Hello, I am very sorry to hear that Molly has a mass on her spleen. I understand how upsetting that news was to hear and how much you have to think about in your decision about whether to put her through surgery or not and whether to let her go on the table should metastases be seen.
In many cases of splenic masses the pup is apparently normal. We find the mass on a routine physical exam and owners are shocked.
That's because 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, so her body is likely doing just fine even with a big tumor. The good news then is if this is a benign growth we don't worry about the spleen not being there creating stress on other organs, they can do very well.
Unfortunately we often cannot see metastases on radiographs because they are often very small, often at the microscopic level. I would still look for metastases on radiographs though.
I also think that an ultrasound of her abdomen is a terrific idea.
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.
It is possible that the surgeon may see more tumors once they open her up surgically, but if we can find them on an ultrasound that would save her going through surgery. If metastases are present though I think a plan to let her go on the table via humane euthanasia is kind.
I do want you to be aware of a few things though in the meantime. 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 fucntion and splenic masses: http://www.marvistavet.com/html/body_splenic_masses.html
If you choose an ultrasound 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.
Please let me know if you have any further questions.