Recently a member of the gimpydog mail list had a question regarding cancer of the spleen and what were the prognosis and treatments.
The following links will provide you with reputable information about masses in the spleen and other spleen issues to help you learn more about the condition and treatment.
Splenic Masses in dogs
Tumors of the Spleen in Dogs
Splenomegaly and Splenic Torsion: Tucker's Story
Video of Splenic Tumor Ultrasound Diagnosis from Tripawds
Visceral Hemangiosarcoma in dogs (specifically spleen)
ACVS resource for Splenic Masses
Canine Cancer Awareness Org Vicsceral Hemangiosarcoma page
Canine Cancer Awareness Org support groups
Support groups are a fantastic way to not only learn about diseases that may affect your dog, but will also provide you with first hand experience with what you are also going through, and sympathetic members who can help you through the process.