1. Introduction
Lung cancer has the highest mortality rate among all cancers [
1], with a 5-year survival of about 17.8% [
2]. The World Health Organization (WHO) categorizes lung tumors into two groups: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC), with the latter covering 85% of cases [
3]. NSCLC is further sub-categorized into lung adenocarcinoma and lung squamous cell carcinoma [
4].
Patients with NSCLC are often diagnosed at advanced stages [
5], with cough and dyspnea as common symptoms. In this group of patients, for whom surgery is not indicated, the availability of predictive biomarkers for target molecular therapy or immunotherapy has opened new treatment possibilities in addition to conventional chemo-radiotherapy. Immunotherapy aims to modulate the immune system, reactivating it against cancer cells. To this end, with immunohistochemical analysis, the expression of the programmed death-ligand 1 protein (PD-L1) by the tumor cells is quantified. The PD-1/PD-L1 axis plays an important role in tumorigenesis and tumor development, since the binding of PD-1, expressed by lymphocytes, to its ligand PD-L1 causes downregulation of the T-cell response, directly suppressing the endogenous anti-tumor cytolytic T-cell activity. Monoclonal antibodies that block the interaction between PD-1 and PD-L1 abrogate the immune tolerance exerted by tumors through the PD-1/PD-L1 pathway.
The pathological test, either performed on tissue obtained through navigational bronchoscopy or computed tomography-guided, is the gold standard to determine the histological subtype of lung cancer and to carry out the immunohistochemical analysis in order to quantify the PD-L1 tumor proportion score (TPS). However, the biopsy is an invasive diagnostic test associated with high perioperative complication rates and severe limitations in patients with poor compliance [
6]; moreover, it samples only a part of the tumor. For these reasons, in recent times, the need has emerged to find a noninvasive alternative way to identify the histologic subtype of lung tumors and quantify PD-L1 expression, providing us with information on the whole tumor rather than on a small sample of it.
The application of multiparametric magnetic resonance imaging (MRI) to lung cancer analysis is a thoroughly explored field since MRI provides better tissue characterization compared to computed tomography (CT) while involving no exposure to ionizing radiation [
7,
8].
T1 mapping is an MRI technique that can quantify the longitudinal relaxation time of water protons in tissues [
9]. It has been used mainly in cardiovascular imaging as an important tool for the characterization of myocardial tissue [
10]. Previous studies have been focused on the use of T1 mapping in pulmonary diseases, specifically on functional assessment in chronic obstructive pulmonary disease (COPD) [
11,
12], on the differentiation between benign and malignant lung lesions, based on the different water content [
13], and on the identification of lung cancer pathological types [
14] and their correlation with Ki-67 expression [
15].
T2 mapping instead is able to calculate the T2 time, i.e., the transverse relaxation time of water protons, displaying it voxel-vice on a parametric map [
9]. It has been applied mainly to the characterization of myocardium-related diseases [
16,
17], but also to many other clinical conditions, including prostate tumors [
18], breast tumors [
19], ovarian cancer [
20], uterine lesions [
21], and osteoarthritis [
22].
To the best of our knowledge, there have been no studies on the correlation between T1 and T2 mapping values and PD-L1 expression of NSCLCs nor on the correlation between T2 mapping values and the histological types of lung tumors. Therefore, the objectives of our study were (1) to investigate the possible association of T1 and T2 mapping values with PD-L1 TPS and (2) to evaluate their potential in distinguishing between the different histological subtypes of NSCLCs.
4. Discussion
In general clinical practice, the histological subtype of lung tumors is evaluated through pathological analysis, while the expression of PD-L1 by the tumor cells is quantified with immunohistochemical analysis. However, a biopsy of the tumor is necessary, which is an invasive diagnostic test that only samples a part of the tumor and carries important limitations in patients with poor compliance. For these reasons, in recent times there have been many attempts to find a noninvasive alternative way to the bioptic test. Magnetic Resonance Imaging (MRI) can provide not only morphologic information but also better tissue characterization with respect to Computed Tomography (CT).
Our results suggest that native T1 mapping times estimated during MRI of the thorax could be used to noninvasively identify the histological subtype of NSCLCs. Furthermore, we demonstrated that T1 and T2 mapping values seem unable to distinguish between tumors with positive PD-L1 expression and those without PD-L1 expression.
We used a protocol to study the lungs with MRI that included a complete morphological assessment and T1 and T2 mapping sequences, followed by the quantification of T1 and T2 times of lung tumors.
T1 mapping is defined by the pixel-to-pixel illustration of absolute T1 relaxation times. Elevated T1 relaxation times have been associated with increased extracellular compartment volume; thus, conditions such as protein deposition or fibrosis can be detected using native T1 mapping [
23,
24,
25,
26]. Yang et al. demonstrated that T1 mapping could distinguish benign from malignant lung lesions due to their different water content; however, the range of T1 values for malignant lung lesions was broad, and the reason could lie in the fact that it varies according to the different pathological types of lung cancer [
13]. Recently, many studies have been focused on T1 mapping as an independent imaging biomarker for characterizing histological lung cancer type. Li et al. have reported a significant difference in T1 values between SCLC and ADK, and between SCC and ADK; specifically, they found higher T1 values in SCC and SCLC compared to ADK. Additionally, they demonstrated a statistically significant difference in apparent diffusion coefficient (ADC) and T1 values between the moderately and highly differentiated group and the poorly differentiated group of lung tumors [
15]. Jiang et al. observed significantly higher T1 values in SCLC compared to ADK and SCC; however, there was no significant difference between ADK and SCC [
14]. In our study, for the
periphery ROI,
core ROI and
whole tumor ROI, T1 values of ADKs were significantly higher with respect to those of SCCs. This could be related to the different extracellular matrix (ECM) composition of the two histological subtypes of tumors, with the ADKs included in our study being more fibrotic than the SCCs. Indeed, due to their different anatomical locations, adenocarcinoma and squamous cancer cells are exposed to different ECM, which is an important regulator of cell behavior in multiple cancer types [
27]. Primary lung tumors, both the ADK and SCC subtypes, have increased and altered fibrillar collagen deposition, consistent with a fibrotic response. On one hand, tenascin-C, a glycoprotein that, if activated, promotes collagen deposition, is significantly upregulated in fibrotic lungs and in lung ADK [
28]; on the other hand, the glycoprotein periostin, expressed by activated fibroblasts, is higher expressed in SCCs compared to ADKs [
29].
Since the association between ECM expression and prognosis has been documented in general in NSCLCs [
30], T1 mapping values could also be a surrogate for ECM quantification to correlate with prognosis, but further studies are necessary.
T2 relaxation time represents the time constant governing the exponential decay of transverse magnetization. The fractional increment in T2 is larger than the one in T1 when the water content is increased, making T2 mapping a tool for detecting edema [
17]. In our study, T2 values did not show significant differences between histological subtypes of NSCLC; however, we reported the T2 mean values of the different subtypes in order to have reference values for future research.
We selected five different ROIs for estimating T1 and T2 values in order to evaluate potential differences in tissue composition among different regions of the tumor, i.e., the whole extent of the tumor, its peripheral portion avoiding the possible necrotic center, its central portion, and the lung tissue strictly adjacent to the tumor. However, there was not a statistically significant difference between the different tumor regions.
Furthermore, no statistically significant difference was identified between PD-L1 tumor proportion score (TPS) neither with T1 nor with T2 mapping values, potentially highlighting the fact that there were no differences in ECM composition or cellular inflammatory infiltrates detectable with MR between the two groups, even considering different regions of the tumors; further studies on the cellular microenvironment of specific lung tumors’ subtypes are needed to confirm our results, as well as the development of new and more advanced MR sequences with better sensitivity to small modifications of imaging parameters.
Tumor heterogeneity is a challenge of modern oncology since it can have a negative impact on clinical outcomes. Yet, the assessment of heterogeneity on the histological sample following biopsy may be tainted by a high level of sampling bias. T1 mapping may be a promising MRI sequence for evaluating the intrinsic properties of tumors, also in association with other MRI sequences. Firstly, MRI can be repeated regularly, also under therapy, without the use of ionizing radiation; furthermore, the T1 mapping sequence we used can be performed in short scanning times and, thus, it can be easily incorporated into MR exams.
Our study was the first research project with the aim to investigate the relationship between PD-L1 expression and T1 or T2 mapping values, however, it presents several limitations: in the first place, we used a set of measurements which were collected by a single trained radiologist, so the independent work of two or more radiologists could have been more useful in evaluating the presence or absence of inter-observer agreement and further increasing the strength of our data. Moreover, low proton density, susceptibility artifacts, and motion artifacts may affect the reproducibility of native T1 mapping in lung lesions.
Another factor to be considered is the heterogeneity both within the neoplastic tissue and at the interface of tumor–normal lung or tumor–atelectasis, which could lead to some bias in the measurement of T1 and T2 values; the ROI areas have been traced on tumors of different sizes and shapes, so they may have included smaller or larger necrotic regions that could have affected our measurements. Finally, this is a monocentric study with a small sample size.
Further studies are necessary to validate T1 mapping values in determining the histological subtype of NSCLCs and to investigate the potential correlation of T1 and T2 values with PD-L1 TPS.