Preoperative differentiation between benign and malignant SRMs is important for treatment selection[24]. In this study, we developed a binary logistic regression model to combine T1 mapping, conventional MRI images, and clinical characteristics, demonstrating that the pre-enhanced T1 mapping is an independent predictor. For the identification of benign renal tumors, T1 mapping demonstrated an AUC of 0.697 (0.596–0.785), and achieved 0.832 (0.743–0.899) when combined with the clinical features.
Recent studies have focused on the quantitative evaluation of MRI findings associated with the differentiation of renal masses[23, 25]. Adams et al.[26] used T1 mapping to differentiate between low-grade and high-grade ccRCC. The results showed the reduction in T1 value after contrast agent administration in higher grade ccRCC (ISUP grades 3–4) was significantly higher than lower grade ccRCC (ISUP grades 1–2)[27]. But their study populations were only 27, which might have been more conclusive if more cases were enrolled. Wang et al.[28] also applied T1 mapping to differentiate different renal tumors, their study included 56 cases of renal tumors: 46 tumors were pathologically proven (including 40 RCCs and 10 AMLs respectively), but 6 AMLs were diagnosed only through MRI. The results show the statistical significance of different T1 mapping based parameters (pre- and post-contrast T1 mapping, the reduction of T1 mapping, and the reduction ratio of T1 mapping) in identifying renal tumors. Those methods have certain defects. In addition to the small sample size, they did not integrate conventional imaging features and clinical characteristics. In comparison, our study combined clinical and imaging data and focused more on the small solitary renal tumors which were indistinguishable using plain MRI. Further, a relatively larger sample size was included in our study, and all tumors were confirmed by surgical pathology. The role of T1 mapping in identifying renal tumors is still in the initial stage of research and is worthy of further exploration.
However, what could be the potential explanation for the efficacy of pre-enhanced T1 mapping in the detection of benign renal tumors in our study? It may be explained by the following reasons. Firstly, malignant renal tumors are always more heterogeneous than benign tumors, exhibit higher levels of necrosis, and are more likely to contain cysts. Although we have avoided these areas in our ROIs, it may still be difficult to exclude macroscopic zones[29]. Secondly, the upregulation of genes and proteins within the extracellular matrix could play a significant role in malignant renal tumors. It is common for poorly differentiated renal tumors to exhibit irregular tumor cells and loose intercellular spaces[22]. In contrast, fat-poor angiomyolipoma is characterized by spindle cells or epithelioid smooth muscle cells with abnormal thick-walled blood vessels in variable proportions, which would indicate tiny intercellular spaces. This may demonstrate a benign renal tumor with lower T1 relaxation time[22, 30].
An interesting observation is that the native T1 mapping was an independent influence factor for diagnosing benign renal tumors, but the enhanced T1 mapping was not. In general, enhanced T1 mapping can improve the accuracy of blood T1 values and can consequently increase the measurement accuracy of extracellular volume fraction. We speculate that these results may be related to the blood supply of both group tumors. Among our groups, the ccRCC group has the highest percentage (61.5%) of malignant tumors, while poor fat angiomyolipoma predominates (71.4%) in benign tumors. They all have a rich blood supply. This outcome may be more conducive to the clinical promotion of T1 mapping. Native T1 mapping can assist in detecting malignant renal tumors in patients with chronic kidney disease, thereby reducing the risk of adverse effects from contrast agents on renal function, as well as decreasing financial burdens[31].
Moreover, several regular sequences for MRI inspection were analyzed in this study, such as the T2-weighted imaging and the strengthen formal of tumors, which have been proven to be helpful in the differentiation of benign from malignant tumors[18]. However, these characteristics were not independent influence factors for SRMs in our study. A meta-analysis by Shang et al. shows the sensitivity and specificity of routine MRI for the detection of small malignant masses achieved 0.85 (95% CI 0.79–0.90) and 0.83 (95% CI 0.67–0.92), respectively[32]. We analyzed and discussed as follows. First, the advantage of a larger sample size for meta-analysis can’t be ruled out. Second, we only used a 1.5T MR scanner, while the 3.0T instrument may provide more information. Third, we excluded renal cysts and typical fat-containing renal lesions and included patients with small solid tumors, which were difficult to make a definite diagnosis in the clinic. Fourth, qualitative assessment based on the radiologist’s decision might be inconsistent, especially when the sign was equivocal on the image[33]. This study hopes to be able to perform such assessments quantitatively and objectively.
This study has some limitations. The first is the retrospective and single-center design of the study, which has certain limitations and may have retrospective bias. Second, we did not conduct experiments to validate the relationship between T1 mapping and tumor pathophysiological changes. Third, there is significant imaging variation in different types of renal tumor subtypes (such as clear cell RCC, papillary RCC, and chromophobe RCC). Our study is based only on the imaging manifestations between the benign and malignance SRMs. Fourth, there is no unified calculating parameter on T1 mapping. Therefore, a collaborative infrastructure development for multicenter studies is needed, so that the performance of T1 mapping techniques at different magnetic field strengths can be evaluated, and the histopathology of SRMs types can be comprehensively studied.