convert rectangle to circle after training process

I applied this code to draw a rectangle on the region of interest that i want after taining using faster RCNN .I want to convert the drawn rectangle to be circle in the next step , could you help me please ?

pp=alexnet;
ppl=pp.Layers;
pp=pp.Layers(1:19);
ppp=[pp
    fullyConnectedLayer(2)
    softmaxLayer()
    classificationLayer()]
options = trainingOptions('sgdm',...
    'InitialLearnRate',1e-3,...
    'MaxEpochs',10,...
    'CheckpointPath',tempdir);
 train1 = trainFasterRCNNObjectDetector(gTruth,ppp,options, ...
        'NegativeOverlapRange',[0 0.1], ...
        'PositiveOverlapRange',[0.5 1], ...
        'SmallestImageDimension',300);
    
 a = imread('US0018_0131.png');
 a = imresize(a,[227 227]);
[bbox,score,label] = detect(train1,a);
detect= insertShape(a,'rectangle',bbox);
figure
imshow(detect)