Transoral robotic surgery for parapharyngeal space tumors

Introduction

Since its US Food and Drug Administration (FDA) approval in 2009, the utility of transoral robotic surgery (TORS) as a minimally invasive technique has continued to expand. Initially approved for resection of oropharyngeal tumors, its indications have now broadened to include tumors of the larynx, hypopharynx, nasopharynx, skull base, and, more recently, the parapharyngeal space (PPS). ^,

The favored surgical approaches for PPS tumor resection have been transcervical, transcervical/parotid, and transcervical/mandibulotomy techniques. ^, This has in part been influenced by the relative inaccessibility of the PPS and its proximity to vital neurovascular structures, including the internal carotid artery (ICA), internal jugular vein, and associated nerves. ^, Transoral approaches have been described and were developed to avoid the morbidity associated with open approaches, particularly if mandibulotomy was required. These include visible skin scarring, facial nerve injury, first bite syndrome, nasogastric tube placement, increased length of stay, and, rarely, tracheotomy. This morbidity had to be balanced against the less morbid transoral approach with its limited access, potential for tumor rupture, incomplete excision, poor visualization, and neurovascular injury. Hence, only a small number of series have been reported. ^,

The introduction of the da Vinci robotic platform with high definition 3D visualization and small wristed instrumentation with 6 degrees of movement has revolutionized the transoral approach, making TORS an attractive alternative to open procedures. Furthermore, for experienced head and neck surgeons performing TORS oropharyngectomy for benign and malignant tumors, access to the PPS is an extension of an already familiar approach. In experienced hands, TORS for selected PPS tumors is a feasible, safe, and direct approach associated with low morbidity.

Anatomy of the parapharyngeal space

The PPS is a complex anatomical compartment which conceptually can be viewed as an inverted pyramid. The base forms the roof or superior limit, which is the inferior aspect of the temporal bone containing three bony landmarks: the scaphoid fossa, spine of sphenoid bone, and the styloid process. Inferiorly, the greater cornu of the hyoid bone defines the apex. Laterally, it is bordered by the fascia overlying the medial pterygoid muscle, ramus of the mandible, and deep lobe of the parotid from anterior to posterior. The superior pharyngobasilar fascia, superior pharyngeal constrictor, and tensor veli palatini form the medial border. ^, The posterior limit is the prevertebral fascia, musculature, and cervical spine.

The PPS is divided into two compartments separated by the tensor veli palatini fascia which runs from the scaphoid fossa to the styloid process. Anterolateral is the prestyloid space and posteromedial is the poststyloid space. This division is of clinical importance as different tumors typically arise in each space, as predicted by their contents. The prestyloid space contains parotid tissue, minor salivary glands, fat, and lymph nodes. The poststyloid includes cranial nerves IX to XII, the sympathetic chain, the ICA, internal jugular vein, and lymph nodes. Tumors arising from the poststyloid space were previously considered a contraindication to TORS. However, as skills, knowledge, and technology have improved, some poststyloid tumors can be accessed and removed safely in the hands of experienced robotic surgeons. ^,

Preoperative assessment

Clinical presentation

Most patients with PPS tumors remain asymptomatic, and tumors are discovered incidentally on cross-sectional imaging performed for other reasons. This is consistent with the reduced cost and easy availability of sophisticated imaging across the globe. Previously, the most common presentation was with an intraoral or neck mass. ^, This reflects the deep and hidden location of the PPS, so tumors may not be appreciated until they are at least 3 cm in diameter, presenting as a neck mass or with medialization of the tonsillar fossa. ^, ^, The majority of tumors are benign and behave in an indolent manner. However, pain or cranial nerve dysfunction should raise the suspicion of malignancy.

A comprehensive head and neck examination is required for all patients with suspected or radiologically confirmed PPS tumors. Transoral examination may reveal a mass displacing the tonsillar fossa or distortion of the posterior pharyngeal wall especially if the mass is poststyloid. A thorough flexible endoscopic examination of the upper aerodigestive tract looking for fullness of the PPS and assessment of the lower cranial nerve is required.

Pathology

Tumors of the PPS are rare and compromise only 0.5% of all head and neck neoplasms. ^, The vast majority are benign lesions (80%), in which salivary gland tumors are most common (50%), followed by neural tumors (20%). There have been more than 70 different pathologic subtypes described in the literature. In total, 60% of tumors represent pleomorphic adenomas, paragangliomas, and schwannomas. ^, Pleomorphic adenomas arise from either the deep lobe of the parotid gland (dumbbell tumors) or from ectopic islets of salivary tissue within the PPS. Neurogenic tumors are most commonly post-styloid in location. Of these, the majority are paragangliomas (49%), schwannomas (31%), and neurofibromas (9%). Importantly, patients with neurogenic lesions requiring surgical excision should be appropriately counseled that this will result in a neuropathy of the nerve of origin postoperatively. Table 46.1 outlines the most common tumors arising in the pre- and poststyloid spaces of the PPS.

TABLE 46.1

Differential Diagnosis of Parapharyngeal Space Tumors Based on Location ^,

	Prestyloid	%	Poststyloid	%	No Specific Location	%
	Salivary	50	Neurogenic	20	Miscellaneous	30
Benign		80		95		70
	Pleomorphic adenoma Other benign	65 30	Paragangliomas Schwannomas Neurofibromas	50 30 10	Branchial cyst Aneurysm Hemangioma	10 15 10
	Pleomorphic adenoma Other benign	65 30	Other benign	5	Other benign	35
Malignant		20		5		30
	Adenoid cystic	<10	Paraganglioma	1	Lymphoma	20
	Mucoepidermoid Other malignant	<5 1	Nerve sheath Other malignant	1 3	Carcinoma Other malignant	10 <10
	Mucoepidermoid Other malignant	<5 1	Nerve sheath Other malignant	1 3	Carcinoma Other malignant	10 <10

The most common paraganglioma truly within the PPS is a glomus vagale. Carotid body tumors are the most common paraganglioma of the head and neck which can grow superiorly into the PPS. Glomus jugulare are the second most common and can grow inferiorly into the space. It is important to note that multicentricity in the head and neck occurs in up to 20% of cases. They can be associated with other neuroendocrine tumors in the chest, abdomen and pelvis. Approximately 40% of cases will have a genetic cause, most commonly related to mutations in the succinate dehydrogenase (SDH) complex. Consequently, whole body imaging is recommended in the form of a DOTA PET-CT scan to rule out the possibility of other neuroendocrine tumors, particularly pheochromocytomas. Only 2% to 3% of head and neck paragangliomas are functional. If suspected, serum and urinary catecholamine studies should be performed.

Imaging

High resolution cross-sectional imaging of the skull base and PPS is imperative for the complete workup of patients with a tumor. Imaging enables localization of the tumor, assessment of tumor extent, involvement of surrounding tissues, and vascularity. Furthermore, information on the proximity to the skull base and major neurovascular structures can be obtained from accurate imaging. In combination, this information permits precise management, surgical planning, and patient counseling.

Magnetic resonance imaging (MRI) is the modality of choice. The majority of tumors can be diagnosed on MRI based on their characteristic appearances on different sequences. Salivary gland tumors are most commonly T2 hyperintense and schwannomas T1 iso/hypointense with gadolinium enhancement. Paragangliomas have a characteristic “salt and pepper” appearance on MRI, due to their vascularity and consequent flow voids which are best seen on T2 sequences. Computer tomography (CT) is considered complimentary and may be an adequate alternative. Both modalities allow for angiography.

The most important feature on imaging is to determine whether the tumor is pre- or poststyloid. Careful analysis of the displacement of fat and the ICA helps determine the site of origin and the differential diagnosis of PPS tumors. ^, Classically, prestyloid masses lie anterolaterally to the great vessels and displace the ICA posteriorly. Post-styloid tumors lie posteriorly to the great vessels and displace the ICA anteriorly. ^, More recent studies have looked at the nuances of the relationship of the lower cranial nerves to the carotid sheath noting that not all poststyloid tumors result in displacement of the ICA in an anterior direction. As the majority of these tumors are neurogenic, the nerve of origin and its relationship to the carotid sheath may result in anterior, anterolateral, or posterolateral displacement of the ICA.

Other investigations

Fine needle aspiration biopsy (FNAB) may be contributory when managing PPS tumors. Its primary utility is when the diagnosis is not clear on imaging or there is suspicion of malignancy. If the tumor is visualized on transoral examination or easily palpated in the neck, an FNAB may be performed in the office.

Indications and contraindications for tors resection of parapharyngeal space tumors

There are no current consensus guidelines on the indications and contraindications for TORS resection of PPS tumors despite several authors proposing algorithms for this approach. ^, ^, ^, There is, however, some agreement on which a tumor may not be resected without morbidity, incomplete resection, or tumor spillage. This includes tumors with radiological evidence of osseous skull base erosion, large malignant infiltrative tumors which cannot be resected with a negative margin, and carotid encasement. There are now well documented approaches for tumors passing through the stylomandibular tunnel arising from the deep lobe of parotid that combines external and transoral approaches. Included in this are patient factors inhibiting successful transoral surgery as outlined below in Table 46.2 .

TABLE 46.2

Contraindications for TORS Resection of Parapharyngeal Space Tumors ^, ^, ^, ^,

Absolute Contraindications	Relative Contraindications
Internal carotid artery or internal jugular vein encasement Carotid body tumor Dumbbell tumors arising from the deep lobe of the parotid (unless combined approach) Osseous skull base involvement Malignant tumors where TORS would not provide a negative margin	Patient factors—8 Ts of endoscopic access: T eeth, T rismus, T ransverse dimension (mandibular), T ori (mandibular), T ongue, T ilt (atlanto-occipital extension), T reatment (prior radiotherapy), and T umor All paragangliomas (except glomus vagale) Malignant PPS tumors Tumors within 10 mm of the skull base Lateral extension of tumors into the stylomandibular tunnel

PPS, Parapharyngeal space; TORS, transoral robotic surgery.

Involvement of the skull base is a contraindication to a TORS approach. Boyce et al. proposed that tumors extending to within 10 mm of the skull base require an open approach due to the risk of tumor disruption using a TORS approach. This would appear to be an issue specific to pleomorphic adenoma or other potentially friable tumors. Neurogenic tumors such as schwannomas and glomus vagale extending toward the skull base are more amenable to a TORS approach due to their firmer, well encapsulated consistency. The traditional transoral incision through the anterior pillar is extended into the soft palate by curving the incision along the lateral aspect of the soft palate to reach the hard palate. Some authors have described this as a “J” shaped incision. ^, It provides wider access to the superomedial aspect of the PPS and, importantly, easy identification and control of the ICA. Malignancy in the setting of small well-circumscribed tumors does not preclude a TORS approach. ^, Transoral excision of malignant tumors with infiltrative growth pattern is contraindicated, as is skull base invasion and larger tumors which similarly cannot be completely removed.

The question of tumor size, which initially concerned critics of the transoral approach, is less of an issue with TORS. ^, There is currently no predetermined size criteria. All of the recent published series on TORS PPS tumor resection do not include size in isolation as a contraindication to successful treatment. ^, ^, ^, ^,

The ultimate decision on TORS resectability of PPS tumors involves careful patient selection. This is a clinical decision integrating patient factors, tumor characteristics, and cross-sectional imaging information. Ultimately, one of the most important factors is surgeon experience, and PPS tumor resection should be reserved for surgeons with significant experience in TORS. ^,

Instruments and patient positioning

We use the da Vinci Robotic Surgical System (Intuitive Surgical Inc., Sunnyvale, CA) for TORS. We use the Xi robotic platform. The patient is positioned supine, with neck extension, and head placed in a head ring, similar to TORS oropharyngectomy. General anesthesia is performed using a nasal or oral endotracheal tube positioned contralateral to the lesion. The patient is draped in standard fashion. A 2-0 silk suture is placed in the anterior tongue for retraction. Strips of DuoDerm (ConvaTec) are cut to size and molded to the upper teeth and a raytec is placed over the lower teeth for protection when using the FKWO (Feyh-Kastenbauer-Weinstein O’Malley) retractor (Gyrus Medical Inc., Tuttlingen, Germany). The Crowe-Davis mouth gag with integrated suction provides a good alternative to expose the oropharynx.

The da Vinci Xi robotic surgical system is docked from the left side, adjacent to the head. We place a 0-degree endoscope and two robotic arms equipped with an 8 mm Maryland bipolar forceps contralateral to the lesion side and an 8 mm spatula tip monopolar diathermy on the lesion side.

Tors technique

We use a bedside surgical assistant to suction any blood or surgical plume, apply hemostatic clips to veins and arteries, as well as retracting tissue to provide exposure.

An incision is made along the pterygomandibular raphe extending into the soft palate with monopolar cautery ( Fig. 46.1 ). The tonsil and superior constrictor muscle are retracted medially exposing the PPS fat ( Fig. 46.2 ). Any tonsillar arterial and venous branches are ligated with both bipolar cautery and hemostatic clips. A combination of blunt and sharp dissection is used to identify the tumor capsule in the PPS. Dissection proceeds circumferentially to mobilize the tumor initially superiorly at the level of the styloid and infratemporal fossa and inferiorly along the palatoglossal fold. Following superior, inferior, lateral, and medial mobilization, the posterior aspect of the tumor is left until last. We often use 1 × 3 cm neuropatties to help mobilize the tumor with the Maryland forceps ( Fig. 46.3 ). This is an atraumatic technique which helps to avoid tumor capsule breach or spillage. Care must be taken to avoid excess direct manipulation of the tumor. Often with larger tumors, we use blunt finger dissection to perform the final mobilization and delivery. Monopolar cautery dissection should not be used in the vicinity of the ICA, as injury may result in catastrophic bleeding.

Fig. 46.1, Left Pterygomandibular Raphe Incision to Access Parapharyngeal Space.

Fig. 46.2, Left parapharyngeal space fat is exposed between the medial pterygoid muscle laterally and superior constrictor muscle medially.

Fig. 46.3, Left parapharyngeal tumor is identified and mobilized with the help of neuro patties.

Once removed, the specimen is assessed to ensure capsular integrity. The operative field is then irrigated with copious amounts of warmed saline wash. A 30-degree telescope is used to inspect the operative cavity. Meticulous hemostasis is performed. The surgical bed is then carefully inspected for any evidence of residual tumor ( Fig. 46.4 ). The pharyngotomy is closed with interrupted 3-0 Vicryl sutures ( Fig. 46.5 ). The patient is then awakened, extubated, and sent to the recovery room. demonstrates TORS approach to PPS tumors.