Transoral Approach to Parotid Tumors: A Review of the Literature

Different surgical techniques have been proposed for parapharyngeal space tumors, including transcervical, transparotid, trans-mandibular, infratemporal, and transoral. The choice of the correct approach depends on the size, localization and nature of the tumor. The transoral approach can be used for benign prestyloid masses, such as tumors of the deep lobe of the parotid gland. It guarantees a short hospitalization without skin scars. The narrowed access represents the main limitation of this technique. This review will summarize and analyze the current knowledge about the transoral approach to parotid lesions. Thirty-seven studies were included in a qualitative and quantitative synthesis. The novelty of this review is the quantitative analyses of the clinical data reported in the included studies.


Introduction
Parapharyngeal space (PPS) is located laterally to the pharynx and has the shape of an inverted pyramid extended from the cranial base to the greater cornu of the hyoid bone [1,2]. PPS is divided into prestyloid and retrostyloid compartments by stylohyoid ligament and muscles. Prestyloid space contains the deep lobe of the parotid gland, lymph nodes, ascending pharyngeal artery, ascending palatine artery, and fat, while retrostyloid space contains internal jugular vein, internal carotid artery, cranial nerves (from IX to XII), and lymph nodes ( Figure 1) [2].
PPS tumors are uncommon, representing only 0.5% of head and neck neoplasms. Most of them are benign and originate from the deep lobe of the parotid gland [3][4][5]. Magnetic resonance imaging (MRI) with gadolinium and fine needle aspiration cytology (FNAC) represent the gold standard for determining the diagnosis and thus the correct management of the patient. Other radiological examinations may include computed tomography (CT) with contrast, ultrasound scan and magnetic resonance angiography, which is recommended especially for vascularized tumors [6].
Different approaches to PPS tumors have been proposed, such as transcervical, transparotid, transmandibular, infratemporal, and transoral [7,8]. The choice of the surgical approach is based on the type and localization of the tumor and its relationships with neurovascular structures [7,9]. The transoral approach can be used for benign prestyloid masses, such as tumors of the deep lobe of the parotid gland [8].
The transoral approach is performed through an incision near the anterior tonsillar pillar mucosa, thus exposing prestyloid space under the superior pharyngeal constrictor muscle. Stylopharyngeal and styloglossus muscles are important landmarks for the transoral approach to PPS, since they divide pre-and retrostyloid spaces. Therefore, they represent the structures that should not be crossed to avoid severe vascular and nervous damages [10].
This review aims to summarize and analyze the current knowledge about the transoral approach to parotid lesions, with particular attention to tumor features and potential complications.

Materials and Methods
A review of the English literature was performed through several databases (Pub-Med, Scopus, accessed on 30 October 2022) to identify articles published before 30 October 2022. A primary search was performed using the terms "parotid AND transoral AND (tumor OR cancer OR adenoma OR warthin)". Search strategies were adapted for each database. The references of selected publications were then examined to identify further reports that were not found by database searching, and the same selection criteria were applied.
The inclusion criteria were clinical trials, cohort studies, case-control studies, case series, and case reports, regarding the transoral approach for parotid gland tumors. Exclusion criteria were as follows: non-human studies, non-English literature, transoral approach for non-salivary tumors, and insufficient clinical data (i.e., tumor origin) reported in the paper.
The abstracts of all relevant articles were examined using the inclusion criteria for applicability. The references of the selected publications were reviewed to identify further reports that were not found by database searching. Two independent reviewers (AL, AB), working separately, extracted the data from all the eligible studies, which were subsequently cross-checked. All retrieved full-texts articles were included in the review by a consensus of all the authors.

Materials and Methods
A review of the English literature was performed through several databases (PubMed, Scopus, accessed on 30 October 2022) to identify articles published before 30 October 2022. A primary search was performed using the terms "parotid AND transoral AND (tumor OR cancer OR adenoma OR warthin)". Search strategies were adapted for each database. The references of selected publications were then examined to identify further reports that were not found by database searching, and the same selection criteria were applied.
The inclusion criteria were clinical trials, cohort studies, case-control studies, case series, and case reports, regarding the transoral approach for parotid gland tumors. Exclusion criteria were as follows: non-human studies, non-English literature, transoral approach for non-salivary tumors, and insufficient clinical data (i.e., tumor origin) reported in the paper.
The abstracts of all relevant articles were examined using the inclusion criteria for applicability. The references of the selected publications were reviewed to identify further reports that were not found by database searching. Two independent reviewers (AL, AB), working separately, extracted the data from all the eligible studies, which were subsequently cross-checked. All retrieved full-texts articles were included in the review by a consensus of all the authors.

Review of the Literature
A total of 400 published papers were identified through database searches. After abstract screening for eligibility, 63 articles were considered eligible. Among these, we included 37 articles in qualitative and quantitative synthesis after full-text assessment. The other 26 papers were excluded for the following reasons: non-parotid tumors (n = 6), not transoral approach (n = 8), and incomplete clinical data (n = 12) ( Figure 2).

Review of the Literature
A total of 400 published papers were identified through database searches. After abstract screening for eligibility, 63 articles were considered eligible. Among these, we included 37 articles in qualitative and quantitative synthesis after full-text assessment. The other 26 papers were excluded for the following reasons: non-parotid tumors (n = 6), not transoral approach (n = 8), and incomplete clinical data (n = 12) ( Figure 2). Among the papers that matched the inclusion criteria, 14 publications were case reports and 23 were retrospective studies. One hundred and thirty-nine cases were included in the review (Table 1).
Based on available data, the mean age was 49.44 ± 15.73 years (range 14-78 years), with a male/female ratio of 67/72 (48.2% male, 51.8% female). The maximum tumor size ranged from 10 to 90 mm (mean 48.01 ± 15.29 mm, calculated on 116 patients). The maximum size was greater than 50 mm in 59 cases (42.4%). The mean tumor volume, calculated on 100 patients, was 36.52 ± 32.20 cm³ (range 0.35-141.99 cm³). Therefore, the transoral approach may be a feasible technique also for large tumors of the deep parotid lobe that involve PPS.
Pleomorphic adenoma was reported in most cases (116 patients, 84.1%). Other histotypes were basal cell adenoma (9 patients, 6.5%), carcinoma ex pleomorphic adenoma (6 patients, 4.3%), Warthin tumor (3 patients, 2.2%), mucoepidermoid carcinoma (2 patients, 1.4%), oncocytoma (1 patient, 0.7%), and adenocarcinoma (1 patient, 0.7%). Histology was not reported in one case (0.7%). The transoral approach was used for benign masses in most cases (129 patients, 92.8%, Figure 3). Indeed, benign capsulated masses may be removed through a blunt dissection with a lower risk of injury of vascular PPS structures than malignant lesions that infiltrate surrounding tissues. Preoperative imaging (MRI) is mandatory to evaluate the presence of a tumor capsule or pseudocapsule and correctly select patients for the transoral approach (well-circumscribed masses with a clear cleavage plane from great vessels). Extensive tumors without smooth borders and/or surrounding the facial nerve or major vascular structures may not be suitable for this approach. Moreover, the transoral approach has not been used for retrostyloid masses in the selected studies. Among the papers that matched the inclusion criteria, 14 publications were case reports and 23 were retrospective studies. One hundred and thirty-nine cases were included in the review (Table 1).
Based on available data, the mean age was 49.44 ± 15.73 years (range 14-78 years), with a male/female ratio of 67/72 (48.2% male, 51.8% female). The maximum tumor size ranged from 10 to 90 mm (mean 48.01 ± 15.29 mm, calculated on 116 patients). The maximum size was greater than 50 mm in 59 cases (42.4%). The mean tumor volume, calculated on 100 patients, was 36.52 ± 32.20 cm 3 (range 0.35-141.99 cm 3 ). Therefore, the transoral approach may be a feasible technique also for large tumors of the deep parotid lobe that involve PPS.
Pleomorphic adenoma was reported in most cases (116 patients, 84.1%). Other histotypes were basal cell adenoma (9 patients, 6.5%), carcinoma ex pleomorphic adenoma (6 patients, 4.3%), Warthin tumor (3 patients, 2.2%), mucoepidermoid carcinoma (2 patients, 1.4%), oncocytoma (1 patient, 0.7%), and adenocarcinoma (1 patient, 0.7%). Histology was not reported in one case (0.7%). The transoral approach was used for benign masses in most cases (129 patients, 92.8%, Figure 3). Indeed, benign capsulated masses may be removed through a blunt dissection with a lower risk of injury of vascular PPS structures than malignant lesions that infiltrate surrounding tissues. Preoperative imaging (MRI) is mandatory to evaluate the presence of a tumor capsule or pseudocapsule and correctly select patients for the transoral approach (well-circumscribed masses with a clear cleavage plane from great vessels). Extensive tumors without smooth borders and/or surrounding the facial nerve or major vascular structures may not be suitable for this approach. Moreover, the transoral approach has not been used for retrostyloid masses in the selected studies.    A pure transoral approach was performed in 125 cases (89.9%), while a combined transcervical/transparotid and transoral approach was used in 14 patients (10.1%). Robotic surgery was reported in 55 cases (39.5%) of pure transoral approach and 11 cases (7.9%) of combined approaches. An endoscopic-assisted transoral approach was described in 41 cases (29.5%). Robotic systems and endoscopes were used to improve the visualization of the mass and PPS structures. Indeed, the main limitations of the transoral approach include a poor visualization of PPS major neurovascular structures with possible uncontrollable bleeding, capsule disruption and tumor spillage, and incomplete tumor removal. According to the size and localization of the mass, 0-, 30-, and 45-degree endoscopes were used for the dissection of tumors' lateral, superior, and deep margins. The development of endoscopy and surgical robotics reduced such disadvantages and increased the feasibility of the transoral approach. However, surgeons must be able to convert the procedure to a transcervical/transparotid approach if needed (impossibility of complete transoral removal, uncontrollable bleeding). Furthermore, trismus is a contraindication for the transoral approach.
Complications were reported in nine cases (6.5%). They were represented by pharyngeal dehiscence (two cases), deep neck space seroma/sialocele (two cases), facial nerve palsy (two cases), and hyperemia of the skin near angulus mandibulae (one case). Two cases described by Boyce et al. had a complication. However, since other patients with non-parotid tumor histology were reported without matching complications, we could not identify those related to parotid tumors ( Figure 4) [31]. All the reported complications were solved. No severe bleeding was reported. However, surgeons must be aware of possible severe bleeding from great vascular structures and ready for a prompt transcervical ligation of external carotid artery. A medial displacement of the internal carotid artery should be considered a contraindication for the transoral approach. Severe postoperative pain was not reported in any cases. Wound closure was usually performed in more than one layer to reduce the risk of postoperative wound dehiscence.  A pure transoral approach was performed in 125 cases (89.9%), while a combined transcervical/transparotid and transoral approach was used in 14 patients (10.1%). Robotic surgery was reported in 55 cases (39.5%) of pure transoral approach and 11 cases (7.9%) of combined approaches. An endoscopic-assisted transoral approach was described in 41 cases (29.5%). Robotic systems and endoscopes were used to improve the visualization of the mass and PPS structures. Indeed, the main limitations of the transoral approach include a poor visualization of PPS major neurovascular structures with possible uncontrollable bleeding, capsule disruption and tumor spillage, and incomplete tumor removal. According to the size and localization of the mass, 0-, 30-, and 45-degree endoscopes were used for the dissection of tumors' lateral, superior, and deep margins. The development of endoscopy and surgical robotics reduced such disadvantages and increased the feasibility of the transoral approach. However, surgeons must be able to convert the procedure to a transcervical/transparotid approach if needed (impossibility of complete transoral removal, uncontrollable bleeding). Furthermore, trismus is a contraindication for the transoral approach.
Complications were reported in nine cases (6.5%). They were represented by pharyngeal dehiscence (two cases), deep neck space seroma/sialocele (two cases), facial nerve palsy (two cases), and hyperemia of the skin near angulus mandibulae (one case). Two cases described by Boyce et al. had a complication. However, since other patients with non-parotid tumor histology were reported without matching complications, we could not identify those related to parotid tumors (Figure 4) [31]. All the reported complications were solved. No severe bleeding was reported. However, surgeons must be aware of possible severe bleeding from great vascular structures and ready for a prompt transcervical ligation of external carotid artery. A medial displacement of the internal carotid artery should be considered a contraindication for the transoral approach. Severe postoperative pain was not reported in any cases. Wound closure was usually performed in more than one layer to reduce the risk of postoperative wound dehiscence. The mean follow-up was 26.57 ± 36.58 months (range 1-192 months). However, follow-up was not reported in 31 cases (22.3%). One case of pleomorphic adenoma recurred after 39 months. The low recurrence rate suggests that the transoral approach for selected benign masses has a low risk of capsule disruption and/or incomplete tumor removal. One case of mucoepidermoid carcinoma with positive margins after transoral removal underwent a superficial parotidectomy with traditional open approach. An indication to a pure transoral approach for malignant tumors of the deep parotid lobe involving PPS must be accurately evaluated and reserved to highly selected cases.
The success of the transoral approach for tumors of the deep parotid lobe depends on the correct identification and exposure of the mass to allow the complete removal, prevent recurrence and ensure good functional outcomes, taking into account the risks of PPS surgery.
The main limitations of the published studies include the small number of patients, the short follow-up and the lack of a randomized control group that underwent an external approach.

Conclusions and Future Perspectives
Selecting the best approach for PPS tumors originating from the parotid deep lobe is based on the volume and extension of the mass to be removed and its putative nature. With these two parameters being established, the less invasive approach that provided sufficient exposure with a low and acceptable complication rate was then chosen. The transoral approach can be selected for benign lesions of the deep parotid lobe that involve prestyloid space. The literature reported a low complication rate with a good recovery in such cases. The novelty of this review was the quantitative analyses of the clinical data reported in the included studies. Further studies with larger samples are mandatory to better understand the role of the transoral approach for PPS tumors with a particular focus on robotic and endoscopic-assisted surgery. The mean follow-up was 26.57 ± 36.58 months (range 1-192 months). However, follow-up was not reported in 31 cases (22.3%). One case of pleomorphic adenoma recurred after 39 months. The low recurrence rate suggests that the transoral approach for selected benign masses has a low risk of capsule disruption and/or incomplete tumor removal. One case of mucoepidermoid carcinoma with positive margins after transoral removal underwent a superficial parotidectomy with traditional open approach. An indication to a pure transoral approach for malignant tumors of the deep parotid lobe involving PPS must be accurately evaluated and reserved to highly selected cases.
The success of the transoral approach for tumors of the deep parotid lobe depends on the correct identification and exposure of the mass to allow the complete removal, prevent recurrence and ensure good functional outcomes, taking into account the risks of PPS surgery.
The main limitations of the published studies include the small number of patients, the short follow-up and the lack of a randomized control group that underwent an external approach.

Conclusions and Future Perspectives
Selecting the best approach for PPS tumors originating from the parotid deep lobe is based on the volume and extension of the mass to be removed and its putative nature. With these two parameters being established, the less invasive approach that provided sufficient exposure with a low and acceptable complication rate was then chosen. The transoral approach can be selected for benign lesions of the deep parotid lobe that involve prestyloid space. The literature reported a low complication rate with a good recovery in such cases. The novelty of this review was the quantitative analyses of the clinical data reported in the included studies. Further studies with larger samples are mandatory to better understand the role of the transoral approach for PPS tumors with a particular focus on robotic and endoscopic-assisted surgery.