How can I connect two different contours?

Attached below you can see the image I am working on, a CEUS image. So far, I managed to detect the contour of the tumor using morphological operations, because I also need to emphasize the contrast. I need to detect all the tumor and it consists on both the contrast I have already detected and also the area enclosed by it. I do not know how to close this contour I already have and combine the small contours into one big contour that is basically the tumor. Any idea will he appreciated. Thank you in advance!

[José-Luis]

I would look either into convex hulls or alpha shapes.