Primary Myelofibrosis/drug therapy; Thalidomide/analogs & derivatives; Administration, Oral; Adult; Aged; Anemia/blood; Anemia/complications; Anemia/drug therapy; Anemia/genetics; Anemia/pathology; Anemia, Myelophthisic/blood; Anemia, Myelophthisic/complications; Anemia, Myelophthisic/drug therapy; Anemia, Myelophthisic/genetics; Anemia, Myelophthisic/pathology; Female; Hemoglobins/analysis; Humans; Janus Kinase 2; Lenalidomide; Male; Neovascularization, Pathologic/blood; Neovascularization, Pathologic/complications; Neovascularization, Pathologic/drug therapy; Neovascularization, Pathologic/genetics; Neovascularization, Pathologic/pathology; Neutropenia/blood; Neutropenia/chemically induced; Neutropenia/genetics; Neutropenia/pathology; Platelet Count; Point Mutation; Primary Myelofibrosis/blood; Primary Myelofibrosis/complications; Primary Myelofibrosis/genetics; Primary Myelofibrosis/pathology; Protein-Tyrosine Kinases/genetics; Proto-Oncogene Proteins/genetics; Remission Induction; Sequence Deletion; Splenomegaly/blood; Splenomegaly/complications; Splenomegaly/drug therapy; Splenomegaly/genetics; Splenomegaly/pathology; Thalidomide/administration & dosage; Thalidomide/adverse effects; Thrombocytopenia/blood; Thrombocytopenia/chemically induced; Thrombocytopenia/drug therapy; Thrombocytopenia/genetics; Thrombocytopenia/pathology