How can detect the Left Ventricle (LV) Cavity into the MR dicom file?
I'm looking at this DICOM now and it is very hard to tell where I am in the brain (since there is only one slice). Anyhow, tumours tend to be bright irregularities. The thing I think is a tumour certainly isn't in the LV.

You can just draw an ROI over it using something like FSLVIEW if you are planning to do it manually, or you can have some fun and use something that looks for irregularities across hemispheres in the edges as detected by an algorithm.

http://www.medicalimagingandgraphics.com/article/S0895-6111(11)00079-6/abstract

Hope this helps...