Is it nessesary to use OTSU's thresholding after Image segmentation by region growing

I have chest radiograph and I segmented image using "single seed Region Growing Algorithm". The seed point is the center of the image and segmented image is attached herewith.The white is segemnted object. The image is properly segmented.When I counted number of blobs using "regionprops" function it returns only 1 blob. If I complement the image and using "regionprops" it returns @ 140 blobs. If 1) I add original image with region growing segmented Image 2) segment it again using Otsus thresholding method I get properly segmented lungs.