Recurrent Nasopharyngeal Cancer: Critical Review of Local Treatment Options Including Recommendations during the COVID-19 Pandemic

Simple Summary Options for the curative treatment of locally recurrent nasopharyngeal carcinoma include surgery or re-irradiation. Both approaches have been scientifically explored, yet there is no consensus on the indication or definitive preference of the above two salvage treatments. The aim of this review is to summarize the current evidence on the local treatment of recurrent nasopharyngeal carcinoma. The feasibility, safety, and efficacy of salvage surgery and radical re-irradiation are discussed. Recommendations on treatment modifications during the coronavirus disease 2019 pandemic are included as well. Abstract Recurrent nasopharyngeal carcinoma represents an extremely challenging therapeutic situation. Given the vulnerability of the already pretreated neurological structures surrounding the nasopharynx, any potential salvage retreatment option bears a significant risk of severe complications that result in high treatment-related morbidity, quality of life deterioration, and even mortality. Yet, with careful patient selection, long-term survival may be achieved after local retreatment in a subgroup of patients with local or regional relapse of nasopharyngeal cancer. Early detection of the recurrence represents the key to therapeutic success, and in the case of early stage disease, several curative treatment options can be offered to the patient, albeit with minimal support in prospective clinical data. In this article, an up-to-date review of published evidence on modern surgical and radiation therapy treatment options is summarized, including currently recommended treatment modifications of both therapeutic approaches during the coronavirus disease 2019 pandemic.


Early Detection of the Recurrence and Restaging
Early detection of recurrent disease is of paramount importance. The initial assessment is better carried out by rigid nasopharyngeal endoscopy; according to prospective trials comparing this procedure to computed tomography (CT), a significantly better agreement with the histological findings has been reported [9,10]. The rigid endoscopic examination provides a better view than flexible endoscopy, to the extent that even submucosal bulges can be seen [11]. Narrow-band imaging (NBI) increases the diagnostic value of endoscopy and differentiates early nasopharyngeal recurrences on the basis of distinctive morphological characteristics of mucosal capillary vessels in such lesions [12]. A recent meta-analysis provided reliable evidence that the level of plasma EBV DNA, as a tumor marker for NPC with high sensitivity and specificity, has a high diagnostic value in the detection of recurrent NPC (rNPC), and monitoring of the levels of plasma EBV DNA is recommended in the follow-up of this disease [13]. Magnetic resonance imaging (MRI), aside from providing better images of soft tissues, also shows tumor size and extension to the base of the skull and intracranial cavity.
Prior to any optimal treatment decision-making, a precise restaging is absolutely necessary, as up to 56% of all patients diagnosed with locally recurrent NPC may suffer from a synchronous distant disease [6,7,14,15]. The cumulative incidence of distant failure exceeds 25% even for recurrent T1 tumors on Tumor-Node-Metastases (TNM) classification [16]. The restaging workup should include a whole-body positron emission tomography/computed tomography (PET/CT) scan to exclude distant metastases. In the presence of negative findings on both PET/CT and MRI, the regional nodal and distant disease can be reasonably, although not absolutely, excluded.

Patient Selection for Retreatment with Curative Intention
A carefully selected subset of patients with rNPC may achieve long-term survival after local retreatment. However, the location of the nasopharynx and its proximity to vital organs and radioresistance induced by previous RT make the therapy challenging. Significant acute and late toxicity must be expected.

Factors Affecting Prognosis
The exact definition of prognostic factors may guide the provision of individualized treatment and lead to a higher chance of local salvage. The first ever validated prognostic index for risk stratification in patients with radioresistant NPC who present with an isolated local recurrence following primary RT was developed by two Asian academic institutions on the basis of a training cohort of 251 patients and a validation cohort of 307 patients who had undergone salvage treatment using IMRT from 2001 to 2015 [17]. Clinical indices (factors) of retreatment using IMRT were independently validated for survival and grade 5 toxicity-free rate. Covariates that were adversely associated with overall survival (OS) in the training cohort were, in ascending order of weight: larger gross tumor volume (GTV) of the recurrence (p = 0.001), higher age at the time of recurrence (p = 0.008), repeated IMRT equivalent dose in 2-Gy fractions (EQD2) of ≥68 Gy (p = 0.03), prior RT-induced grade ≥3 toxicity (p = 0.001), and T3-T4 classification of the recurrent disease (p = 0.005). The prognostic index was able to dichotomize the cohort into low-risk and high-risk subgroups that were correlated with survival outcomes and treatment-related mortality after salvage IMRT [17]. Low-risk patients with an estimated 3-year overall survival (OS) in 69-74% represent ideal candidates for curative re-irradiation using IMRT, whereas novel clinical trials are recommended in the unfavorable high-risk subgroup with the reported 3-year OS in 15-31% only. To encourage its widespread clinical use, the prognostic index presented herein is a publicly available online tool at http://prancis.medlever.com [18].
Factors with a major impact on OS in the recursive partitioning analysis (RPA) published by Ward [19].
The volume of the recurrent tumor is another independent prognostic factor. Five-year OS rates are poorer in patients with a tumor volume >38 cm 3 than in patients with a tumor volume ≤38 cm 3 (30.1% vs. 55.9%, p < 0.001) [20]. World Health Organization histologic type also determines the outcome in rNPC. Locoregional progression-free survival (LRPFS, p < 0.035) and OS (p < 0.0001) are both significantly better for patients with WHO type III disease than with WHO type I or II disease [21].
The prognostic significance of biomarkers, including pretreatment circulating plasma EBV DNA titer [22][23][24] and the neutrophile-to-lymphocyte ratio (NLR) [25,26] that serve as biologic surrogates of occult metastases, is uncertain in locally or regionally recurrent NPC. However, studies have shown that a detectable circulating EBV DNA following upfront CRT is associated with poor prognosis of EBV-associated NPC, and, conversely, patients with undetectable circulating EBV DNA may have an excellent prognosis [5,24]. Furthermore, in a retrospective study of 552 patients from an endemic region, the patients with a low NLR (≤2.29) reached a significantly worse survival in comparison to the patients with a higher NLR (>2.29) (HR, 1.46; 95% CI, 1.14-1.85; p = 0.002) [27].

Surgical Treatment
Salvage surgery can be an option for carefully selected patients with a local recurrence or an isolated relapse in the neck [28,29]. A new surgical staging system of recurrent nasopharyngeal carcinoma has been proposed to help determine the therapeutic outcome in resected patients. The "surgical" stage model is based on the recurrent Tumor-Node-Metastases (rTNM) staging system for rNPC and classifies local recurrences as minimal (stage I), limited (stage II), extensive (stage III), and disseminated (stage IV). The survival curves are distinctly separated within these stages (5-year OS 72.0%, 55.1%, 21.1%, and 10.1% for surgical stages I, II, III, and IV, respectively; p < 0.001) [30].
Three-year survival rates up to 60% after salvage surgery have been reported, with survival benefit being most significantly observed in patients with T1 and T2 recurrent disease on TNM classification [31][32][33][34]. Locally advanced stages with skull base, cranial nerve, dural or brain involvement, positive surgical margins, and concurrent nodal metastases are associated with poor prognosis [35,36].
Some evidence suggests that surgical salvage may be superior to re-irradiation using IMRT (re-IMRT) in terms of LC, OS, quality of life (QoL), and even treatment cost [34,37]. In a case-matched cohort analysis, surgical treatment was associated with significantly better OS compared to re-IMRT (5-year OS 77.1% vs. 55.5%, p = 0.003), higher QoL (mean global health status score 57.6 vs. 29.8, p < 0.001), and decrease in severe post-treatment complications (12.5% vs. 65.3%, p < 0.001) [37]. To date, there is no consensus on the indications or definitive preference of either surgery or re-irradiation in this clinical setting [38].

Recommendations for Surgical Treatment during the COVID-19 Pandemic
In general, curative treatment of any cancer is challenging not only for the patient but also for the healthcare system, with physicians and other healthcare providers in the pivotal role. This is highly relevant during any era of limited resources. The current coronavirus disease 2019 (COVID-19) pandemic is a real example of a situation where even developed countries with an advanced healthcare system quickly reach a state of limited resources. For this reason, the discussion and guidance of treatment approaches during limited resources is becoming the new unmet medical need. In this article, we held on the current COVID-19 situation, although some parts of the following discussion may be generalizable.
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the COVID-19 pandemic with the first reported outbreak in China in 2019. It is found to a high extent in the upper respiratory tract mucosa, especially in the nasopharynx [39]. A high risk of infection is imposed on all head and neck surgeons when treating patients with rNPC. There is general concern over personal protective equipment shortage rises as the number of confirmed COVID-19 cases grows worldwide [40].
All patients should be in a home isolation or, if feasible, hospital quarantine at least 14 days prior to any surgical salvage within the nasopharynx. Two sets of real-time reverse transcription polymerase chain reaction (rRT-PCR) tests for the qualitative detection of nucleic acid from SARS-CoV-2 should be collected 24 hours apart. If the individual remains asymptomatic during the 14 days of isolation or quarantine followed by two separate negative rRT-PCR results, surgery can be performed with standard precautions [41][42][43]. The hospital infection control team, in cooperation with microbiologists and pulmonary physicians, should consult further management of the patients who are tested positive for COVID-19. It is recommended that surgery be postponed until symptoms have resolved and two separate rRT-PCR tests have remained negative. According to some authors, a 12-week waiting time is acceptable for patients with rNPC during the COVID-19 pandemic [40].

Endoscopic Nasopharyngectomy
A worldwide shift from an open to a minimally invasive endoscopic approach is widely observed in the surgical treatment of rNPC [38]. Indications for endoscopic nasopharyngectomy (ENP) using a transnasal approach include early-stage local recurrences (T1, T2 on TNM classification) located at the nasopharyngeal roof that invade minimally to the parapharyngeal space. The major goal of surgery is to achieve microscopically negative margins while preserving the surrounding fragile neurological and vascular structures. With a careful patient selection, ENP may achieve results comparable to re-IMRT, with a reduced rate of severe treatment-related complications [37,[44][45][46][47][48][49][50][51][52][53].
Using the modern three-dimensional (3-D) endoscopic navigation system, resection of locally advanced tumors located at the base of the skull is facilitated, with an easier approach to the adjacent pterygopalatine fossa. Safe isolation of the petrous and paraclival portion of the internal carotid artery (ICA) is enabled as well [49,50,53].
Transoral robotic surgical (TORS) nasopharyngectomy is performed using a transpalatal wound, with the help of a high-resolution 3-D camera and two Endowrists. In the case of the involvement of the base of the skull, the transoral approach may be combined with the transnasal endoscopic approach in order to achieve a better treatment outcome [51]. Unfortunately, with the use of TORS nasopharyngectomy, the crucial tactile sensation is disabled, which may increase the risk of iatrogenic damage to the ICA [40,48,51].
During the COVID-19 pandemic, concerns regarding the safety of the endoscopic approaches to the nasopharynx arise as a high viral load of SARS-CoV-2 in the upper respiratory tract mucosa is documented [39]. The risk of aerosolization of blood and irrigation fluids is not negligible in this situation [55,56]. In order to minimize the risk, constant irrigation and smoke, fluid, and blood suction using a high-efficiency particulate air filter is recommended [57].

Maxillary Swing Nasopharyngectomy
Following maxillary swing nasopharyngectomy (MSNP), the observed 5-year LC and 5-year OS can reach up to 74% and 62%, respectively, with reported severe complications such as facial numbness, trismus, palatal fistulae, and otitis media with middle ear effusion in up to 38% of the patients [58][59][60] ( Table 1). The maxillary swing approach involves a Weber Ferguson Longmire incision followed by osteotomies of the anterior and medial wall of the maxillary antrum and the lower portion of the zygomatic arch and separation of pterygoid plates from the maxillary tuberosity [40]. During the procedure, the maxillary osteocutaneous unit is unfolded and its blood supply is secured by the branches of the external carotid artery. With the use of a state-of-the-art MSNP, a complex exposure of the nasopharyngeal, oropharyngeal, and parapharyngeal space can be achieved and the acquirement of microscopically negative resection margins is potentially enhanced.
Throughout the MSNP, the ICA can be identified by manual palpation or with the use of intraoperative ultrasound. If there is ICA exposure, the surgeon may opt for a microvascular tissue transfer, which may result in a decreased risk of bleeding or carotid artery blow-out [40]. Tumors that encircle the ICA are technically challenging, but they are potentially resectable using an extracranial-intracranial bypass [61]. Therefore, the indication of MSNP can be extended to more clinical situations and MSNP may be safely offered to a carefully selected subset of patients with locally advanced T3-T4 rNPC [58][59][60][61][62].

Neck Dissection
Most regionally recurrent NPCs are technically resectable, and neck dissection (ND) is the treatment of choice for patients with isolated regional failure with the observed 5-year OS in 41% [63]. Currently, there are no uniform standards on the extent of the ND in this clinical setting.
Radical, modified radical, or selective neck dissection can all be considered for an isolated neck recurrence after initial RT. Radical neck dissection (RND) has long been considered the mainstay of treatment for the regional failure of NPC [64][65][66][67]. However, RND may worsen QoL of the patients because of the potential damage of the spinal accessory nerve and chronic shoulder stiffness and pain [68]. Whether such an extensive operation such as RND is truly necessary to achieve control of the neck disease is highly debatable, as cervical metastases in rNPC frequently present as a solitary node, and, sometimes, subsequent examination of the radical neck dissection specimen reveals no tumor in any of the lymph nodes removed [69]. Some authors suggest that as the neck nodes in level I are affected only in 4-5% of the patients with regionally recurrent NPC, a less extensive RND sparing level I might be considered [64,70,71].
Selective neck dissection (SND) may offer similar OS and disease-free survival (DFS) for patients with residual nodal disease in a single node following curative RT compared to patients that had undergone RND in this clinical setting, with the advantage of decreased peri-and postoperative morbidity [64,70]. Further studies are needed to determine and confirm which patients would benefit most from the SND procedure.

Recommendations for Radiotherapy during the COVID-19 Pandemic
To safeguard treatment capacity from staff shortages and mitigate the risk of patient infection by SARS-CoV-2 from daily hospital attendances, a large number of publications in the COVID-19 pandemic recommend consideration of hypofractionated radiation therapy (lower number of fractions with a higher dose per fraction) [117][118][119][120]. Indeed, although hypofractionated schemes have been proposed in head and neck cancer patients, the lowest currently accepted number of fractions for curative-intent treatments using IMRT according to the ASTRO-ESTRO Consensus Statement published in July 2020 is 20 [119]. However, if feasible and indicated, the choice to address the patient to mini-invasive endoscopic surgery would have the advantage to prevent patients from multiple accesses to the hospital, and the risk of cross-contamination would be limited [121].
With the use of re-IMRT, severe ≥ grade 3 toxicity was variably reported in 0-74%, predominantly in the form of hearing loss, cranial nerve palsy, temporal lobe necrosis, trismus, and dysphagia, in descending order. Treatment-related deaths were observed in 0-65% of the patients, which are alarming numbers that emphasize the need for very careful patient selection. However, these data should be interpreted with caution, as treatment mortality rates differ substantially between the Chinese studies [17,20,[75][76][77][78][79][80]82,83] (Table 2) and studies from the rest of the world [74,[84][85][86] (Table 2). In the series from endemic regions, the most common cause of treatment-related death was massive hemorrhage from the ulcerated nasopharyngeal mucosa, temporal necrosis, and cranial nerve palsy, in descending order. With the cautious patient selection and meticulous RT planning and delivery, which is a prerequisite for safe rNPC re-irradiation, a low rate of treatment-related mortality can be achieved [74,[84][85][86] (Table 2).
Treatment-related hemorrhage was slightly reduced with the use of hyperfractionated re-IMRT compared to standard fractionation (30.0% vs. 0 %, p = 0.060) [87]. A model to predict the risk of lethal nasopharyngeal necrosis after re-IMRT in nasopharyngeal carcinoma patients was developed by Chinese authors [88]. It was concluded that female sex (p = 0.008), necrosis before re-irradiation (p = 0.008), accumulated total prescription dose to the GTV ≥145.5 Gy (p = 0.043), and recurrent tumor volume ≥25.38 cm 3 (p = 0.009) were independent risk factors for lethal nasopharyngeal necrosis. Future scientific research may suggest strategies to prevent the development of severe complications following re-IMRT in patients struggling with rNPC.

Stereotactic Radiosurgery
Stereotactic radiosurgery is a highly accurate form of radiation therapy that delivers a single dose of precisely targeted radiation using focused gamma-ray beams that converge solely on the specific area where the tumor resides, minimizing the amount of radiation to healthy tissues. Limited evidence on the use of linac-based SRS [89] or Gamma Knife [90,91] as a single treatment modality in rNPC is available.
With the use of a single fraction of 12.5-18 Gy, satisfactory treatment outcomes were observed with reported 1-, 3-, and 5-year OS in 90-98%, 66-77%, and 47%, respectively [89][90][91] (Table 3). The time interval from previous RT, T classification of the recurrence, and tumor volume are all predictive factors of local control and survival. A prognostic scoring system for SRS in the treatment of rNPC was designed on the basis of these predictive factors. The 5-year local failure-free rate in patients with good, intermediate, and poor prognostic score was 100%, 42.5%, and 9.6%, respectively. The corresponding 5-year OS was 100%, 51.1%, and 0%, respectively [89].
Severe late toxicity reported in 0-33% of the patients after SRS for rNPC included brain necrosis, pituitary insufficiency, and cranial nerve palsy, in descending order, with no treatment-related deaths observed in the published studies [89][90][91].

Stereotactic Body Radiotherapy
Fractionated SBRT is a modification of SRS, which enables irradiation to be delivered in several fractions (typically 24-48 Gy/6 fr in rNPC) without losing the advantage of the mechanical precision and accuracy as well as dose conformity of SRS [92]. Published evidence suggests that tumor size reduction following ablative doses of RT could be largely dependent on T cell response [122]. Given that the activation of T lymphocytes in regional lymphatics is dramatically increased in response to ablative doses of RT, the treatment outcome could be enhanced by immunotherapy [93,122]. Furthermore, a common feature of EBV-associated NPC is the dense infiltration of lymphocytes in the tumor stroma and positive programmed death-ligand 1 expression in tumor cells, making it an especially attractive target for immune checkpoint inhibitors [123].Compared to SRS using a single fraction of high-dose irradiation, fractionated SBRT may be superior in terms of tumor control (3-year local failure-free rate 51% for SRS vs. 83% for SBRT, p = 0.003) but not OS (3-year OS in 66% for SRS and 61% for SBRT, p = 0.31). The incidence of severe late toxicity was 33% in the SRS group compared to 21% in the SBRT group [91]. The incidence of late ≥grade 3 toxicity in 4-57%, mostly in the form of nasopharyngeal, temporal lobe, and brainstem necrosis, was reported by other authors treating rNPC with SBRT [91,[94][95][96][97][98][99] (Table 3), with treatment-related deaths occurring in 2-16%, mostly due to massive hemorrhage. Tumors involving the fossa of Rosenmüller and invading deeply to the foramen lacerum are the most important predisposing factors of fatal hemorrhage in this clinical setting [98].
Despite the significantly improved rate of severe toxicity and a decline in treatment-related deaths compared to re-IMRT in the treatment of rNPC, it cannot be definitely concluded that one technique is superior to the other; the available evidence on the use of SBRT is strongly biased by a short follow-up. The use of SBRT is generally recommended over re-IMRT for Ward's RPA group III patients (<2 years from previous RT with organ dysfunction) [19]. This recommendation is based upon the observed 8 months median survival in this subgroup of patients with the aim to reduce overall treatment time in contrast to the 7-week IMRT. Albeit, no advantage in survival can be expected with the use of SBRT compared to re-IMRT [100].

Proton Beam Radiotherapy
The proton beam delivers the majority of its dose at the end of the range (Bragg peak) and almost no dose afterward, allowing a much better normal tissue sparing before and beyond the tumor region. High linear energy transfer (LET) protons are considered to be more effective than photons as they inflict more direct double-strands breaking DNA damage. A higher LET yields a higher relative biological effectiveness (RBE). Currently, most proton therapy treatments and studies assume a fixed RBE of 1.1 [101], but a biologically isoeffective dose is typically delivered to the patients, and therefore any definitive radiobiological advantage of protons is doubtful. The biological dose of proton/heavy-ion therapy is usually expressed as "Gy equivalent" (GyE) after taking the RBE into account.
Only a few authors report the use of PBT specifically in the treatment of rNPC [102,103] (Table 3). With the use of double scattering PBT, the 2-year OS and LC were both 50%. A significant association between adequate tumor coverage (defined as ≥90% of the treated volume receiving ≥90% of the prescribed dose) and OS was described [102]. With the use of active scanning PBT technique, the reported 1.5-year OS and LC were 54% and 66%, respectively. Severe late toxicity occurred in 23% of the patients in the form of hearing loss, dysphagia, and cervical vertebral osteoradionecrosis, in descending order [103]. More data and longer follow-up are required to refine the application of PBT in patients with rNPC.

Carbon Ion Radiotherapy
Carbon ion radiation therapy is still considered experimental for many tumor sites, but guidelines for its clinical indications are being established [104,105]. The accepted RBE for carbon ions used in clinical RT is generally estimated to be 2.5-3, however, values as high as 5 have been reported [106]. Complex DNA damage following CIRT, mostly in the form of double-strand breaks, is refractory to repair as it involves the clustering of multiple types of DNA lesions in close proximity to one another [109]. Given the physical and biological characteristics of carbon ions, CIRT has the potential to improve LC and reduce normal tissue complications in treating cancer patients, especially those with deep-seated and traditionally radioresistant tumors that have poor outcomes with standard photon therapies [107].
Initial clinical outcome on the use of CIRT in 75 patients with rNPC reported 1-year OS in 98% and 1-year local relapse-free survival in 87%. Severe toxicity was infrequent (12%) and included mucosal necrosis, xerostomia, and temporal lobe necrosis [108] (Table 3).
Carbon ion radiotherapy offers a promising alternative to patients with a short time interval from previous radical RT, and with its higher RBE and distinctive DNA damage, it bears the potential to cure even the most difficult-to-treat radioresistant tumors. However, more data and long-term follow-up is needed to refine the optimal dose and fractionation of CIRT in patients with rNPC.

Brachytherapy
The inherent physical and dosimetric characteristics of BRT allow the delivery of a very high dose to the nasopharynx while minimizing the dose to the adjacent structures. Current treatment options include brachytherapy alone with either interstitial [110][111][112] or intracavitary [113] approach, or in combination with external beam radiation therapy (EBRT) [114], SRS [115] and SBRT [116].
Outcomes of interstitial and intracavitary brachytherapy as single modalities in the treatment of small T1 and T2 recurrences may be comparable to ENP in terms of LC and OS and may be superior to re-IMRT concerning the reported rate of severe complications and treatment-related deaths [72]. However, published data on the use of BRT are very limited, and expertise in the procedure of radioactive seed implantation is not widespread and is generally restricted to highly specialized comprehensive cancer centers.

Conclusions
To the best of our knowledge, surgery or RT are the only curative treatment options for patients with recurrent non-metastatic NPC. The treatment of patients with rNPC is challenging, and precision radiation technologies such as SRS, SBRT, or BRT can be used effectively and safely to treat early stage and low-volume recurrent foci only. Conventionally fractionated IMRT is the most commonly indicated radiation treatment technique in this clinical setting. However, results from some series indicate that the use of re-IMRT is accompanied with a very high risk of radiation-induced morbidity and mortality. In the largest meta-analysis published to date, summarizing predominantly clinical experience from endemic areas, it was concluded that 33% of the patients with rNPC treated with re-IMRT died as a consequence of the treatment, whereas <30% died of local or distant failure (treatment period 2001-2012, 99% of analyzed cases coming from China) [72]. In this meta-analysis, however, due to issues with a moderate to high risk of bias in the methodological quality of the included studies, the overall quality of summarized evidence was deemed to be very low. Yet, a few series from the rest of the world provide reassurance of safe delivery of IMRT in terms of the risk of treatment-related mortality. Late morbidity of re-irradiation in the base of the skull region may be significant, and'a high level of clinical and radiotherapy planning expertise is required. Despite limitations, given the widespread availability of the technology, IMRT seems to be the most feasible modality for the treatment of rNPC. In early local (T1, T2) and neck recurrences, retreatment with salvage surgery should always be considered in the view of low morbidity.
In the era of the COVID-19 pandemic, techniques of extreme hypofractionation (SRS, SBRT) and time-sparing interstitial or intracavitary BRT (if feasible) should be reconsidered as the risk of cross-contamination of both the patient and medical staff is definitely lower compared to 6-7 week normofractionated IMRT or heavy-ion therapy. We acknowledge that ENP yields superior results compared to IMRT in T1-T3 recurrent disease in terms of LC, QoL, and rate of severe complications. In the era of COVID-19, the advantages of surgical treatment are especially highlighted as it successfully prevents patients from multiple accesses to the hospital, and the risk of cross-contamination is probably the lowest with careful and repeated pre-operative SARS-CoV-2 virus testing.
Although the aim of this review was to present local treatment methods in the curative therapy of rNPC, the importance of comprehensive multidisciplinary treatment must be acknowledged. As new immunotherapeutic agents are being rapidly adopted in many cancers including NPC, promising response rates to the anti-programmed cell death-1 (PD1)-targeted therapy in the heavily pretreated NPC patients with locally recurrent disease have been demonstrated [124][125][126]. Future clinical trials evaluating a potential role in the curative treatment of rNPC are highly warranted [123]. Among the exciting scientific progress in the treatment of NPC, vascular endothelial growth factor inhibition and EBV-specific tumor antigens offer promising alternatives to patients with a locoregionally incurable disease.

Conflicts of Interest:
The authors declare no conflict of interest.