Exceptions to every rule

Speaking with my mother about cancer the other day, I told her that cancers rarely metastasize to the spleen.  In fact I have never seen it nor had I even heard of it.  Not more than 2 days later was an interesting case presented at one of our conferences.  A CT scan showed a large tumor growing out of the left side of the pancreas (known as the "tail"), and into the spleen.  There also appeared to be what were isolated nodules in the spleen which might have been metastases.

The surgeons suspected that the cancer was a pancreatic adenocarcinoma, so they removed the left half of the patient's pancreas and spleen (which is a pretty standard operation, a "distal pancreatectomy and splenectomy").  In the operating room however the surgeons noticed that the mass was softer, and did not feel fibrotic like how they might expect a pancreatic adenocarcinoma to feel.

Actually, it turns out the tumor was not a pancreatic adenocarcinoma; it was a diffuse large B cell lymphoma.  In retrospect, that might have been higher on the differential diagnosis.  It is indeed very rare for a GI-derived adenocarcinoma to metastasize to the spleen.  Adenocarcinomas are cancers that are derived from cells that line the surfaces of the body, and the GI tract is actually a surface if you think about it - it is exposed to the outside elements.  After all, the GI tract is just a long tube running through the body; the lumen of the tube isn't actually IN the body.

 Alternatively, lymphomas are cancers that are derived from lymphoid-lineage hematopoietic cells, which is a scientific way of saying white blood cells.  The spleen is a major lymphoid organ in the body, and lots of white blood cells end up there to be "trained" to respond to infectious pathogens.  Think of the spleen as a gymnasium for your immune system.  In retrospect, it isn't so surprising that a large tumor invading and "metastasizing" to the spleen actually turned out to be a lymphoma, instead of a carcinoma.  So what I told my mother is only true with a qualifier:  it is extremely rare for [a cancer] an adenocarcinoma of GI origin to metastasize to the spleen.

You might be wondering, if the spleen is so important for your immune system, how can we take it out without pause?  Patients with splenectomy by and large retain immune function, but they are actually more vulnerable to specific types of bacterial infection.  Especially bacteria which have capsules can pose a challenge to a patient without a spleen, as encapsulated organisms require a very specific and focused immune response to defeat, the sort of which requires splenic function.  To compensate, we can vaccinate patients who are going to get a splenectomy for the most common encapsulated infectious organisms:  streptococcus pneumoniae, neisseria meningitidis, and haemophilus influenzae.  Patients without a spleen do not have any problem leading a normal life.