRE [16] and enable accurate guidance and dissection.
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Conclusion
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The application of SNG in nasopharyngectomy for recurrent local NPC via a minimally invasive transnasal approach has allowed us to protect the internal carotid arteries, thus rendering the procedure a safe, less morbid and more cost effective technique than open transfacial approaches.
Received for publication April 11, 2001.
Accepted for publication January 15, 2002.
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Abstract
Introduction
