Computed Tomography-Based Radiomics Signature for the Preoperative Differentiation of Pancreatic Adenosquamous Carcinoma From Pancreatic Ductal Adenocarcinoma

Purpose The purpose was to assess the predictive ability of computed tomography (CT)-based radiomics signature in differential diagnosis between pancreatic adenosquamous carcinoma (PASC) and pancreatic ductal adenocarcinoma (PDAC). Materials and Methods Eighty-one patients (63.6 ± 8.8 years old) with PDAC and 31 patients (64.7 ± 11.1 years old) with PASC who underwent preoperative CE-CT were included. A total of 792 radiomics features were extracted from the late arterial phase (n = 396) and portal venous phase (n = 396) for each case. Significantly different features were selected using Mann–Whitney U test, univariate logistic regression analysis, and minimum redundancy and maximum relevance method. A radiomics signature was constructed using random forest method, the robustness and the reliability of which was validated using 10-times leave group out cross-validation (LGOCV) method. Results Seven radiomics features from late arterial phase images and three from portal venous phase images were finally selected. The radiomics signature performed well in differential diagnosis between PASC and PDAC, with 94.5% accuracy, 98.3% sensitivity, 90.1% specificity, 91.9% positive predictive value (PPV), and 97.8% negative predictive value (NPV). Moreover, the radiomics signature was proved to be robust and reliable using the LGOCV method, with 76.4% accuracy, 91.1% sensitivity, 70.8% specificity, 56.7% PPV, and 96.2% NPV. Conclusion CT-based radiomics signature may serve as a promising non-invasive method in differential diagnosis between PASC and PDAC.

PASC patients demonstrate a slight male preponderance, with neoplasms frequently located in the head of the pancreas, which is commonly seen in PDAC patients (2). The clinical symptoms and manifestations of PASC, such as abdominal pain, body weight loss, and jaundice, are also similar to those of PDAC (3,5), making it difficult to differentiate between the two entities. However, PASC is considered to be more aggressive than PDAC, which has a higher frequency to simultaneous metastasis to the lymph nodes and the liver (2,6). Consequently, patients with resected PASC have a poorer prognosis than those with PDAC (median survival: 12 vs. 16 months) (2). Surgical resection with R0 margin offers the only potential chance of a cure in patients with PDAC, which is also recommended for patients with PASC (3,7). However, patients with PASC can also benefit from radiation therapy as squamous cancer tissue is sensitive to radiation therapy. Some conventional CT or MRI imaging features, including a round and lobulated shape, cystic changes, tumor thrombus in the portal vein system, and ring-enhancement pattern, were useful in differential diagnosis between PASC and PDAC (4,6), but the enrolled number of patients was small (4) and discriminative sensitivity was relatively low (6). Ultrasound-guided fine-needle aspiration (US-FNA), used to sample the mass, is a sensitive and safe method in the diagnosis of solid pancreatic tumors (8). However, there has been much debate about the fact that sometimes a biopsy sample could not reflect the entire extent of the phenotype of the whole tumor due to sampling errors (9). Even EUS-FNA biopsies had 12-14% false negative rates, which can cause delayed patient care (10). Therefore, accurate preoperative discrimination between PASC and PDAS using non-invasive imaging is very important for choosing the optimal treatment and for prognosis prediction.
CT has become an important non-invasive method for the diagnosis and the evaluation of different diseases, owing to its capability to reflect the biological and the physiological characteristics of different organs. CT texture analysis (CTTA) is an emerging field of investigation which is capable of assessing tissue gray-level intensity within an image and allows accurate characterization of tumors by quantification of the intra-tumoral heterogeneity (9,11). An investigation of CTTA has been carried out in different tumors, and the original results are encouraging (12)(13)(14).  However, no studies have been conducted with CT-based radiomics signature to preoperatively differentiate PASC from PDAC. This study assessed the predictive ability of CT-based radiomics signature in differential diagnosis between PASC and PDAC.

Patient Selection
This retrospective study was approved by the Medical Ethical Committee with a waiver of patients' approvals, which identified a total of 35 patients with pathologically confirmed PASC through surgical resection from three institutions between January 2010 and January 2019. This study also identified a total of 106 patients with pathologically confirmed PDAC through surgical resection from three institutions between January 2017 and January 2019. The inclusion criteria were as follows: (1) the patients underwent dual-phase contrast-enhanced CT (late arterial and portal venous phases) within 30 days prior to surgery, (2) the patients had a definite pathological diagnosis of PASC or PDAC, (3) the patients had optimal CT images for further radiomics analysis, and (4) the patients did not receive pre-surgical treatment such as chemoradiotherapy. Patients with PASC (n = 31) and PDAC (n = 81) were finally selected. The exclusion criteria and the acquisition of the study cohort are demonstrated in Figure 1.

Image Acquisition
In 16 PASC patients, CE-CT was performed in our institution.

Technical Workflow
A general technical workflow, including tumor segmentation, radiomics feature extraction and selection, and radiomics signature construction and validation, is displayed in Figure 2.

Tumor Segmentation
The most current NCCN guidelines (version 2.2019; April 9, 2019) recommend dual-phase pancreatic CT protocol (pancreatic phase/late arterial phase: 40-50 s; portal venous phase: 65-70 s) as necessary for the optimal evaluation of primary pancreatic  tumors (7). The tumors were manually segmented from the late arterial and portal venous CT images using ITK-SNAP software (version 3.6.0) 1 . The segmentation of the tumor was based on the total volume of the tumor and done by a board-certified abdominal radiologist. The radiologist had no prior knowledge of the patients' pathological diagnosis and had used ITK-SNAP for a prior study of 109 patients (13,15). Moreover, during the region-of-interest delineation, the blood vessels and the lymph nodes around the tumor were avoided.

Extraction and Selection of Radiomics Features
Two pre-processing steps were applied to CT images before feature extraction. The first one was resampling the image into 1-mm × 1-mm × 1-mm spacing to eliminate the difference of revolution and slice thickness. Second, gray level discretization was used to merge the neighboring gray levels into one level so as to eliminate the random fluctuation of the gray value, with a final 256 bins. Then, a total of 792 radiomics features were extracted from the late arterial phase (n = 396) and portal venous phase (n = 396) for each case using Analysis Kit software (version V3.0.0.R, GE Healthcare). In order to reduce the redundancy of the radiomics features, Mann-Whitney U test was firstly applied to explore features that are significantly different between the two groups; then, univariate logistic regression method was applied to explore the discriminative features between the two groups. Finally, minimum redundancy maximum relevance (MRMR) method 1 http://www.itksnap.org/ (13,15,16), allowing the retention of the 10 most predictive radiomics features with minimum redundancy maximum relevance, was adopted.

Construction and Validation of Radiomics Signature
Ten radiomics features, including seven features from late arterial phase images and three from portal venous phase images, were combined to construct a radiomics signature using random forest (RF) method (17), which contains a specific combination of multiple classification and regression trees comprising of independent diagnostic algorithms. The discriminative ability of the radiomics signature was recorded.
To explore the robustness and the reliability of the radiomics signature, we performed 10-times leave group out crossvalidation (LGOCV) analysis (18), where the patients were randomly divided into training and testing sets with a ratio of 7:3 for 10 times. During each time, the training group was used to train a new model, and the testing set was used as an independent set with which to evaluate the model. The average performance of the 10 newly built models can be used to prove the stability and the reliability of the radiomics signature.

Statistical Analysis
Statistical analysis was performed using SPSS version 24.0 (IBM SPSS Inc., Chicago, IL, United States) and R software, version 3.6.1 2 . A p value below 0.05 was considered to indicate statistical significance.

Patient and Tumor Characteristics
Thirty-one PASC and 81 PDAC patients were analyzed in this study. The patient and tumor characteristics are listed in Table 1. No significant difference was found in terms of age, sex, tumor size, tumor location, or clinical symptoms between both groups ( Table 1).

Selection of Radiomics Features
A total of 396 radiomics features primarily extracted from each phase CT image could be divided into six categories  ( Figure 3). Finally, a total of 792 radiomics features derived from dual-phase CT images were obtained and analyzed. All radiomics features were compared using Mann-Whitney U test between both groups, the p values of which were displayed by using Manthattan (Figure 4). As a result, a total of 295 radiomics features with a p value <0.05 were selected. Then, univariate logistic regression analysis was performed to explore the discriminative features between both groups, and 159 features were found. Finally, 10 radiomics features at the late arterial phase (named A_) and portal venous phase (named V_), including A_Compactness2, A_SurfaceVolumeRatio, A_RunLengthNonuniformity_angle135_offset1, A_LongRun LowGreyLevelEmphasis_angle45_offset7, A_Correlation_angle 135_offset7, V_LongRunEmphasis_angle45_offset7, V_Short RunHighGreyLevelEmphasis_angle135_offset4, A_Inverse DifferenceMoment_AllDirection_offset4_SD, V_Compactness2, and A_GLCMEntropy_AllDirection_offset4_SD, were retained using MRMR method. Two ways, including AUC barplot ( Figure 5A) and heat map (Figure 5B), were used to show the remaining 10 radiomics features selected using MRMR method at the late arterial and portal venous phases. The comparisons of the 10 radiomics features between PASC and PDAC groups are summarized in Table 2. Moreover, the classification performance of each CT radiomics feature was evaluated using a receiver operating characteristic (ROC) curve ( Table 3).

Construction and Validation of Radiomics Signature
Subsequently, RF method was used to construct the radiomics signature which displayed differential diagnosis ability between PASC and PDAC with 10 radiomics features. The results revealed that the radiomics signature was able to discriminate between PASC and PDAC. The classification performance of the radiomics signature had 94.5% accuracy, 98.3% sensitivity, 90.1% specificity, 91.9% PPV, and 97.8% NPV, with an AUC of 0.98 (Figure 5C). The importance of the 10 radiomics features is shown in Figure 5D. Then, we performed 10-times LGOCV analysis to explore the robustness and the reliability of the radiomics signature. The radiomics signature was proved to be robust and reliable using LGOCV method, the accuracy, sensitivity, specificity, PPV, and NPV of which were 76.4, 91.1, 70.8, 56.7, and 96.2%, with an average AUC of 0.82. Figure 6 shows the results of 10-times LGOCV analysis in differentiating PASC from PDAC using ROC curves, which proved that the radiomics signature was relatively robust and reliable.

DISCUSSION
PASC is a rare malignant neoplasm of the pancreas, characterized by a mixture of glandular and squamous differentiation, which FIGURE 6 | Receiver operating characteristic (ROC) curves of 10-times LGOCV analysis for differentiating pancreatic adenosquamous carcinoma from pancreatic ductal adenocarcinoma, the mean AUC of which was 0.82. The radiomics signature was proved to be robust and reliable.
is very difficult to be discriminated from PDAC (3). In a population-based analysis, it was reported that the postoperative overall survival in patients with PASC after surgical resection is significantly worse than that of patients with PDAC after surgical resection (2). An accurate preoperative diagnosis is of great importance in the patients' prognosis prediction. There are two major obstacles in accurately differentiating between PASC and PDAC: One is that these two types of tumors share similar clinical symptoms, such as abdominal pain and jaundice (3,5); the other is that very few features with high sensitivity and specificity on imaging studies are found in the preoperative discrimination between both groups (4,6,(19)(20)(21).
Malignant tumors display heterogeneity with variable internal spatial organization due to differences in cellularity and angiogenesis. Tumors with highly aggressive behavior and subsequent poor prognosis have high intratumoral heterogeneity (22,23). Evidence for such histologic characteristics could be embedded into the pixels of CT or MRI images, which could be evaluated by texture analysis, as it provides a potential method for the quantification of tumor spatial heterogeneity (9). The tumor classification of PASC and PDAC depends on the microscopic evaluation of uniformity and heterogeneity. Microscopic heterogeneity could also be reflected by tumors grossly. Interestingly, texture analysis provides a plausible method to non-invasively evaluate macroscopic heterogeneity (15,24). Li et al. (25) have explored the potential application of CT-based texture analysis in discriminating atypical pancreatic neuroendocrine tumors (PNETs) from PDAC. D'Onofrio et al. (12) have shown the potential of 3D CT texture analysis in PNET grading. Similarly, in our previous studies, CT texture analyses have been proven to be a plausible quantitative method to differentiate between pancreatic lesions which share similar conventional imaging findings (13,14).
Thus far, to our best knowledge, no studies have evaluated the potential value of CT-based radiomics signature to preoperatively differentiate PASC from PDAC. Toshima et al. (4) analyzed the CT and MR imaging findings in eight PASC and 33 PDAC patients. They found that PASC demonstrated a higher frequency of a round and lobulated-shaped (100 vs. 57.6%) tumor thrombus in the portal vein system (37.5 vs. 6.1%) and necrotic portions (100 vs. 39.4%) as compared with PDAC, but the number of enrolled PASC patients was relatively small. In a study by Imaoka et al. (6), they concluded that PASC showed a higher frequency of a smooth outline, ring-enhancement pattern, and cystic changes as compared with PDAC, and the most predictive feature was the ring-enhancement pattern, which showed a low sensitivity of 65.2% and a specificity of 89.6%. In our study, seven radiomics features from late arterial phase images and three from portal venous phase images were finally selected. All radiomics features showed an acceptable classification performance for differentiating PASC from PDAC, with a range from 0.660 to 0.755 of AUC. In addition, RF method was used to construct the radiomics signature, which showed 94.5% accuracy, 98.3% sensitivity, 90.1% specificity, 91.9% PPV, and 97.8% NPV, with an AUC of 0.98. Finally, 10-times LGOCV method was used to validate the robustness and the reliability of the radiomics signature. A high pooled sensitivity of 91.1%, specificity of 70.8%, and accuracy of 76.4%, with an average AUC of 0.82, indicated a stable and reliable radiomics signature.
We are aware of some limitations. First, the number of PASC patients was small for radiomics analysis due to its low incidence rate, which only accounts for 1-4% of pancreatic adenocarcinoma (2). Second, an inherent selection bias cannot be avoided due to the retrospective nature of the study. Third, different scanners were adopted. However, similar parameters were used during CT scanning, and pre-processing steps were adopted before feature extraction. Despite the use of different tube currents, a study by Mulé et al. (26) indicated that different tube currents have a little influence on the CT texture parameters. Fourth, conventional imaging features were not investigated in the study as we thought that there is appropriate evidence in the literature concerning this topic. Fifth, an external validation or a multi-center validation of these promising outcomes will be needed in the future. Finally, a core issue concerns the context in which the texture was filed as deciphered of the radiomics features, even though they were somehow validated (27).
In conclusion, our study developed and validated a CT-based radiomics signature in differential diagnosis between PASC and PDAC, which may serve as a promising non-invasive method.

DATA AVAILABILITY STATEMENT
All datasets presented in this study are included in the article/supplementary material.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by the ethics committee of the Eighth Affiliated Hospital, Sun Yat-Sen University. Written informed consent for participation was not required for this study in accordance with the national legislation and the institutional requirements.