Introduction

Non-small cell lung cancer (NSCLC) accounts for approximately 80–90 % of cases of primary lung cancer [1, 2], which is the leading cause of cancer deaths in most industrialized countries, including Japan. Although surgery is the optimal therapeutic modality for the cure of NSCLC, the prognosis following complete resection remains unsatisfactory [2, 3] (Table 1). Accordingly, a number of clinical trials of adjuvant therapy after surgery have been conducted to improve the prognosis. A meta-analysis, reported in 1995, showed the potential benefit of cisplatin-based adjuvant chemotherapy (a 13 % reduction in the risk of death, equivalent to an absolute benefit of 5 % at 5 years); however, the survival benefit was not statistically significant (p = 0.08), with a hazard ratio (HR) of 0.87 [95 % confidence interval (CI), 0.74–1.02] [4]. Recently, randomized controlled trials (RCTs) have revealed a significant survival benefit from adjuvant chemotherapy [5], and adjuvant chemotherapy following surgery has been established as the “standard treatment of care” [6]. In this article, we review the recent clinical evidence and discuss the current issues and future perspectives of adjuvant therapy in the treatment of NSCLC.

Table 1 Postoperative survival of primary lung cancer patients according to pathologic (p-) stage (UICC version 7 classification [2])
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Adjuvant chemotherapy with cytotoxic agents

Platinum-based chemotherapy

Since the 1995 meta-analysis [5], many large-scale RCTs of adjuvant chemotherapy using cisplatin-based regimens have been conducted [7] (Table 2). Among them, an international trial conducted by the International Adjuvant Lung Cancer Trial Collaborative Group (IALT) was the first trial to show that cisplatin-based adjuvant chemotherapy significantly improved overall survival (OS) [8]. In this trial, a chemotherapy regimen was selected from four cisplatin-based regimens by each participating center, and postoperative radiotherapy was allowed after the completion of chemotherapy according to each center’s policy. The JBR 10 trial [9] and the ANITA01 trial [10] were subsequent “positive” trials, in which cisplatin plus vinorelbine was employed as an adjuvant chemotherapy regimen.

Table 2 Randomized controlled trials of adjuvant cisplatin-based chemotherapy for non-small cell lung cancer (NSCLC) included in the LACE meta-analysis [7]
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The Lung Adjuvant Cisplatin Evaluation (LACE) Collaborative Group conducted a pooled analysis of 5 adjuvant trials, 3 positive trials (IALT, JBR10, ANITA01), and 2 negative trials (ALPI, BLT), and showed that cisplatin-based chemotherapy significantly improved postoperative OS [HR, 0.89 (95 % CI, 0.82–0.96); p = 0.005) with an absolute benefit of 5.4 % at 5 years after surgery. The benefit varied according to stage [HR for stage IA, 1.40 (95 % CI, 0.95–2.06); HR for stage IB, 0.93 (95 % CI, 0.78–1.10); HR for stage II, 0.83 (95 % CI, 0.73–0.95), HR for stage III, 0.83 (95 % CI, 0.72–0.94)], suggesting that postoperative adjuvant cisplatin-based chemotherapy only provided a survival benefit for advanced-stage (stages II–III) NSCLC patients. Regarding toxicity, 19 treatment-related deaths (0.9 %) have been reported to be associated with cisplatin-based adjuvant chemotherapy [7].

As a less toxic chemotherapy, carboplatin plus paclitaxel was employed in some adjuvant trials. In the Cancer and Leukemia Group B (CALGB) 9633 trial, 344 patients with completely resected pathologic (p-) stage IB (T2N0M0) NSCLC were randomly assigned to receive no chemotherapy (observation group) or adjuvant chemotherapy with carboplatin plus paclitaxel (chemotherapy group). Although the preliminary results indicated that postoperative adjuvant chemotherapy improved OS and disease-free survival (DFS), the final analysis failed to show a significant benefit in survival [median OS time (MST), 78 months in the observation group vs. 95 months in the chemotherapy group; HR, 0.83 (95 % CI, 0.64–1.08); p = 0.12]. An exploratory analysis demonstrated a significant OS benefit for patients who had tumors of ≥4 cm in diameter [HR, 0.69 (95 % CI, 0.48–0.99); p = 0.043] [11].

The NATCH trial, another trial with carboplatin plus paclitaxel, was a unique trial, which examined whether postoperative adjuvant chemotherapy or preoperative chemotherapy improved DFS as the primary endpoint. A total of 624 patients with clinical (c-) stage IA, with tumor size of >2 cm in diameter, IB, II, or T3N1 NSCLC were randomly assigned to surgery-alone group (control group), surgery followed by adjuvant chemotherapy with carboplatin plus paclitaxel (postoperative adjuvant group), or chemotherapy with carboplatin plus paclitaxel followed by surgery (preoperative induction group). No differences were observed between the control group and the postoperative adjuvant group in DFS [5-year DFS rate, 34.1 vs. 36.6 %; HR, 0.96 (95 % CI, 0.75–1.22); p = 0.74] or OS [5-year OS rate, 44 vs. 45.5 %; HR, 0.99 (95 % CI, 0.75–1.3); p = 0.93] [12].

Based on the evidence above, postoperative adjuvant chemotherapy with cisplatin-based regimens, especially cisplatin plus vinorelbine, is now recommended for p-stage II and IIIA patients, but not for p-stage I patients [6, 13, 14]. For unresectable c-stage IV patients with non-squamous NSCLC, pemetrexed in combination with a platinum agent is recommended as a first-line chemotherapy regimen [15], because treatment-by-histology analyses in a RCT indicated that pemetrexed plus cisplatin was superior to gemcitabine plus cisplatin for the treatment of non-squamous NSCLC patients [16]. In the postoperative adjuvant setting, whether pemetrexed plus cisplatin is the optimal chemotherapy regimen for non-squamous NSCLC patients remains unclear. This will be revealed by an ongoing Japanese trial comparing pemetrexed plus cisplatin with vinorelbine plus cisplatin (JIPANG trial).

Uracil-tegafur (UFT)

Uracil-tegafur (also referred to as UFT), the combination drug of uracil and tegafur at a molar ratio of 4:1, is an oral antimetabolite agent; tegafur is a pro-drug that is gradually converted to 5-fluorouracil (5-FU), and uracil is an inhibitor of dihydropyrimidine dehydrogenase (DPD) that is responsible for 5-FU degradation [17–20]. Uracil-tegafur can generate a higher serum and intra-tumoral concentration of 5-FU released from tegafur through the inhibition of DPD activity by uracil. Unfortunately, for advanced NSCLC patients, chemotherapy with uracil-tegafur failed to provide a significant anti-tumor effect, with response rates of only 6–8 % [18, 20].

In contrast, for completely resected NSCLC patients, especially those with p-stage I disease, adjuvant chemotherapy with uracil-tegafur was shown to provide a significant OS benefit in several RCTs conducted in Japan (Table 3) [21–27]. The 2nd-study conducted by the West Japan Study Group for Lung Cancer Surgery (WJSG) was the first study to clearly show a significant survival benefit of adjuvant chemotherapy with uracil-tegafur. In the trial, patients with completely resected p-stage I-III NSCLC were randomly assigned to the control group (surgery alone), the UFT group (surgery followed by chemotherapy with uracil-tegafur), or the CVUft group (surgery followed by a chemotherapy regimen that consisted of cisplatin plus vindesine followed by uracil-tegafur). The UFT group showed the most favorable survival with a statistically significant difference in comparison with the control group (p = 0.022) [21].

Table 3 Randomized controlled trials of adjuvant tegafur-uracil chemotherapy for non-small cell lung cancer (NSCLC)
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The Japan Lung Cancer Research Group on Postsurgical Adjuvant Chemotherapy (JLCRG) trial was the largest adjuvant trial of uracil-tegafur to be conducted in Japan. In the trial, only p-stage I adenocarcinoma patients were randomly assigned to the control group (surgery alone) or the UFT group (surgery followed by uracil-tegafur), and the UFT group showed a significantly favorable survival benefit in comparison with the control group [HR, 0.71 (0.52–0.98); p = 0.04]. The survival benefit was significant in p-stage IB (T2) patients (5-year OS, 74 % for the control group vs. 85 % for the UFT group; HR, 0.48; P = 0.005), but was not seen in p-stage IA (T1) patients [5-year OS, 90 vs. 89 %; HR, 0.97 (95 % CI, 0.64–1.46); p = 0.87] [26].

A meta-analysis of six randomized trials was conducted which compared surgery-alone with surgery followed by uracil-tegafur. A total of 2003 patients, most with p-stage I disease, were included. The 5-year and 7-year overall survival rates were higher in the surgery plus uracil-tegafur group (81.5 and 76.5 %, respectively) than in surgery-alone group (77.2 and 69.5 %) with a significant difference (p = 0.011 and p = 0.001, respectively). The overall pooled HR was 0.74 [(95 % CI, 0.61–0.88); p = 0.001] [28]. A further exploratory analysis of the data from the meta-analysis showed that chemotherapy with uracil-tegafur was effective for the T1b subset (tumor diameter, >2 to ≤3 cm) of p-stage IA disease [29]. No treatment-related deaths (TRD) were reported in any of the adjuvant trials, indicating the clinical advantage of the use of uracil-tegafur, which has a mild toxicity profile.

In Japan, it is generally accepted that postoperative uracil-tegafur chemotherapy is effective and it is recommended for the T1b subset of p-stage IA disease and for p-stage IB disease. Because all adjuvant trials with uracil-tegafur were conducted in Japan, the use of uracil-tegafur after complete resection is not recommended outside of Japan [6].

Adjuvant chemotherapy with targeting agents

Today, two major classes of targeting agents, those that inhibit tumor angiogenesis and those that inhibit “driver oncogenes,” play important roles in chemotherapy for patients with unresectable metastatic NSCLC [30]. In the postoperative adjuvant setting, chemotherapy using targeting agents may provide a survival benefit, but the safety and efficacy remain unclear and should be investigated.

Angiogenesis inhibitors

Bevacizumab, a monoclonal antibody against vascular endothelial growth factor (VEGF), is the first clinically available angiogenesis inhibitor. In a phase 2 trial for advanced NSCLC, life-threatening pulmonary hemorrhage occurred in 6 of 66 patients who received bevacizumab, and serious hemorrhage events appeared to be more common among patients with predominantly squamous cell carcinomas [31]. Based on these results, only non-squamous NSCLC patients were enrolled in further randomized trials [32]. The ECOG 4599 is a landmark trial showing a significant improvement in OS with the addition of bevacizumab to carboplatin plus paclitaxel as the first-line chemotherapy for advanced non-squamous NSCLC [median OS time (MST), 10.3 vs. 12.3 months; HR, 0.79 (95 % CI, 0.67–0.92); p = 0.003] [33]. A meta-analysis of four randomized trials, including the ECOG 4599 trial, showed that bevacizumab significantly improved OS when it was combined with platinum-based chemotherapy [HR, 0.90 (95 % CI, 0.81–0.99; p = 0.03)] [31], and the addition of bevacizumab was recommended in first-line chemotherapy for selected non-squamous NSCLC patients [15]. Ramucirumab, an antibody against a certain receptor of VEGF (VEGFR-2), is another angiogenesis inhibitor. A phase 3 trial comparing docetaxel alone and docetaxel plus ramucirumab in second-line chemotherapy for advanced NSCLC of all histologic types, including squamous cell carcinoma, showed a significant OS benefit from the addition of ramucirumab (MST, 9.1 vs. 10.5 months) [34]. In December 2014, the addition of ramucirumab was approved in the USA.

Whether the addition of bevacizumab to cisplatin-based chemotherapy can provide a survival benefit postoperative adjuvant setting is currently being evaluated in an ongoing phase 3 trial (ECOG1505). [35]. Patients with completely resected p-stage IB (≥4 cm in diameter)–IIIA NSCLC are eligible for participation in the study and are randomly assigned to chemotherapy-alone group or chemotherapy plus bevacizumab group. Patients assigned to the chemotherapy-alone group receive four cycles of a cisplatin-based chemotherapy, which is chosen (by physicians) from four different regimens: cisplatin plus either vinorelbine, gemcitabine, docetaxel, or pemetrexed (pemetrexed is only used for the treatment of non-squamous NSCLC patients). Patients assigned to the chemotherapy plus bevacizumab group receive 15 mg/kg of bevacizumab every 3 weeks for 1 year in addition to four cycles of cisplatin-based chemotherapy. An interim analysis of the trial showed no unexpected toxicity caused by the addition of bevacizumab in the postoperative adjuvant setting; however, treatment-related mortality seemed high in both groups (2.5 % in the chemotherapy-alone group and 3.8 % in the chemotherapy plus bevacizumab group) [36], considering the relatively low mortality after recent lung cancer surgery (<1 % in Japan) [37]. Patient accrual into the study has been completed, and the safety and efficacy of the addition of bevacizumab in the postoperative adjuvant setting will be revealed in near future.

Tyrosine-kinase inhibitors (TKIs)

The discovery of “driver oncogenes” such as mutations in the epidermal growth factor receptor (EGFR) gene and chromosomal alterations in the anaplastic lymphoma kinase (ALK) gene has provided new insights into chemotherapy for metastatic NSCLC [30, 38–42]. A strong correlation between the presence of EGFR gene mutations (deletions in exon 19 or point mutations in exon 21) activating the tyrosine-kinase activity of EGFR and a significant clinical response to the administration of tyrosine-kinase inhibitor of EGFR (EGFR-TKI, gefitinib) was first reported in 2004 [38, 39]. The subsequent subgroup analyses of a phase 3 study comparing carboplatin plus paclitaxel and gefitinib, as first-line chemotherapies for lung adenocarcinoma patients, showed that the presence of EGFR-activating mutations strongly predicted prolonged progression-free survival (PFS) and a higher response rate [43]. In addition, all phase 3 trials comparing platinum-based regimens with EGFR-TKI (gefitinib [44, 45], erlotinib [46, 47], or afatinib [48, 49]) as the first-line chemotherapy for patients with NSCLC harboring EGFR-activating mutations showed a significantly longer PFS in patients treated with EGFR-TKI. Similarly, as a first-line treatment for metastatic NSCLC with ALK-alteration, a phase 3 trial comparing pemetrexed plus platinum agent (cisplatin or carboplatin) and an ALK-TKI (crizotinib) showed a significantly better PFS and response rate in patients treated with crizotinib [50]. Based on these results, systemic treatment with TKIs is currently recommended for patients with metastatic NSCLC who harbor “driver oncogenes” [15].

Several phase 3 studies comparing surgery-alone and surgery followed by EGFR-TKI administration have been conducted in a postoperative adjuvant setting (Table 4) [51–53]. The V-15-3 trial (conducted in Japan) and the BR 19 trial (conducted in North America) were RCTs of adjuvant gefitinib administration for mutation-unselected NSCLC patients. All patients with completely resected p-stage IB, II, or IIIA NSCLC were eligible for inclusion in the study, regardless of EGFR-status, because EGFR-activating mutations had not been discovered when the studies were planned. The V-15-31 trial was halted very early because of increased incidence of interstitial lung disease (ILD). The safety data for the 38 recruited patients showed no unexpected adverse events related to gefitinib treatment; however, it should be noted that one of the 18 patients assigned to the gefitinib group died of ILD [51].

Table 4 Randomized controlled trials of adjuvant treatment with EGFR-TKI for non-small cell lung cancer (NSCLC)
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The BR 19 trial was also closed early, after the random assignment of 503 of the 1242 planned patients (252 to the placebo group and 251 to the gefitinib group), because two RCTs for advanced NSCLC [Iressa Survival Evaluation in Lung Cancer (ISEL) [54] and S0023 [55]] showed no survival benefit from the administration of gefitinib. The survival data analyses for all patients enrolled in the BR 19 trial showed no difference in OS (primary endpoint) or DFS (secondary endpoint) between the placebo group and the gefitinib group. The exploratory analyses showed no survival benefit from gefitinib administration, even in patients with tumors harboring EGFR-activating mutations [52].

In the RADIANT trial, an international trial of adjuvant erlotinib treatment for p-stage IB, II, or IIIA NSCLC, patients were eligible only when the expression of EGFR was found to be positive by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH). DFS for all eligible patients, the primary endpoint of the trial, was similar in the placebo group and in the erlotinib group (HR, 0.90). DFS for patients with EGFR mutations, an added secondary endpoint, favored the erlotinib group (HR, 0.61); however, the difference was not statistically significant [53].

Accordingly, adjuvant EGFR-TKI administration should not be indicated for mutation-unselected patients, but might be evaluated in clinical trials for patients with EGFR mutations. An RCT comparing cisplatin plus vinorelbine and gefitinib as the postoperative adjuvant chemotherapy for p-stage II-III NSCLC patients with EGFR mutations (IMPACT trial) is currently underway in Japan [56].

Adjuvant radiotherapy

A meta-analysis of postoperative radiotherapy (PORT) conducted in 1998 showed a significant adverse effect of postoperative radiotherapy on survival [HR, 1.21 (95 % CI, 1.08–1.34)] [57], and the updated meta-analyses showed similar results indicating the detrimental effect of postoperative radiotherapy [58–60]. However, subgroup analyses showed that postoperative radiotherapy was increasingly detrimental with decreasing stage and lower nodal stage and that it was not detrimental in the treatment of p-N2 disease (HRs, 1.41, 1.21, and 0.96 for N0, N1, and N2 diseases, respectively) [60].

Some studies have shown that postoperative radiotherapy might improve survival in p-N2 disease [60, 61]. A retrospective review of the population-based data [data from the surveillance, epidemiology, and end results (SEER) database, in the USA] showed a significant association between the performance of postoperative radiotherapy and favorable survival [HR, 0.855 (95 % CI, 0.762–0.959); P = 0.0077] [61]. A subgroup analysis of the ANITA01 trial, an RCT of adjuvant chemotherapy with cisplatin plus vinorelbine in which postoperative radiotherapy was optional [10], showed that postoperative radiotherapy significantly improved survival both in patients assigned to receive adjuvant chemotherapy (5-year OS rate, 34 vs. 47 %) and in those assigned to receive no chemotherapy (5-year OS rate, 17 vs. 21 %) [10, 62]. These results suggest that postoperative radiotherapy may benefit some patients with p-N2 disease, but the role of postoperative radiotherapy remains unclear due to a lack of high-level clinical evidence. This will be clarified in ongoing RCTs such as the Lung Adjuvant Radiotherapy Trial (LUNG ART) [63]. Accordingly, postoperative radiotherapy is not recommended for unselected p-N2 patients, but might be considered for carefully selected p-N2 patients with a high risk of local recurrence [13].

Future perspectives: biomarker-oriented individualized adjuvant chemotherapy

Today, adjuvant chemotherapy with cytotoxic agents [uracil-tegafur for p-stage IA disease (tumor diameter >2 cm) and IB disease, and cisplatin-based regimens for p-stage II-IIIA disease] has been established as the “standard treatment of care” for completely resected NSCLC. However, adjuvant chemotherapy was shown to provide only a modest survival benefit of 4 % at 5 years (from 60 to 64 %) in a meta-analysis [5]. In other words, only 4 of 100 patients who receive adjuvant chemotherapy will benefit from the therapy. The 5-year survival rate of 60 % in the surgery-alone group means that 60 of 100 patients survive without adjuvant chemotherapy and that the remaining 40 patients actually need adjuvant chemotherapy. The 5-year survival rate of 64 %, which corresponds to a 5-year mortality rate of 36 %, in the adjuvant chemotherapy group means that 36 of the 40 patients die from tumor relapse despite undergoing treatment with adjuvant chemotherapy. Simply put, in a group of 100 patients who receive adjuvant chemotherapy, the “evidence-based” findings suggest that adjuvant chemotherapy would not be necessary for 60 patients and not be effective for 36. Adjuvant chemotherapy would only be beneficial for the 4 remaining patients (Fig. 1). In addition to the modest survival benefit, adjuvant chemotherapy, especially cisplatin-based chemotherapy, may cause adverse effects that are severe and sometimes fatal. Accordingly, 96 of 100 patients who received adjuvant chemotherapy to prevent 4 lung cancer deaths would experience certain, sometimes fatal, adverse effects without any individual benefit from the treatment.

Fig. 1
figure 1

Survival curves showing the survival benefit from adjuvant chemotherapy following surgery. Adjuvant chemotherapy is essentially unnecessary for 60 % of patients, who can be expected to survive without adjuvant chemotherapy, and is ineffective for 36 % of patients who can be expected to die despite receiving adjuvant chemotherapy. As a result, adjuvant chemotherapy has been found to only be beneficial for 4 % of the patients who received the treatment, although the survival benefit of 4 % at 5-years after surgery was found to be statistically significant in clinical trials

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To improve the risk–benefit balance of adjuvant chemotherapy, a promising approach may be biomarker-oriented individualization. After selecting patients by biomarkers that predict prognosis (prognostic markers) as well as those that predict the efficacy or toxicity of chemotherapy (predictive markers), adjuvant chemotherapy is prescribed only to the selected patients who are likely to benefit from the treatment (Fig. 2). Prescribing adjuvant chemotherapy only to patients in whom a higher risk of recurrence is identified by prognostic markers means that the other patients will not receive unnecessary chemotherapy. The ability to prescribe adjuvant chemotherapy only to patients who are expected, due to the presence of predictive markers, to receive a significant survival benefit from chemotherapy means that the patients who are not expected to benefit will not receive ineffective chemotherapy. A number of biomarkers such as excision repair cross-complementation group 1 (ERCC1) have been investigated in postoperative adjuvant setting [64–73], but no optimal biomarker has been established (Table 5) [64]. The establishment of new biomarkers is necessary to realize individualized adjuvant chemotherapy with a favorable risk–benefit balance.

Fig. 2
figure 2

Individualized adjuvant chemotherapy based on prognostic and predictive biomarkers. The selection of patients who truly need adjuvant chemotherapy by prognostic markers and those who will truly benefit from adjuvant chemotherapy by predictive markers may improve the risk–benefit balance of adjuvant chemotherapy

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Table 5 Biomarkers investigated in adjuvant chemotherapy trials (modified from Tables 3, 4, 5 in [64])
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ERCC1

ERCC1 is an important enzyme that is responsible for the repair of damage to deoxyribonucleic acids (DNA). It has been investigated as a potential prognostic and predictive marker in a number of studies [65]. The IALT Bio study was conducted to assess whether IHC-determined biomarkers can predict a significant survival benefit from adjuvant cisplatin-based chemotherapy [66]. Among all of the 1867 patients enrolled in the IALT trial, adjuvant cisplatin-based chemotherapy provided only a modest survival benefit, with a 4.1 % improvement in 5-year OS [8]. The IALT Bio study showed a significant association between a survival benefit from adjuvant chemotherapy and the absence of ERCC1 expression (test for interaction, P = 0.009). When stratified by ERCC1 expression status, the survival benefit was greater among patients with ERCC1-negative tumors [5-year OS rate, 39–47 %; HR, 0.65 (95 % CI, 0.50–0.86); P = 0.002], but was not seen in patients with ERCC1-positive tumors [5-year OS rate, 55–50 %; HR, 1.14 (95 % CI, 0.84–1.55); P = 0.40]. Among patients assigned to receive no chemotherapy, patients with ERCC1-positive tumors survived significantly longer than those with ERCC1-negative tumors [5-year OS rate, 46 vs. 39 %; HR, 0.66 (95 % CI, 0.49–0.90); P = 0.009]. These results suggest that the absence of ERCC1 expression is a significant prognostic factor in the prediction of a poor prognosis [66].

Nevertheless, a subsequent study to validate the prognostic and predictive effects of ERCC1 expression status failed to show a significant association between ERCC1 expression status and the prognosis or efficacy of cisplatin-based adjuvant chemotherapy [67]. A meta-analysis of published studies that investigated the clinical significance of ERCC1 status in NSCLC showed that high ERCC1 expression was associated with poor survival in patients treated with platinum-containing chemotherapy. In spite of these findings, there is no clear evidence to support the use of ERCC1 status in clinical practice, mainly due to small sample size of each study as well as the non-standardized protocols that they employed, including the assay and cutoff values for ERCC1 expression [64]. The evidence that is currently available suggests that ERCC1 status should be investigated within clinical trials. To evaluate the feasibility of individualized adjuvant chemotherapy based on ERCC1 expression and EGFR mutations, a randomized phase 2 trial [IFCT-0802, the phase 2 part of the tailored postsurgical therapy in early-stage NSCLC (TASTE)] was conducted in France. The primary endpoint of the study, the feasibility as defined by 80 % of patients being able to start adjuvant therapy within 2 months after surgery, was met. However, the phase 3 part of the TASTE was canceled due to the poor reliability that the IHC assay demonstrated in the evaluation of ERCC1 expression [67, 74].

Gene signatures

Microarray studies have provided a large number of gene signatures that may be used to predict clinical outcomes by evaluating the expression of multiple genes (not a single gene) in tumor tissues. In breast cancer, the prognostic significance of gene signatures evaluated by commercially available assays such as “Oncotype DX” and “MammaPrint” has been established, with gene signatures being used in decision-making related to the administration of adjuvant chemotherapy [75].

A number of studies have identified different gene signatures that may be used to predict the prognosis or survival benefit associated with adjuvant chemotherapy in NSCLC; however, no signatures have been established for use in clinical practice [76, 77]. For example, Potti et al. [78] reported that a gene signature (the “metagene model”) predicted the risk of recurrence following surgery, but the article was retracted because the results were not validated.

Recently, some promising gene signatures, which have been validated in independent cohorts, were identified. The prognostic significance of a 15-gene signature, identified as a prognostic and predictive marker in a cohort of patients enrolled in an adjuvant cisplatin plus vinorelbine chemotherapy trial (JBR10) [72], was validated in an independent cohort of resected NSCLC patients [73]. In addition, another gene signature incorporating 14 genes was identified as a prognostic marker in a cohort of resected non-squamous NSCLC patients, which was validated in 2 independent cohorts [79]. The “14-gene signature assay” is the only commercially available laboratory test to evaluate a prognostic gene signature in NSCLC (Perveniom RS test, Life Technologies, Inc., Grand Island, NY), but the test may not be useful in prediction of a benefit from adjuvant chemotherapy.

Circulating tumor cells (CTCs)

Circulating tumor cells (CTCs) are tumor cells that are shed from a primary tumor and circulate in the peripheral blood [80]. CTCs, which can even be detected in the early stage of the development and progression of malignant tumors, can be a surrogate of micro-metastasis. The presence of CTCs may indicate the presence of micro-metastatic foci, which will develop into clinically apparent metastatic foci. Accordingly, among all patients who underwent complete resection, patients who truly need adjuvant chemotherapy due to a high-risk of recurrence can be identified and selected as “CTC positive” by a CTC test (Fig. 3). In addition, a CTC test can be repeatedly and noninvasively performed by peripheral blood sampling, which may indicate another advantage of the CTC test: It can be used for the real-time monitoring of the therapeutic effects of adjuvant chemotherapy.

Fig. 3
figure 3

Circulating tumor cells (CTCs) are tumor cells that are shed from a primary tumor and circulate in the peripheral blood. The detection of CTCs by the peripheral blood sampling (CTC test) may predict the presence of micro-metastatic foci, which will develop and result in postoperative recurrence

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Despite such potential advantages of CTCs as a biomarker, the detection of rare CTC contaminants in a large number of normal blood cells may present a technical challenge. The CellSearch System (Veridex LLC, Raritan, NJ) is a semi-automated system for the quantitative evaluation of CTCs, in which CTCs are immunomagnetically captured with an antibody against epithelial cell adhesion molecule (EpCAM) [81]. The most important advantage of the CellSearch system is the reproducibility of its results across different laboratories [82], A CTC test using the CellSearch system has been approved in the USA by the Food and Drug Administration (FDA) for the monitoring of blood from metastatic breast and colon cancer patients. In lung cancer, however, little has been reported. We assessed the clinical usefulness of the CTC test in a series of prospective studies [83–85] and showed that CTCs were more frequently detected in small cell lung cancer (SCLC) patients than in NSCLC patients (66.7 vs. 27.6 %) [84] and that the CTC test provided a significant prognostic performance in SCLC [85]. The diagnostic performance of the CTC test in predicting the presence of distant metastasis was significant [area under receiver operating characteristic curve (AUC-ROC), 0.783 (95 % CI, 0.669–0.886); P < 0.001), but the sensitivity was only 71.0 % [84]. The relatively low sensitivity (<100 %) in predicting the presence of “clinically detectable” metastasis may indicate that the CTC test is not sensitive enough to predict the presence of micro-metastasis. For the clinical application of a CTC test in decision-making related to the performance of adjuvant chemotherapy for completely resected NSCLC patients, it will be essential to develop detection systems with a higher degree of sensitivity.

Conclusions

Adjuvant chemotherapy for completely resected NSCLC has been established as the “standard treatment of care” based on the abundant evidence that has been yielded by RCTs. However, current adjuvant chemotherapy provides only a modest survival benefit and sometimes causes fatal side effects when it is prescribed for all patients according to “evidence-based” guidelines. Future studies may focus on the individualization of adjuvant chemotherapy based on biomarkers such as gene signatures, which will improve the risk–benefit balance of adjuvant chemotherapy.