Lung cancer is the leading cause of cancer death worldwide,1,2 and extensive research on this cancer has been conducted to date. Recent progress in the field of medical treatment of advanced lung cancer has been focusing on molecular changes.
Genetic alteration of the epidermal growth factor receptor (EGFR), which is predominantly found in adenocarcinomas,3 is a representative marker in determining the appropriate treatment for advanced lung cancer.3,4 The recognition of EGFR mutation indicates clinical treatment with tyrosine kinase inhibitors such as gefitinib and erlotinib.5–7
Among distant metastases, brain metastasis is one of the most devastating complications of lung cancer threatening affected patients.8 Although magnetic resonance imaging (MRI) is used to detect brain metastases, its cost effectiveness remains uncertain.9 Moreover, there is controversy over the use of routine brain MRI in staging asymptomatic patients with non–small-cell lung cancer (NSCLC), particularly when they have early thoracic diseases.10,11 Thus, in studies of brain metastases, potential biases arise from overlooking patients with brain metastases.
Extensive research has proved the association between the genetic alteration of EGFR and distant metastases in various cancers such as those of the breast and rectum.12–14 In patients with pulmonary adenocarcinomas, different pulmonary metastatic patterns of EGFR-mutated tumors were also reported.15 However, because of the limited data available, it has not been possible to determine different brain metastases according to EGFR mutation status in pulmonary adenocarcinomas.
Several studies have tried to identify the predictive factors for brain metastasis in patients with NSCLC. Carcinoembryonic antigen,16 size of primary tumor, nodal stage,17 and presence of bone metastases18 have been proposed as predictive factors for the presence of brain metastasis in patients with NSCLC. A histopathology of nonsquamous cell carcinoma is also known to be a predictive factor, which means that pulmonary adenocarcinoma has a higher tendency to metastasize to the brain than squamous cell carcinoma.17 However, the clinical implications of EGFR mutation status in terms of brain metastasis in patients with pulmonary adenocarcinoma have not yet been examined.
In this study, we investigated the clinical association between EGFR mutation and brain metastasis in patients with pulmonary adenocarcinoma who were examined by using brain MRI.
PATIENTS AND METHODS
A total of 314 patients diagnosed with adenocarcinoma of the lung were consecutively enrolled between October 2005 and December 2011 at the Korea Cancer Center Hospital. We evaluated the patients’ data from chest computed tomography (CT) scan, bone scan, and MRI of brain, as routine staging workup, for which expenses were shared by national health insurance. The tumor stage was classified using recently revised tumor, node, metastasis (TNM) system proposed by American Joint Committee on Cancer (8th edition). T and N stage were determined on the basis of the findings of CT with or without additional fiberoptic bronchoscopy. Mediastinal lymph nodes larger than 1 cm on transaxial CT images were considered positive. Stage was not based on the findings of additional positron emission tomography scan because it was not performed in all patients. Distant metastases were categorized as metastasis to the brain and those to extracranial sites only. Patients with squamous cell and adenosquamous carcinoma were excluded. Recurrence of brain metastases in patients who had been treated with curative resection were suspected on the basis of clinical symptoms and confirmed by results of brain MRI.
Genomic DNA was extracted from paraffin-embedded tissues, as described previously.19 In patients whose only available tissue was cytologic sample at initial diagnosis, methanol-fixed cytologic specimens were used for DNA extraction.20 EGFR mutation analysis was carried out by direct sequencing before March 2009 (135 patients), and pyrosequencing was performed for the detection of EGFR mutation after March 2009 (179 patients), as described previously.21
Statistical analysis of data was performed using STATA version 11 (Stata Corp, College Station, TX). The associations between EGFR mutation status and clinical features, specifically age, sex, smoking, TNM stage, and metastatic site, as well as brain metastasis, were analyzed using logistic regression analysis. Clinical factors known to be associated with EGFR mutation status in pulmonary adenocarcinoma and staging factors of TNM were included in the multivariate analysis.
The associations between the spread pattern of brain metastases and EGFR mutation status were also evaluated using Kruskal–Wallis test. The prognostic implications of EGFR mutation on the occurrence of brain metastasis in patients who were treated with surgical resection were analyzed using competing risk regression analysis of Fine and Gray.22 Deaths without cranial disease were considered as competing events. A p value less than 0.05 (two-sided) was considered statistically significant.
This study protocol was reviewed and approved by the Institutional Review Board of the Korea Cancer Center Hospital (Insititutional Review Board No: K-1206-002-030). The recommendations of the Declaration of Helsinki for biomedical research involving human subjects were followed.
A total of 314 patients (151 men and 163 women) with a median age of 64 years (range, 25–84 years) were consecutively enrolled in our study. Adenocarcinoma was the histologic subtype in all patients. Almost half of the patients (153 patients, 48.7%) had metastatic disease. Among the 153 patients with distant metastases, 51 patients (33.3%) had cranial metastases and 102 patients (66.7%) had only extracranial metastases (Table 1).
A total of 138 patients (43.9%) had mutations, of which exon 19 deletion was the most common (79 patients, 57.2%), followed by L858R mutation in exon 21 (51 patients, 37.0%). The other eight patients (5.8%) had a G719X mutation in exon 18.
EGFR Mutation and Baseline Characteristics
In univariate analysis, EGFR mutation was found more frequently in women (61.4% versus 25.2% [men], p < 0.001) and never smokers (60.6% versus 26.6% [ smokers], p < 0.001). The difference in age according to EGFR mutation (64 years versus 61 years [wild type], p = 0.12) was not statistically significant.
We further evaluated the association between stage and EGFR mutation. A reverse correlation between advanced nodal stage and EGFR mutation was observed (36.2% [N2–3] versus 47.9% [N0–1]). However, statistical significance was marginal in univariate analysis (p = 0.050). The frequency of EGFR mutation did not differ by T stage (44.7% [T1–2] versus 41.4% [T3–4], p = 0.63), whereas the size of primary tumor was inversely correlated with EGFR mutation (3.0 cm versus 3.4 cm [wild type], p = 0.024) (Table 1). Metastatic disease (M1) was not associated with EGFR mutation in univariate analysis (39.8% [M0] versus 48.4% [M1], p = 0.12).
EGFR Mutation and Brain Metastasis in Multivariate Analysis
At the time of initial diagnosis of pulmonary adenocarcinoma, the frequency of EGFR mutations was statistically different according to the site of metastases (39.8% [no metastases] versus 40.2% [extracranial metastases] versus 64.7% [brain metastases], p = 0.005). The percentage of EGFR mutations was also significantly different in subcohorts of metastatic disease (40.2% [extracranial metastases] versus 64.7% [brain metastases], p = 0.004).
Next, we assessed the significance of EGFR mutation with respect to brain metastases by using the multivariate model. A strong association between EGFR mutation status and brain metastasis was observed (adjusted odd ratio = 3.83, 95% confidence interval [CI]: 1.72–8.55, p = 0.001), whereas there was no association between EGFR mutation and extracranial metastases (adjusted odd ratio = 1.73, 95% CI: 0.94–3.20, p = 0.079) (Table 2). In the final model, nodal stage (p = 0.025) and tumor size (p = 0.024) remained significant factors, whereas smoking history was no longer significant (p = 0.088).
EGFR Mutation and Physical Features at Metastatic Brain Lesion
To investigate the potential difference in physical features of brain metastases according EGFR mutation, we performed a subgroup analysis in patients with brain metastases (n = 51). The number and size of brain metastases stratified by EGFR mutation status are described in detail in Table 3. Multiple brain metastases (≥2 metastatic lesions) were more frequently detected in EGFR-mutated tumors than in EGFR wild-type tumors (78.6% versus 47.8%, p = 0.022). In addition, the number of brain metastases according to EGFR mutation status showed a statistical significance (p = 0.029). However, the size of the largest metastatic brain lesion was not significantly different according to EGFR mutation status, in contrast with the primary tumor size (Table 3).
Risk of Brain Metastasis after Surgical Resection
To evaluate prognostic significance of EGFR mutation status with respect to brain metastases, we performed a subgroup analysis for the risk of brain metastasis in patients who were treated with curative resection of pulmonary adenocarcinoma (133 patients). The last follow-up review of this group of patients was done on January 2, 2013. The median follow-up duration in this cohort was 28.6 months (range, 0.3–74.4). The recurrent brain metastases suspected on the basis of clinical symptoms during follow-up period were confirmed by performing brain MRI. To reach a concrete conclusion, the pathologic N stage was included in the final multivariate analysis. Brain metastases were developed in a total of 10 patients among 133 patients who were treated with curative resection. Deaths without cranial disease (n=17) were considered as competing events. Our result showed that EGFR-mutant tumor had a significantly higher risk of recurrence of brain metastasis (hazard ratio = 4.49, 95% CI: 1.20–16.80, p = 0.026) adjusted by pathologic N stage (0–1 versus 2, hazard ratio =4.75, 95% CI: 1.37–16.49, p = 0.014) (Fig. 1).
In this study, we evaluated different features of brain metastases according to EGFR mutation status. To exclude the impact of histology, this study was confined to data on pulmonary adenocarcinoma. Of note, the risk of brain metastases increased in EGFR-mutated tumors at the time of diagnosis as well as during disease course after surgery. Compared with wild-type tumors, EGFR-mutated tumors showed wide spread of brain lesions. To date, there have been few studies that focused on the implications of EGFR mutation on brain metastasis in a homogeneous population of pulmonary adenocarcinomas.
This is the first study to suggest that EGFR mutation is significantly associated with a higher likelihood of brain metastases in patients with pulmonary adenocarcinomas at initial presentation. Some investigators have shown clinical evidence for the impact of EGFR mutation on distant metastasis. Preliminary results from a Chinese study suggested different metastatic patterns in the brain.23 In contrast, in another study evaluating three different oncogenes (EGFR, V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog [KRAS], and anaplastic lymphoma receptor tyrosine kinase [ALK]),24 EGFR mutation was not significantly associated with brain metastasis.24 However, different diagnostic timing of metastatic presentation and histological heterogeneity in the previous studies23,24 should be considered. It seems difficult to draw conclusions from previous data about the clinical implication of EGFR mutation on brain metastases.23,24
Further analysis of data from patients with brain metastases showed that EGFR mutation was significantly associated with extended tumor spread in terms of the number of brain metastases. However, the size of brain metastases was not associated with EGFR mutation status, neither were the specific subtypes of EGFR mutations correlated with the number and size of brain metastases (data not shown). In contrast with our study, Sekine et al.25 proposed that the miliary spread to the brain might be correlated with exon 19 deletion rather than exon 21 point mutation. Compared with the previous study,25 our study had a different patient population. Our study population consisted of patients with pulmonary adenocarcinoma, and we collected data from patients who received routine brain MRI, minimizing a potential bias from using different methods for brain imaging.
Of interest, the increased risk of brain metastases in EGFR-mutated tumors was also observed at the time of follow-up after resection. It is well known that tumors with adenocarcinoma histology, in general, have a higher tendency of brain metastasis than lung cancer involving other cell types such as squamous cell carcinoma.17,26 However, data on the potentially prognostic role of EGFR mutation with respect to recurrence in the brain in resected patients are limited. Although the limitation of this study, its retrospective nature, cannot exclude a potential bias during the disease course under treatment, we believe that the prognostic impact of EGFR mutation on brain metastases is worth examining in further studies.
In our data, associations of EGFR mutations with primary tumor size and nodal stage, in addition to smoking history and sex, were suggested. In a previous study, EGFR mutations were predominantly seen in smaller primary tumors.27 The negative correlation between EGFR mutation status and advanced nodal stage is also consistent with findings from a recent study.28 Considering the result that frequent metastases to the brain are expected in large primary tumors and advanced nodal stages,17 the association between brain metastases and EGFR mutations cannot be explained by tumor extent only. Rather, it is likely that metastatic preference to the brain may result from a distinct molecular pathway in EGFR-mutated tumors.
The molecular aspect explaining the link between EGFR mutation and brain metastases is unknown. However, the data from several studies let us infer possible hypotheses explaining why EGFR mutation–harboring pulmonary adenocarcinomas easily metastasize to the brain. An experimental study from China29 showed that EGFR inhibition significantly decreased brain metastases in a brain-trophic clone of human DMA-MB-231 breast carcinoma cell line. That study iterated that the action of EGFR in brain metastasis might be through phosphoinositide 3-kinase/protein kinase B and phospholipase C γ downstream pathways. Another report performed by Benedettini et al.30 showed that Met expression and phosphorylation induced by gefitinib treatment were also associated with development of late brain metastasis in patients with NSCLC, indicating that potential role of another genetic change such as Met activation or a modification of the downstream signaling pathway below EGFR on brain metastases. Further clinicomolecular studies need to performed to examine this further.
Our study has several limitations. First, the retrospective nature of our study was one of the potential pitfalls. Second, we could not evaluate other clinically important genetic changes besides EGFR mutations, for example, KRAS mutation, Met amplification, or echinoderm microtubule-associated protein-like 4 anaplastic lymphoma kinase translocation. Despite these limitations, our study is valuable in view of new insights about the clinical association between EGFR mutation status and brain metastasis in pulmonary adenocarcinoma.
Because of the devastating outcomes of brain metastases,8 the results of our study theoretically support the idea that physicians should pay attention to brain metastases, particularly in patients with EGFR-mutated pulmonary adenocarcinoma. Considering the uncertainty of cost effectiveness of brain MRI,9 EGFR mutations may be useful for early detection of brain metastases. However, its predictive role should be determined through further studies.
In conclusion, this study suggests an association between EGFR mutation and increased risk of brain metastases at the time of diagnosis and during the disease course after curative resection. Despite its retrospective nature being a limitation, this study is the first to indicate distinct clinical features of EGFR-mutated tumors in terms of brain metastases. Molecular studies should be conducted on this finding in the future.
This work was supported by Institute-Supported Research Project appointed by Korea Cancer Center Hospital. This study was presented in part at the 37th ESMO Congress, Vienna, Austria, 28 September to 2 October 2012.
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Epidermal growth factor receptor mutation; Pulmonary adenocarcinoma; Brain metastasis; Cranial metastasis
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