Patients
This retrospective study was approved by the institutional review board at Affiliated Cancer Hospital & Institute of Guangzhou Medical University, and the requirements of patients’ informed consent were waived. Between January 2008 to December 2015, clinical, pathologic, and radiological data of 6451 consecutive NPC patients after RT were reviewed. A total of 123 patients that showed emerging cervical spine lesions on follow-up MRI were selected for further analysis. The inclusion criteria were as follows: (a) underwent pre-treatment MRI and showed no abnormal signal changes in the cervical spine; (b) after lesion detection, histopathology assessment or MRI follow-up at least 6 months were performed for confirming the nature of the lesions; (c) no distant metastasis apart from cervical spine that necessitated systemic chemotherapy, because the chemotherapy may alter imaging features of the cervical spine lesion; (d) no history of cervical spine trauma during follow-up. According to the inclusion criteria, 52 patients were excluded due to the following reasons:(1) Pre-treatment MRI was unavailable (n = 8); (2) showed abnormal signal changes in the cervical spine at pre-treatment MRI (n = 12); (3) insufficient MRI was performed to confirm the diagnosis of the cervical spine lesions (n = 25); (4) receive systemic chemotherapy due to distant metastasis (n = 5); (5) underwent cervical spine trauma (n = 2). Patients’ inclusion flowchart was displayed in Fig. 1.
Consequently, 71 patients were enrolled in this study, 46 NPC patients (ORN, n = 30; metastasis, n = 16) gathered from January 2008 to December 2012 were assigned to the training set, and 25 NPC patients (ORN, n = 14; metastasis, n = 11) gathered from January 2013 to December 2015 were assigned to the validation set.
MR image acquisition, segmentation and radiomics feature extraction
MR images were acquired using a 1.5-T system unit (Intera Achieva; Philips). The MRI protocol included an axial turbo spin echo (TSE) T1-weighted, an axial TSE T2-weighted, a coronal short time inversion recovery (STIR) sequence, and an axial and a sagittal contrast-enhanced TSE T1-weighted sequence. Contrast-enhanced T1WI was performed after intravenous administration of 0.1 mmol/kg gadopentetate dimeglumine (Magnevist, Bayer Schering). Details of the MRI acquisition were showed in Supplemental Materials (Table S1).
All lesions showed contrast-enhancement, thus axial enhanced T1-weighted images were retrieved from PACS in the “.dicom” format for image feature extraction. Segmentation for regions of interest (ROIs) was performed using a software package MaZda 4.6 (URL: http://www.eletel.p.lodz.pl/programy/mazda/). Before ROIs placement, the gray-level of image was normalized by adjusting image intensities in the range ofu±3σ (u, gray-level mean; σ, gray-level standard deviation) [26, 27]. All lesions ROIs were manually delineated in the largest cross-sectional area of lesion (Fig. 2a). In total, 279 radiomics features derived from six statistical image descriptors (Histogram, Grey-level co-occurrence matrix, Run-length matrix, Absolute gradient, Autoregressive model and Wavelet) were extracted (Fig. 2b). Details of radiomics feature information are in the Supplementary Data (Table S2).
The inter-observer reproducibility of radiomics feature extraction was estimated using interclass correlation coefficients (ICC). The ROI segmentation was performed independently by two radiologists experienced in skeleton MRI interpretation (J.X.Y. and B.G.L. with 10 years of experience). An ICC value > 0.75 indicates good agreement of the feature extraction [14, 28].
Feature selection and radiomics nomogram development
A radiomics nomogram was constructed in the training set. To identify the most discriminating radiomics feature for differentiating cervical spine ORN from metastasis, feature selection was performed before radiomics signature development. ICC was calculated for the 279 radiomics features, and those features that showed ICC value greater than 0.75 were selected for subsequent procedure. Then the remaining features were reduced by using a combination feature selection algorithm (combination of fisher coefficient [FC], classification error probability combined with average correlation coefficients (CEP+ACC) and mutual information [MI]; FCM) that comprised 30 radiomics features with the most discriminative ability [27, 29].
The least absolute shrinkage and selection operator (LASSO) logistic regression algorithm using10-fold cross-validation based on minimum criteria was adopted for final feature selection for radiomics nomogram development [15, 28]. A formula was created using a linear combination of the selected features that were weighted by their respective LASSO coefficients; then a radiomics nomogram was constructed based on the radiomics score calculated by formula that reflected the possibility of ORN. The procedure of feature selection and radiomics nomogram development was showed in Fig. 2c. The calibration of the nomogram was assessed using a calibration curve, and the Hosmer–Lemeshow test was performed to assess the goodness-of-fit of the nomogram [30]. The diagnostic efficiency of the nomogram for discrimination of ORN and metastasis was assessed by ROC analysis in the training set, and the diagnostic sensitivity and specificity was also calculated.
Validation of the radiomics nomogram
Validation of the radiomics nomogram was accomplished with the validation set. A radiomics score was calculated for each lesion in the validation set using the formula constructed in the training set. The diagnostic efficiency and calibration of the nomogram model were also assessed in the validation set.
Clinical utility of the radiomics nomogram
To assess the clinical use of the nomogram, we used the decision curve analysis (DCA) to calculate the net benefits for a range of threshold probabilities in the combined training and validation set. The net benefit is identified as the proportion of true positives minus the proportion of false positives, weighted by the relative harm of false-positive and false-negative results [31].
Reference Standard
The pathological assessment was performed for only one ORN patient. The reference standard without pathological assessment was based on the MRI and clinical follow-up for confirming the diagnosis of the lesions [9, 10]. Lesions with progressive enlargement that presented as soft-tissue masses were identified as bone metastasis. Lesions that shrank or remained stable on MRI for more than 6 months without further treatment were interpreted as ORN. If a lesion’s diagnosis could not be identified based on the follow-up procedure, it would be eliminated.
Statistical Analysis
LASSO logistic regression was performed by using R statistical software (version 3.3.1, http://www.rproject.org/), the "glmnet" package was adopted. Nomogram construction and calibration plots were performed using the "rms" package, and the Hosmer–Lemeshow test was conducted using the "generalhoslem" package. DCA was performed using the "dca.R." Other statistical analysis was performed using the SPSS 16.0 (SPSS Inc., Chicago, IL, USA), the overall performance was determined by assessing the area under the receiver operating characteristic (ROC) curve (AUC). Mann–Whitney U test and Pearson chi-square test (or Fisher test) were used for continuous and categorical variables, respectively. Statistical tests; P < 0.05 indicated statistical significance.