I am using CT scans to classify lung cancer into one of two types (Adenocarcinoma vs. Squamous carcinoma; we can abbreviate them A & B). I am applying LASSO penalized logistic regression to a data set containing 756 CT-derived texture features and 12 radiologist identified categorical (yes/no) features. The latter are based on literature for relevance whereas the former are computer generated with no prior proof of being useful. I have 107 cases so my final dataframe (df) is 107 x 768 dimensional:
i)Texture features (mathematical quantities n=756) are continuous variables scaled and centered. Their names are stored in list
ii)Semantic features(Qualitative features subjective assessed by experienced radiologist, n=12). These are usually categorical binary inputs of yes / no type. Their names are stored in list
Following comments from community on my original (very different) model Is my high dimensional data logistic regression workflow correct?, I performed my LR development in three steps:
1)Feature selection: I used principle components analysis to reduce texture feature-space from 756 to 30. I kept 4 most relevant (from literature) semantic features. This gave me 34 final features. I used the following command:
trans = preProcess(df[,texVars], method=c("BoxCox", "center", "scale", "pca"),thresh=.95) # only column-names matching ‘texVars’ are included. neodf2 <- predict(trans,df[,texVars]). neodf.sem <- neodf2[,c("Tumour","AirBronchogram", "Cavity", "GroundglassComponent","Shape")] # this DF is 107 x 4 dimensional, containing only 4 semantic features (most relevant from prior knowledge). neodf.tex <- neodf2[,c("Tumour",setdiff(names(neodf2),names(neodf.sem)))] # this only has the 30 PCA vectors (labelled PC1 – PC30).
2) Model development (LASSO) and penalty term tuning (10fold cross-validation) using cv.glmnet command Deviance was used as determinant of model quality. Using this method, I developed a model incorporating only semantic features, a second model incorporating only texture features, and a third model incorporating both semantic and texture features. Here are the commands:
#Converting to model.matrix for glmnet xall <- model.matrix(Tumour~.,neodf2)[,-1] xtex <- model.matrix(Tumour~.,neodf.tex)[,-1] xsem <- model.matrix(Tumour~.,neodf.sem)[,-1] y <- neodf$Tumour require(glmnet) grid <- 10^seq(10,-2,length=100) lasso.all <- cv.glmnet(xall,y,alpha=1,lambda=grid, family="binomial", type.measure="deviance") lasso.tex <- cv.glmnet(xtex,y,alpha=1,lambda=grid, family="binomial", type.measure="deviance") lasso.sem <- cv.glmnet(xsem,y,alpha=1,lambda=grid, family="binomial", type.measure="deviance")
3) Testing model classification accuracy on entire dataset. The following is the backbone of bootstrap to generate 95% confidence intervals of predictive accuracy:
pred <- predict(lasso.all, newx = xall, s = "lambda.min", "class") tabl <- table(pred,y) sum(diag(prop.table(tabl)))
4) As an alternative means to assess model performance than classification accuracy, I used ROC area under curve on entire dataset and compared AUROC curves from different models using DeLong's method (pROC package)
The results are interesting
LR MODEL BASED ON SEMANTIC FEATURES ALONE: lasso.sem$lambda.min 0.01
Feature Odds Ratio 1 (Intercept) 0.1292604 2 AirBronchogramPresent 0.1145378 3 CavityPresent 35.4350358 4 GroundglassComponentAbsent 4.3657928 5 ShapeOvoid 2.4752881 AUC: .84 =================================================
LR MODEL BASED ON TEXTURE FEATURES ALONE:
Feature OddsRatio 1 (Intercept) 0.6461538 ============================================================
LR MODEL BASED ON TEXTURE + SEMANTIC FEATURES:
Feature OddsRatio 1 (Intercept) 0.3136489 2 PC23 0.9404430 3 PC27 0.8564001 4 AirBronchogramPresent 0.2691959 5 CavityPresent 6.7422427 6 GroundglassComponentAbsent 2.0514275 7 ShapeOvoid 1.5974378 AUC : .88
Having rejected the texture only model, which only contains intercept, i was left with two models - semantic and combined texture+semantic. I created ROC curves for both and compared them using DeLong's method:
pred.sem<- predict(lasso.sem, newx = xsem, s = "lambda.min") pred.all<- predict(lasso.all, newx = xall, s = "lambda.min") roc.sem<- roc(y,as.numeric(pred.sem), ci=TRUE, boot.n=1000, ci.alpha=0.95,stratified=FALSE) roc.all<- roc(y,as.numeric(pred.all), ci=TRUE, boot.n=1000, ci.alpha=0.95,stratified=FALSE)
Outputs of ROC analysis are:
data: roc.sem and roc.all Z = -2.1212, p-value = 0.0339 alternative hypothesis: true difference in AUC is not equal to 0 sample estimates: AUC of roc1 AUC of roc2 0.8369963 0.8809524
Showing that combined model ROC curve is significantly better despite the modest improvement in AUC (83% vs 88%). Questions are:
a) is my methodology airtight from a publication point of view now? Apologies in advance for any gross errors in my presentation of this problem.
b) what is the formal inference that texture model is intercept only.
c) if texture model is useless, how do its variables become useful once added to semantic features and yield a higher overall accuracy in the combined result? perhaps that means the effect of texture features alone is too small to be detected in this small dataset but becomes apparent when combined with a stronger predictor (i.e., semantic features).
Any further comments are welcome.