Introduction

The predominant subtype in the 2015 World Health Organization (WHO) lung tumor classification has been identified as a prognostic indicator for patients with resected lung adenocarcinoma in various countries [1,2,3,4,5]. The histologic subtype can be used to stratify patients with lung adenocarcinoma into three prognostic groups: low grade (lepidic predominant), intermediate grade (acinar or papillary predominant), and high grade (solid or micropapillary predominant) [2]. Moreover, classification and stratification by the predominant pattern are suggested to be predictive of response to adjuvant chemotherapy [6].

Pulmonary adenocarcinomas are histologically heterogeneous and present with multiple combinations of patterns and proportions. When classified according to the predominant pattern alone, the acinar subtype is the most prevalent (estimated at 40–50% of patients) and carries the widest spectrum of prognoses [2, 3, 7,8,9]. In addition to the five major histologic patterns, several other patterns such as the cribriform pattern have been reported in the lungs. The cribriform pattern in lung adenocarcinoma was recognized in the 2015 WHO classification; however, a new subtype was not created, and this pattern was rather described as a part of a high-grade pattern of the acinar subtype [10]. These complex glandular patterns (high-grade acinar) have been found to be associated with a high mitotic rate, tumor necrosis, and lymphovascular invasion in the lungs [11, 12]. Furthermore, current evidence supports that these complex glandular patterns carry a poor prognosis similar to that of high-grade histologic types (solid and micropapillary) [9, 11,12,13,14,15,16]. However, the lack of appreciation of these patterns may have led to uncertainty in tumor classification and poor reproducibility because some investigators may have classified these patterns as intermediate grade (acinar) or high grade (solid). Thus, it is crucial to identify these “nontraditional patterns” and to classify them as complex glands separately from the conventional acinar pattern.

Consequently, the International Association for the Study of Lung Cancer (IASLC) pathology committee conducted a systematic study to evaluate a set of histologic features that had been described as prognostic indicators and established a grading system for resected invasive pulmonary adenocarcinoma. The IASLC grading system proposed the following: Grade 1, well-differentiated adenocarcinoma (lepidic predominant tumors with no or < 20% of high-grade patterns [solid, micropapillary, and/or cribriform patterns]); Grade 2, moderately differentiated adenocarcinomas (acinar or papillary predominant tumors with no or < 20% of high-grade patterns); and Grade 3, poorly differentiated (any tumor with ≥ 20% of high-grade patterns) (Online Resource 1) [17].

To further validate the prognostic significance of tumors graded by the IASLC grading system, we analyzed a series of 471 Japanese patients with resected lung adenocarcinoma and compared the IASLC grade with the 2015 WHO classification (low grade, lepidic predominant; intermediate grade, acinar or papillary predominant; and high grade, solid or micropapillary predominant).

Methods

Patients

This retrospective study was approved by the Institutional Review Board of Kagawa University. The need for informed consent was waived by the Institutional Review Board due to the retrospective nature of the study. We reviewed the data of 471 patients with therapy-naïve stage I–IIIA lung adenocarcinoma who had undergone a radical surgical resection by more than a segmentectomy with systematic lymph node dissection at Kagawa University between 1999 and 2016. Cases with multifocal invasive carcinomas, stage IIIB–IV lung carcinomas, adenocarcinoma in situ, minimally invasive adenocarcinomas, multifocal adenocarcinomas, invasive mucinous adenocarcinoma, and other variants of adenocarcinoma were excluded from the study.

Clinical data were collected from a prospectively maintained lung carcinoma database. Initial CT was performed 3 months after surgery; thereafter, chest and abdominal CT were routinely performed every 6 months. Disease recurrence was confirmed by clinical, radiological, or pathological assessment. The disease TNM (tumor, node, metastasis) staging was based on the eighth edition of the American Joint Committee on Cancer TNM Staging Manual [18].

Histologic evaluation

Hematoxylin and eosin-stained slides were reviewed by two pathologists blinded to the patients’ clinical outcomes using an Olympus BX53 upright microscope (Olympus Corporation, Tokyo, Japan) with a standard 22-mm-diameter eyepiece. The tumors were graded using both the IASLC grading system and an architectural approach based on the predominant subtypes of the 2015 WHO lung tumor classification [1]. The presence of lymphatic and vascular invasions was noted if at least one tumor cell cluster was visible.

Statistical analyses

Associations between categorical variables were analyzed using the Chi-square test. Recurrence-free probability (RFP) was defined as the time from surgical resection to the date of disease recurrence. Overall survival (OS) was defined as the time from surgical resection to the date of death or last follow-up. RFP and OS were estimated using the Kaplan–Meier method, and nonparametric group comparisons were performed using the log-rank test. Multivariate analyses were performed using a Cox proportional hazards regression model. Furthermore, multivariate models were built to include factors that were significant in the univariate analysis. Any association between pathologic factors were checked, and a factor was included in the model only if any strong association was discovered. All statistical tests were two-sided, with a significance level of 0.05. Statistical analyses were conducted using SPSS Statistics for Windows (version 23.0; IBM Corporation, Armonk, New York).

Results

Patient clinicopathologic characteristics and outcomes

The median age of the 471 patients was 70 years (range 26–92 years), and more than half of the patients were male individuals (n = 255, 54%). Most patients had pathologic stage I disease (n = 357, 76%). Furthermore, 428 (91%) patients underwent at least lobectomy, and 43 (9%) underwent segmentectomy. Ninety-three patients (20%) received adjuvant therapy. During the study period, 123 patients (26%) experienced recurrence and 104 (22%) died. The median follow-up period for the patients who were alive at the time of the last follow-up was 60 months (mean 64 ± 38 months).

Among all patients, higher pathologic stage (p < 0.001), lymphatic invasion (p < 0.001), vascular invasion (p < 0.001), and tumor spread through air spaces (STAS; p < 0.001) were found to be significantly associated with a lower RFP. Male sex (p = 0.002), surgical procedure (p = 0.017), higher pathologic stage (p < 0.001), lymphatic invasion (p < 0.001), vascular invasion (p < 0.001), and STAS (p < 0.001) were found to be significantly associated with a worse OS (Table 1).

Table 1 Clinicopathologic characteristics and their associations with patient's outcomes in all stages
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Association between clinicopathologic features and the IASLC grade

High-grade tumors classified by the 2015 WHO classification were present in 17% (n = 78), whereas Grade 3 tumors classified by the IASLC grade were present in 38% (n = 181) of all tumors. Among all patients, IASLC Grade 3 tumors were more frequently identified in male individuals (p < 0.001) and those who underwent lobectomy (p = 0.045), those with higher pathologic stages (p < 0.001), lymphatic invasion (p < 0.001), vascular invasion (p < 0.001), STAS (p < 0.001), and more distant recurrence (p < 0.001) (Online Resource 2). In the subgroup of stage I patients, IASLC Grade 3 tumors were more frequently identified in male individuals (p < 0.001) and those with higher pathologic stages (p < 0.001), larger invasive tumor size (p < 0.001), lymphatic invasion (p < 0.001), vascular invasion (p < 0.001), STAS (p < 0.001), and more distant recurrence (p < 0.001) (Online Resource 3).

Association between patient outcomes and the IASLC grade

The 5-year RFP of patients with IASLC Grade 3 tumors (45%) was lower than that of patients with Grade 1 and 2 tumors (91% and 83%, respectively) (Fig. 1A). Moreover, we compared the outcomes of previously established grades (low grade, lepidic; intermediate grade, acinar, papillary; and high grade, solid, micropapillary) based on the 2015 WHO classification and IASLC grade among all patients. The number of patients with IASLC Grade 1 was the same as those with 2015 WHO low-grade tumors. This indicated that there was no lepidic-predominant tumor with a significant portion exhibiting a high-grade pattern. In contrast, there were fewer patients with IASLC Grade 2 tumors than those with 2015 WHO intermediate-grade tumors and more patients with IASLC Grade 3 tumors than those with 2015 WHO high-grade tumors (Table 1).

Fig. 1
figure 1

Prognostic significance of tumors classified in accordance with A the IASLC grading system and B 2015 WHO classification. A, B IASLC Grade 3 and 2015 WHO high-grade tumors are statistically significant prognostic factors for RFP, with the difference more pronounced in the IASLC grading system than in the 2015 WHO classification. IASLC International Association for the Study of Lung Cancer, RFP recurrence-free probability, WHO World Health Organization

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Although the 5-year RFP of patients with IASLC Grades 1 and 3 tumors and that of patients with 2015 WHO low- and high-grade tumors were almost the same, the 5-year RFP of patients with IASLC Grade 2 tumors (83%) was higher than that of patients with 2015 WHO intermediate-grade tumors (69%) (Fig. 1A, B). In addition, according to the multivariate analysis for recurrence, IASLC Grade 3 was an independent prognostic factor for a worse RFP (hazard ratio [HR], 1.83; p = 0.018) (Table 3). In a subgroup of stage I patients, the prognostic significance of IASLC Grade3 was confirmed on univariate analysis (Table 2, Supplementary Fig. 1) and on multivariate analysis (Table 3).

Table 2 Clinicopathologic characteristics and their associations with patient's outcomes in stage I
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Table 3 Multivariate analysis
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The 5-year OS for patients with IASLC Grade 3 tumors (51%) was lower than that of patients with IASLC Grade 1 and 2 tumors (90% and 86%, respectively). Moreover, the 5-year OS for patients with IASLC Grade 2 tumors (86%) was higher than that of patients with 2015 WHO intermediate-grade tumors (73%) (Table 1). According to the multivariate analysis for survival, IASLC Grade 3 was an independent prognostic factor for worse OS (HR, 2.99; p < 0.001) (Table 3). The prognostic significance of IASLC Grade 3 in stage I patients was confirmed on univariate (Table 2) and multivariate (Table 3) analyses.

Moreover, patients with IASLC Grade 2 tumors were stratified according to a < 5% cut-off of high-grade patterns and patients with IASLC Grade 1 and 2 tumors were stratified according to the presence of STAS as high-grade histological features. Among patients with IASLC Grade 2 tumors, the 5-year RFP of those with ≥ 5% high-grade patterns was lower than that of patients with < 5% high-grade patterns (Grade 2, 69% vs. 88%, p < 0.001, respectively) (Fig. 2A). Moreover, the 5-year RFP of patients with STAS-positive tumors was lower than that of patients with STAS-negative tumors (Grade 1, 67% vs. 91%, p < 0.001, and Grade 2, 69% vs. 89%, p < 0.001, respectively) (Fig. 2B, C). According to the multivariate analysis for recurrence, STAS was an independent prognostic factor for worse RFP compared with patients without STAS (HR, 2.45; p < 0.001). The prognostic significance of STAS-positive tumors in stage I patients was confirmed on multivariate analysis (Table 3).

Fig. 2
figure 2

Association between ≥ 5% high-grade patterns, STAS, and RFP. A Among patients with IASLC Grade 2 tumors, the 5-year RFP of patients with ≥ 5% high-grade patterns was lower than that of those with < 5% high-grade patterns (71% vs. 89%, p = 0.001). B Among patients with IASLC Grade 1 and C among patients with IASLC Grade 2 tumors, the 5-year RFP of patients with STAS-positive tumors was lower than that of those with STAS-negative tumors (Grade 1: 46% vs. 95%, p < 0.001; Grade 2: 73% vs. 89%, p = 0.001, respectively). IASLC International Association for the Study of Lung Cancer, RFP recurrence-free probability, STAS spread through air spaces

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Discussion

In this study, we showed that IASLC Grade 3 tumors and 2015 WHO high-grade tumors were present in 38% and 17% of the same set of patients, respectively. We also found that in patients with resected lung adenocarcinoma, IASLC Grade 3 was an independent predictor of worse clinical outcomes (recurrence and survival) after adjustments for significant clinicopathologic factors and was associated with characteristics of aggressive tumor behavior such as larger invasive tumor size, higher pathologic stage, lymphovascular invasion, and histological features such as STAS. Therefore, our findings highlight the significant prognostic value of the IASLC grading system for invasive adenocarcinoma.

Moreover, patients with tumors having ≥ 5% high-grade patterns (solid, micropapillary, and/or complex glandular patterns), even those with IASLC Grade 2 tumors, had a poor prognosis. Although STAS was not included in the IASLC grading system, its incidence was a poor prognostic factor regardless of tumor grade. Based on these results, we suggest that Grade 2 tumors should be recognized as having a poor prognosis if they have a high-grade pattern of ≥ 5% or histological features such as the incidence of STAS.

In the 2015 WHO classification, tumors are graded using an architectural approach based on the predominant subtype: low (lepidic subtype), intermediate (papillary or acinar subtype), or high grade (micropapillary or solid subtype) [2, 6]. Therefore, according to this classification of lung carcinomas, cribriform arrangements are regarded as a pattern of acinar adenocarcinoma despite the previously reported association between the pattern and worse clinical outcomes in patients with lung adenocarcinoma [9, 13,14,15]. Kadota et al. [16] reported that the cribriform-predominant subtype (currently a subcategory of the acinar subtype) is an independent factor of poor prognosis, with respect to recurrence and survival, among patients with resected lung adenocarcinoma, and that it was associated with aggressive pathology. To detect tumors with high-grade patterns, it was necessary to classify them as complex glands separate from the conventional acinar pattern. Based on the IASLC grading system, tumors with ≥ 20% high-grade patterns were classified as poorly differentiated because these tumors behave more aggressively, similar to those with a predominantly high-grade pattern. Hence, this grading system offers a superior prognostic grouping compared to the 2015 WHO classification.

In this study, even in stage I disease, IASLC Grade 3 tumors had more distant recurrences than Grade 1 and 2 tumors. Considering these results, we propose that the high-grade pattern such as IASLC Grade 3 tumors should be accurately detected, and that careful follow-up after surgery and indication for adjuvant therapy should be considered. However, as a potential limitation of our study, it is important to note that the patients’ epidermal growth factor receptor status, which may affect the prognosis of those with lung adenocarcinoma, was unclear.

Conclusion

In conclusion, IASLC Grade 3 tumor is an independent factor of poor prognosis in patients with resected lung adenocarcinoma, with respect to recurrence and survival. Moreover, they are associated with aggressive pathology. This finding was validated in independent cohorts comprising more than 400 patients with resected lung adenocarcinoma.