Patients
The Institutional Ethics Committee approved this retrospective study and waived the need for patient consent. The study included patients who underwent abdominal CT scans and were diagnosed with a renal tumor at our institution between January 2016 and January 2022. The inclusion criteria were as follows: (1) Patients with ccRCC who underwent a partial or radical nephrectomy. (2) Patients who had plain and enhanced CT scans performed prior to surgery. (3) Patients with complete clinical information. The exclusion criteria were as follows: (1) Significant artefacts on CT images. (2) Tumors with a diameter greater than 4 cm. (3) Patients with a history of both kidney tumors and other tumors. (4) Patients who received treatment before the CT scan. A total of 113 patients were enrolled in the study, including 49 with high-grade small ccRCC and 64 with low-grade small ccRCC. In a ratio of 6:4, patients were randomly assigned to a training set (n = 67) and a testing set (n = 46). Figure 1 illustrates the workflow for enrolling the patient cohort.
Ct Imaging Acquisition
Routine clinical CT scans of the kidney are typically performed using 64-slice multidetector CT equipment. The CT scan parameters are as follows: the tube voltage is 120kV-140 kV; tube current is 250mA-400 mA; slice thickness is 5 mm. Approximately 80 to 100 mL of contrast agents is injected into the antecubital vein at a rate of 3.0 mL/s using a high-pressure injector. Four phases of CT images are obtained: the unenhanced phase (UP), the corticomedullary phase (CMP) which is acquired 30 seconds after contrast injection, the nephrographic phase (NP) which is acquired 90 seconds after contrast injection, and the excretory phase (EP) which is acquired 180 seconds after contrast injection.
Traditional Radiological Characteristics Analysis
Two radiologists, Reader 1 and Reader 2, carefully reviewed the CT images. If there was a disagreement, the two radiologists would discuss together to reach a consensus. Without access to clinicopathologic information, the radiologists jointly evaluated the CT findings, including the maximum diameter of the tumor on axial CT images, shape, location, boundary, calcification, necrosis, renal vein invasion and lymph node metastasis.
To determine the CT value of the tumor, a region of interest (ROI) was selected within the parenchyma of the tumor, excluding necrosis, calcification and vascularity. The ROIs in the study were chosen based on NP images as the tumor was clearly contrasted with the renal parenchyma in these images. Reader 1 selected three non-overlapping ROIs, took individual CT measurements for each, and then averaged the results. As CT scans are performed by different operators and patients, systematic inaccuracies in tumor CT value measurement may occur. To mitigate errors, the CT values of the cortex were measured in the cortical region of the kidney on the side of the tumor. Figure 2 shows an example of this method.
The average tumor attenuation value (TAV) for UP, CMP and NP was obtained from the ROI. The CT value measured in the renal cortex at each phase is known as the cortical attenuation value (CAV). The tumor enhancement value (TEV) and the cortex enhancement value (CEV) were calculated by subtracting the CT value of the UP: TEVx = TAVx – TAV0 and CEVx= CAVx – CAV0, where x indicates the phase (0, UP; 1, CMP; 2, NP). The ratio of TEV to CEV was used to define the relative enhancement value (REV): REVx = TEVx/CEVx, representing the degree of enhancement within the tumor relative to the renal cortex[25].
Construction Of The Clinical Model
The differences between clinic-radiological characteristics of high-grade and low-grade small ccRCC were analyzed using univariate analysis. For categorical variables, the Chi-square test or Fisher exact test was used, while for continuous variables, the t-test or Mann-Whitney U test was applied. Statistically significant clinic-radiological characteristics were then used in a multivariate logistic regression analysis to identify the most valuable clinical factors and build a model. The odds ratio (OR) was calculated for each independent factor as a measure of relative risk prediction with a 95% confidence interval (CI).
Tumor Segmentation And Extraction Of Radiomics Features
Figure 3 illustrates the key steps in a radiomics model for renal tumors. The tumor’s volumes of interests (VOIs) were manually defined in ITK-SNAP software (version 3.8, www.itksnap.org) by two radiologists with extensive abdominal diagnostic experience (Fig. 4).
The extraction of features was performed using the Artificial Intelligence Kit software (A.K. Software, version 3.3.0.R). To minimize variability in the radiomics features, prior to extraction, the following image preprocessing techniques were applied: gray-level discretization, intensity normalization and voxel resampling. Subsequently, 1595 features were extracted from the UP, CMP and NP CT images by the open-source PyRadiomics library, respectively.
The intraclass correlation coefficient (ICC) was calculated to evaluate the consistency and reproducibility of the features. Features with ICC greater than 0.75 in both intra- and inter-observer agreement analyses were included in further analysis. Reader 1 and Reader 2 randomly segmented CT images of 20 patients (8 high-grade small ccRCC and 12 low-grade small ccRCC). Two weeks later, Reader 1 segmented these 20 patients once again.
Construction Of The Radiomics Model
To reduce redundant features and prevent overfitting of the developed radiomics model. The following steps were taken: 1) Features with an ICC greater than 0.75 were selected, 2) Univariate logistic analysis was performed to identify the statistically significant features, 3) The most significant features were chosen through a Gradient Boosting Decision Tree (GBDT) and multivariate logistic analysis, and 4) The remaining features were utilized to compute the radiomics score (Rad-score). The radiomics model was finally established through multivariate logistic regression.
Construction Of Radiomics Nomogram And Evaluation Model Performance
Clinical variables and Rad-score were combined to create a nomogram. Calibration curves were utilized to evaluate the calibration of the nomogram. The Hosmer-Lemeshow test was applied to assess the nomogram’s goodness of fit. The receiver operator characteristic (ROC) curves were utilized to evaluate the discrimination ability of the prediction model for high/low small ccRCC. The clinical validity of the clinical radiomics nomogram was further evaluated through decision curve analysis (DCA).
Statistical analysis
Python software (v.3.6.0) and R software (v.3.5.1) were used to perform the statistical analysis. A statistically significant difference between the two was defined as p < 0.05.