I am sorry to hear that Nola has been diagnosed with splenic cancer. Was this via an ultrasound guided aspirate or biopsy of the mass?
If they are only giving her 6 months to live then this is likely a malignant tumor.
Most malignant splenic tumors have spread microscopically by the time we diagnose them. That's because the spleen is filled with blood vessels that allow tumor cells to exfoliate and spread easily to other areas in the body. We may not find the metastases initially because they are only at the cellular level, but they are likely there.
If the mass truly is only on her spleen and it isn't believed to be malignant then surgery to remove her spleen is an option. The spleen is not an essential organ. The functions that it performs can be done by other organs in the body. It is a filter organ, removing damaged red blood cells, and it produces lymphocytes (a type of white blood cell) and stores red blood cells. These functions can also be performed by lymph nodes, the liver and the bone marrow. Don't worry about it not being there creating stress on other organs.
I think that an ultrasound of her abdomen is a terrific idea if that hasn't been done. The most common malignant (spreads to other tissue and is very invasive) mass in the spleen is a tumor called a hemangiosarcoma. This tumor is a tumor of the lining of blood vessels and is easily transferred to other organs via the blood vessels carrying tumor cells. That means by the time we find it, the tumor has most certainly already spread to other tissues, usually the liver, lymph nodes, small blood vessels in the abdomen and lungs. If that is the type of tumor she has then no matter what is done her prognosis is very poor and I wouldn't recommend surgery as her lifespan will be very short, usually less than 2 to 3 months, rarely as long as 6 months. There is no use putting her through major surgery knowing that we cannot change her comfortable life span.
There are other tumors though, such as hemangiomas, mast cell tumors and lymphosarcoma. Knowing what type of tumor is present will help you decide how to proceed.
If a splenic tumor starts to bleed one of two things can happen. It can clot after a small amount of bleeding and while the dog may feel a little woozy from blood loss, depending upon how much blood is lost they may be OK. If the tumor starts to bleed and cannot stop the dog will become hypotensive (low blood pressure) and faint or have such low oxygenation to the brain they will become unaware and pass away. This is not painful for the dog but it is very difficult for owners to have to experience as the dog may vocalize or thrash a bit as this happens. The pet is unaware of what is happening because of lack of oxygen to the brain but it is upsetting for an owner to observe.
Here is a link to an article that accurately discusses spleen function and splenic masses: http://www.marvistavet.com/html/body_splenic_masses.html
I would make sure that the person that does the ultrasound is very experienced, ideally a board certified internist or oncologist, so that you can get the most accurate information from this procedure. Then make a decision based upon that information.
If this tumor was characterized as a hemangiosarcoma on biopsy then another option is a homeopathic treatment with a product called I'm-Yunity which is derived from a particular mushroom that has had some very promising results.
Here are some links to read about a study done at Penn as well as the company's website to buy the product.
Best of luck with your pup, please let me know if you have any further questions.