Can I sue the hospital for ignoring a mass on my spleen?

I am so sorry for your situation and I hope that you are on the road to recovery.  Medical malpractice is a very tricky area of the law.  In many states it requires that there be submitted an affidavit of sorts from a doctor that there was a deviation from the standard of medical care in order to even bring a suit.  It does indeed sound like you have facts that give rise to a deviation from the standard of care necessary when they noted the mass on your spleen in the first hospital but did not address it.  I would consult with an attorney in your area on the matter.  If yo have the medical records from both hospitals that is helpful but the attorney will request them to review and see if you have a case.  They also have to make sure that the acts of the doctor's caused you damage in some way in order to being suit.  Good luck.