Patients
This retrospective study adhered to the tenets of the Declaration of Helsinki and was approved by the local Ethics Committee of our hospital. The requirement for written informed consent was waived. Between January 2008 and January 2018, 124 consecutive patients with pathologically proven RB after enucleation were included in this study. The patient enrollment process for this study is shown in Fig. 1. The inclusion criteria were as follows: (a) all patients had an MRI scan of the orbit with pre- and postcontrast images, and (b) all MRI scans were performed within 4 months of enucleation. The exclusion criteria included (a) poor quality of MR images due to motion artifacts and (b) the RB patients who were negative in PLONI and had received short-term treatment before enucleation. Among the 124 patients, nine patients had bilateral RB tumors, but only one tumor per patient was used in the data analysis (that with the shorter interval time between the MRI scan and enucleation). The pathological assessment of PLONI in the 124 RB patients showed that PLONI was confirmed in 54 globes and absent in 70 globes. Among the 54 RB patients with PLONI, 20 patients had received 1-3 cycles of CEV (carboplatin, etoposide, vindesine) intravenous chemotherapy (IVC). The median time between MR imaging and enucleation for the 20 patients was 54 days (range, 7–119 days). The median time between MR imaging and enucleation for the other 104 patients without any preoperative treatment was 16 days (range, 1–52 days).
The consecutive study population was divided into two groups according to the time point. The training set (recruited from January 2008 to June 2015) consisted of 90 patients (37 with PLONI and 53 without PLONI). The validation set (recruited from July 2015 to January 2018) consisted of 34 consecutive patients (17 with PLONI and 17 without PLONI).
MR image acquisition
All MR images were obtained with a 1.5-Tesla (Signa Highspeed, GE Healthcare, Milwaukee, USA, n=49) or a 3-Tesla (GE HDxt, GE Healthcare, Milwaukee, USA, n=46 or Discovery MR750; GE Healthcare, Milwaukee, WI, USA, n = 29) scanner. Precontrast axial T1-weighted images (T1-WI), T2-weighted images (T2-WI) and postcontrast enhanced T1-WI (CET1-WI) in the axial and coronal planes were acquired for all 124 patients. The imaging parameters are shown in Table 1. CET1-WI was obtained after an intravenous bolus injection of 0.1 mmol/kg gadopentetate dimeglumine. Fat suppression (FS) was used in the axial CET1-WI.
Table 1
MR scanning parameters
Sequence
|
TR (ms)
|
TE (ms)
|
Field of view(mm)
|
Number of slices
|
Slice thickness (mm)
|
Slice gap (mm)
|
NEX
|
Matrix
|
T1-WI
|
400-500
|
8-11
|
160× 160
|
16
|
3.0
|
0.3
|
2
|
256*384(3.0T)
244*288(1.5T)
|
T2-WI
|
2860-4440
|
115-120
|
160× 160
|
16
|
3.0
|
0.3
|
1
|
256*384(3.0T)
244*288(1.5T)
|
CET1-WI
|
400-500
|
8-11
|
160× 160
|
16
|
3.0
|
0.3
|
2
|
256*384(3.0T)
244*288(1.5T)
|
TR: repetition time; TE: echo time; NEX: number of excitations |
Region-of-interest segmentation and radiomics feature extraction
Manual segmentation for each of the 124 RB tumors was performed by two radiologists (Radiologist 1 and Radiologist 2, with 3 years and 15 years of experience, respectively, in reading head and neck images). The segmented region of interest (ROI) covered the whole tumor and was delineated by the radiologists on both the axial T2-WI and CET1-WI on each slice. For each MR sequence, 1029 radiomics features were extracted on the Radcloud platform, which is a useful tool for extracting radiomics features with a large panel of engineered hard-coded feature algorithms (Huiying Medical Technology Co., Ltd. http://mics.radcloud.cn/)(24, 25). The 1029 obtained features can be divided into four main categories: first order, shape feature, texture feature, and higher-order statistical features. First-order statistics such as the mean, standard deviation, variance, maximum, median, and range describe the intensity information in the MRI region of interest. Shape features such as volume, surface area, compactness, and maximum diameter reflect the shape and size of the region. Texture features can quantify the regional differences in heterogeneity. The following higher-order statistical features included the first-order statistics and texture features derived from wavelet transformation of the original images: exponential, square, square root, logarithm and wavelet (wavelet-LHL, wavelet-LHH, wavelet-HLL, wavelet-LLH, wavelet-HLH, wavelet-HHH, wavelet-HHL, and wavelet-LLL). These features complied with the definitions produced by the Imaging Biomarker Standardization Initiative. We used the Radcloud platform to manage the imaging data and perform subsequent radiomics statistical analyses.
Different medical imaging factors cause inconsistencies in the image intensity information of tissues of the same nature. We used the following formula for intensity normalization (where x indicates the original intensity; f(x) indicates the normalized intensity; μ refers to the mean value; σ indicates the variance; s is an optional scaling, and by default, it is set to 1).

Inter- and intraobserver reproducibility evaluation
Interobserver and intraobserver reproducibility of ROI detection and radiomics feature extraction was initially determined by two radiologists using the T2-WI and CET1-WI data of 20 patients chosen by computer-generated random numbers. To assess intraobserver reproducibility, Radiologist 1 repeated the generation of radiomics features twice within a 1-month period following the same procedure.
Feature selection and model construction
Feature selection and model construction were only performed on the training set, and the validation set was only used to evaluate the model performance. We applied the Kruskal-Wallis (K-W) test to compare the distributions of feature values across three different MR scanners, and features with P<0.05 were removed to reduce redundant features and avoid collinearity and overfitting. Then, the least absolute shrinkage and selection operator (LASSO) logistic regression algorithm was used to reduce the dimensionality of extracted features on the training set. The initial alpha was set to 0.1, and the tuning parameter λ was set to zero as the default. Finally, the recursive feature elimination (RFE) with the SVM (kernel: linear) estimator was performed as a multivariate analysis method to select the PLONI-related features. RFE constantly eliminates unimportant features to obtain the optimal feature set by calculating the importance of features . Logistic regression (LR) classifier analysis was conducted to develop a model for the prediction of PLONI in the training set. The performance of the radiomics model was then internally tested in an independent validation set with the formula derived from the training set. The radiomics workflow is shown in Fig. 2.
Radiologist assessment
In this study, abnormal enhancement of the optic nerve in continuity with the tumor on postcontrast MR images regardless of the asymmetrical thickening of the nerve was defined as MRI-based PLONI in RB patients. The same two radiologists who were blinded to the pathologic findings classified the 124 patients into PLONI-positive or PLONI-negative groups based on their imaging interpretations. The performance of radiologist assessment was compared with that of the radiomics model with regard to the prediction of PLONI in RB patients.
Statistical analysis
Univariate and bivariate analyses were performed with SPSS version 20 (SPSS Inc.). Statistical analyses were performed for both the training and validation sets. Student’s t-test and the chi-square test were used to assess differences in age and sex distributions between the training and validation sets. Dice's coefficients were used to evaluate the intra- and interobserver consistency for the ROI segmentation and radiomics feature extraction with 20 randomly selected samples. We interpreted a coefficient of 0.81 to 1.00 as almost perfect agreement, 0.61 to 0.80 as substantial agreement, 0.41 to 0.60 as moderate agreement, 0.21 to 0.40 as fair agreement, and 0 to 0.20 as poor or no agreement. Receiver operating characteristic (ROC) analysis and the area under the curve (AUC) were used to assess the predictive performance of the radiomics model and visual assessment for PLONI. The cutoff values of the ROC curves were determined by the principle of maximizing the Youden index. The AUC of the radiomics model was compared with that of visual assessment by the DeLong test. P<0.05 was considered to indicate a statistically significant difference.