Prognostic significance of laterality in lung neuroendocrine tumors

TNM cancer staging is one of the most relevant and established prognostic tools across several types of tumors. TNM system quantify the extent of the primary tumor (T), lymph nodes (N), and distant metastases (M), providing a stage grouping [29]. TNM stage groups are of crucial importance in oncology [30]. Worsening survival rates with increasing TNM status have been demonstrated in all cancers [31, 32].

In the largest series of NET [33] and NEC [34], that are data obtained from the Surveillance, Epidemiology, and End Results (SEER) program, TNM stage has been confirmed as a fundamental prognostic marker, independently from primary tumor origin.

Given to their low incidence, Lu-NETs do not have a tumor-specific staging system. However, since the 7th edition, the TNM classification for lung cancer has been used for TC and AC, providing a clear stratification of patients with a major impact on patients’ outcomes [35]. In our study, TNM staging has been confirmed as a well-established prognostic factor for survival. This data was consistent with literature data [36].

In this context, a relevant role is represented by nodal status [7]. In a recent retrospective study, including 3.335 Lu-NETs, a positive nodal status emerged as independent negative prognostic factor (HR: 2.3), both for TC and AC [37]. In our study, N+ was associated with worse outcomes with a significant impact on PFS and OS, p = 0.011, HR: 2.598 and p = 0.003, HR: 6.481, respectively.

To date, the diagnosis of TC vs. AC represents the major indicator to guide the clinicians in their management of these patients. TC are associated with a biologically more indolent disease with a significant better prognosis if compared to AC [38‐40]. A reason could be found in the recurrence rates, accounting for 16.6% in AC and 4.9% in TC [41]. A retrospective analysis of 62 AC, demonstrated that after a complete surgically removal, recurrences were observed mostly within the first 5 years of follow-up, within bronchi, mediastinal nodes, the liver, and bones [42]. In our study, the diagnosis of TC vs. AC clearly depicts two different groups, with a well-defined gap in terms of patients’ outcomes. Notably, the diagnosis of AC was confirmed as an independent negative prognostic factor in terms of PFS.

In our analysis, male sex correlated with a more aggressive disease, with a significant impact on survival. This observation, that clearly is showed in Kaplan–Meier curves of PFS and OS (p < 0.0001 and p < 0.0001, respectively). However, its independent value was not confirmed in our multivariant Cox regression model. According to SEER data, female patients were more likely to have a primary NET located in the lungs respect to males [43]. This data is confirmed in our study, where 62.3% of the patients included were female. Notably, a sex difference between TC and AC in males and females has been reported in literature, with a higher percentage of AC between males [44]. Several works report a correlation between male sex and a worse prognosis for Lu-NETs. This data is coherent with what observed in the no-neuroendocrine counterpart of lung tumors [45]. Filosso et al. reported the male sex as a negative prognostic factor for Lu-NETs [46]. In the same direction, the data coming from a retrospective analysis of 293 Lu-NETs, where OS was influenced by male sex. A recent study confirmed sex as independent factor for OS [47]. In this analysis female patients presented better prognosis than male (HR = 0.604, 95% CI = 0.45–0.806). On the other hand, in the Cox regression univariate analysis of 108 Lu-NET patients, sex was not associated with patient survival [48]. Recent analyses [49] have reported clinically relevant differences between male and female in NET originating from other primary sites (i.e., pancreatic NET).

Additionally, our analysis showed a relevant role for patients’ age. Our multivariate model confirmed the independent value of age, associating a higher risk of progression and death for increasing value of this variable. A role for age as a prognostic factor for NET has been previously reported [50, 51]. Focusing on Lu-NET, a retrospective analysis including TC selected from the SEER database, allowed to develop a nomogram to predict the probability of 3- and 5-year OS [52]. In this model longer OS was associated with younger age (p < 0.0001). This data has been confirmed by other studies, as a retrospective analysis of 108 Lu-NET [53] and an Asiatic study, including 64 lung carcinoids [54].

Therefore, considering our results and the available literature data, we suggest that sex and age should be evaluated as potential useful prognostic factors for Lu-NETs. Perspective larger studies are encouraged to validate this observation.

According to 2021 WHO classification, Lu-NETs are divided in two entities (TC and AC) according to two parameters, the mitotic count and the presence/absence of necrosis. These criteria are largely different from the ones for the diagnosis of NET with other origin [55]. For extra-pulmonary (EP) NETs the proliferation activity is quantified trough Ki67 index, which together with morphology, allows the codification of a grade for each case. Therefore, EP-NETs are separated in three classes, with a progressively higher Ki67 value, named G1, G2, and G3 NETs.

For Lu-NETs, several attempts have been made to introduce tumor grading and Ki67 to obtain a uniform classification for all sites NETs [56]. Some data exists about the utility of Ki67 as a prognostic biomarker even for Lu-NETs [57]. However, to date this type of classification has not been validated yet.

In our study, the two traditional pathological features for Lu-NETs, the mitotic count and the necrosis, confirmed their value also as prognostic factors. A mitotic count ≥2 per 10 HPF and the presence of necrosis correlated with a dismal outcome (in terms of PFS p < 0.0001 both, and OS p = 0.002 and p = 0.03).

Notably, in our study, 8 (2.7%) patients had a mitotic count >10 per 10 HPF. This subgroup presented a significant worse prognosis if compared to patients with lower mitotic count. This observation is intriguing, due to the 2021 WHO classification criteria that consider the mitotic count cut-off for Lu-NETs, equal to a maximum of 10 mitosis for 10 HPF. Taking together these data, a more heterogeneous and complex scenario could be hypothesized, comprehending Lu-NET characterized by a WD morphology associated with a high mitotic rate and a poor outcome.

Additionally, in our analysis 9 (3.0%) cases were reported to have a Ki67 > 20%. Among them, 5 presented a mitotic count ≤10 per 10 HPF, 2 a mitotic count ≥10 per 10 HPF (one of 12 and one of 14 per 10 HPF), this data was missing in the remaining 2 cases. This population of patients correlated with dismal outcome, in terms of PFS and OS, p = 0.006 and p < 0.0001, respectively. The Ki67 in this subgroup presented a median value of 30% and a range from 25% to 80%. Therefore, the highest Ki67 value in our study was 80%. This data differs from previous evidences, which reported lower maximum Ki67 index (65% in the study by Rubino et al. [58], 62% in the study by Oka et al. [21], 37% in the study by Kasajima et al. [22]). Arising literature data suggest that Lu-NET with a Ki67 > 20% are associated with a worse clinical outcome, if compared to WD-Lu-NET with lower Ki67 index [19]. An example is the study carried out by Rubino, with mOS of 203 months in patients with Ki67 index ≤5%, 101 months in patients with Ki67 index of 6–20% and 53 months in patients with Ki67 > 20% (p = 0.002) [58].

Therefore, the value of Ki67 as well as the role of mitotic count higher than 10 per 10 HPF, still remains an open and challenging question for Lu-NETs. A more complete diagnosis, taking into account tumor heterogeneity, immunohistochemistry and genomic profile, could help to provide new weapons to better characterize and stratify Lu-NETs, in the era of personalized medicine.

Laterality is a potential significant prognostic factor for lung cancer because of the heterogeneity between left and right lung, in terms of anatomy, hematic and lymphatic circulation, relationship to the surrounding organs. The importance of right-sided vs. left-sided tumors has been clearly demonstrated for colon cancer, which exhibits differences in incidence, pathogenesis, molecular pathways and outcome depending on the location of the tumor [59]. A prognostic difference between right and left cancer has been also observed for the renal cell carcinoma by analyzing the SEER database. The worse prognosis in left than right tumors was related to earlier tumor stage, lower tumor grade, lymph node and distant metastasis [60].

However, a role for the side of primary tumor in the context of lung cancers in general and specifically for Lu-NET has not been established, so far. Previous studies in the field of Lu-NETs have demonstrated that right-sided tumors are more common [61], in analogy to our population (in which 172 cases were right-sided, accounting for 57.3% of the included population). A recent study analyzed the prognostic relevance of laterality in 1465 patients with non-small cell lung cancer undergone pneumonectomy [62]. Although right-sided pneumonectomy after induction therapy was associated with a significantly higher perioperative mortality, the 5-year survival was similar bewteen tumor sides. Anyway, this previous observation is not significant for the present study, due to the very low rate of pneumonectomy performed in lung NETs.

In our study, a highly significant association between tumor side and the presence of necrosis was found. In 39 of 300 cases, focal or diffuse necrosis was observed. 29 of them (74.3%) were detected in left-sided tumors, whereas only 10 (25.6%) in right-sided primaries. We observed also a significant negative impact for left tumor side on patients’ prognosis at the univariant analysis (PFS, p = 0.007 and OS p = 0.014). Therefore, we confirmed left side as an independent negative prognostic factor in terms of PFS (p = 0.043, HR: 3.033 (95% CI, 1.034–8.893) and OS p = 0.071, HR: 1.823 (95% CI, 0.951–3.494) at the multivariate analysis. The distribution of AC was in 44 in the left parenchyma and in 31 cases in right parenchyma. Among them, 39 of 75 cases were positive for necrosis at the histopathological evaluation, whereas the remaining 36 cases were diagnosed as AC due to the mitotic count ≥2 per 10 HPF only. Thereby, the simple distribution of AC cases between left and right parenchyma is not sufficient to explain the notably higher presence of necrosis in left tumors.

The consistence of this association between necrosis and left Lu-NETs could provide the rationale for a differential expression of angiogenesis and hypoxia according to the primary side of the tumor (right-sided vs. left-sided) with potentially relevant implications for patients’ outcomes. Therefore, we believe that this association should be better evaluated in further original and prospective studies to determine if primary tumor side could represent a new prognostic marker for Lu-NETs. Moreover, a better knowledge of these tumors biology and angiogenesis could pave the way for a more personalized approach in the therapeutic algorithm of these tumors.

The current study presents some limitations. First of all, the retrospective nature of patients’ inclusion, the collection of patients’ data and of the analysis. A second limit could be represented by patients’ sample size, although relatively adequate if we consider that NENs are rare tumors and between them, Lu-NET account for about 25–30%. Another limit is the percentage of missing data above all for pathological variables, as IHC for TTF-1 or Ki67 value (missing in 168 and 68 cases, respectively).