Based on our study findings, our data show that the 8th edition has a superior prognostic value for patients with NPC than the 7th edition.
In the treatment of NPC, IMRT has become the optimal radiation technique because of its clear advantage in target dose uniformity and better protection of adjacent organs at risk compared with 2-dimensional radiotherapy (2D-RT) or 3-dimensional conformal radiotherapy (3D-CRT). It can be administered in 2 ways, sequential technology (SEQ-IMRT) [16, 17] or SIB-IMRT [18]. Compared with SEQ-IMRT, SIB-IMRT simply uses a single radiation plan in the entire course of treatment, allowing the simultaneous delivery of different dose levels to different target volumes that reduces the treatment duration and enhances biologically equivalent dose (BED) [12, 19].
The TNM staging system is crucial for predicting prognosis, guiding treatment decisions for different risk groups, assessing treatment efficacy, and evaluating clinical outcomes between different centres. Therefore, the TNM staging system should be updated based on the development of radiation technology. The 7th staging system was based on the information data form the 2D-RT era, and several trials have been conducted to determine its value considering the advent of IMRT [20-22]. Zong [20] et al analysed the data of 1241 NPC patients treated with IMRT and revealed that the differences in local relapse-free survival (LRRFS) between T1 and T2, and between T2 and T3 were not significantly different (P = 0.055 and 0.605, respectively). Additionally, they reported that the hazard ratios for OS and disease-specific survival between T1 and T2 were not statistically significant. The study considered that the TNM staging system should downgrade stage T2 patients to T1 patients. In a study performed by Chen et al [21] on 181 NPC patients with N0 stage, the authors reported that the difference in OS, LRFS, and PFS between T1 and T2, and between T3 and T4 was not statistically significant. In this study, we also confirmed that there were no differences in OS, DFS, and DMFS between T1 and T2 (P = 0.987, 0.984, and 0.191). Fortunately, the 8th staging system was revised after the introduction of IMRT as a treatment option and several previous studies [11, 23-25] have reported its superiority over the 7th edition staging system. Our data confirmed that the 8th edition had better prognostic performance than the 7th edition.
For T categories, our data found that the T-classification in the 8th edition showed better separation between T3 and T2, and T3 and T1 compared to OS and LRRFS, while there were no significant differences in the T-classification in the 7th edition. A retrospective study performed by Pan et al [11] on 1609 patients staged based on MRI findings and treated with IMRT at 2 major centres in Hong Kong and Mainland China (median follow-up of 5 years) found that there were statistically significant differences among OS between T3 and T2 (P = 0.009). Additionally, OuYang et al [24] retrospectively studied 899 patients with NPC (from Hong Kong, Guangzhou, and Guangxi) who were staged based on MRI findings and received IMRT; this study compared the 7th and 8th staging systems and reported that the 8th edition had better differentiation of OS between T3 and T2 (P = 0.003). All these data confirmed that it was reasonable to downstage MP and LP from T4 in the 7th edition to T2 in the 8th edition. This change has increased the survival difference values between T3 and T2, and also resulted in improved classification of patients with NPC.
In terms of N categories, replacing SCF with the lower neck region to differentiate N1-2 and N3 is the main revision in the 8th edition. Ng et al [10] first explored the possibility of replacing the SCF by levels IV and Vb as a demarcating criterion for the N3 category, and found this method potentially useful. A few studies debated that the definition of SCF involvement is primarily based on clinical examination and defining SCF using clinical landmarks is difficult [6-8]. However, the lower neck, as an anatomical landmark, can be reliably defined on the basis of both physical examination and cross-sectional images, thereby making it more convenient in clinical practice.
Several studies [23-25] have reported that the new staging system is useful in predicting outcomes with regard to N categories. In a study performed by Tang [23] that included 1790 NPC patients, the survival curves between different groups were accurately differentiated considering the 8th staging system. Another respective study also confirmed that the T-classification according to the 8th staging system showed better differentiation compared with that performed using the 7th edition [25]. Similarly, our results showed a clear difference between N2 and N3 among OS and DFS according to the new staging system. Moreover, we found no differences between N3a and N3b among OS, DFS, and DMFS considering the 7th staging system, indicating that this subgroup was unnecessary.
In terms of clinical stage, the 8th edition has upgraded IVC to IVB, and merged IVA and IVB from the 7th edition into IVA. Our data showed that the segregation of IVA and IVB in terms of survival was inaccurate in the 7th staging system as IVA and IVB share similar 5-year OS and DFS rates.
Our study included patients with NPC from a centre between year 2009 and 2013 with a relative long follow-up time. However, because of the radiation technique, only 300 patients in our study underwent SIB-IMRT. This small number of patients may result in low end-point events that may weaken the power to convince the differences between both the staging systems. Another limitation was the nature of the study (retrospective), and hence, prospective multicenter studies are required to be performed to confirm the results of our study.