BackgroundDual hematological malignancies, asynchronous or synchronous, are underrecognized entities and are usually suspected when clinical, hematological, or biochemical features cannot be explained by the primary malignancy alone. We present a case of synchronous dual hematological malignancies (SDHMs), where the patient was diagnosed with symptomatic multiple myeloma (MM) and essential thrombocythemia (ET), when excessive thrombocytosis occurred following initiation of MPV (melphalan–prednisone–bortezomib) antimyeloma therapy.Case descriptionAn 86-year-old woman presented to the emergency in May 2016 with confusion, hypercalcemia, and acute kidney injury. She was diagnosed with free light chain (FLC) lambda and Immunoglobulin G (IgG) lambda MM and started MPV (standard of care at that time) treatment with darbopoietin support. At diagnosis, she had normal platelet count since the ET was likely masked by bone marrow suppression due to active MM. After she reached stringent complete remission with no MP detected on serum protein electrophoresis or immunofixation, we noticed that her platelet counts increased to 1,518,000 × 109/L. She was tested positive for mutation in exon 9 of calreticulin (CALR). We concluded that she had concomitant CALR-positive ET. After bone marrow recovery from MM, the ET became clinically apparent. We started hydroxyurea for ET. Treatment for MM with MPV did not affect the course of ET. Presence of concomitant ET did not decrease the efficacy of sequential antimyeloma therapies in our elderly and frail patient.ConclusionThe possible mechanism underlying the occurrence of SDHMs is unclear but is likely due to stem cell differentiation defects. SDHMs can be challenging to treat and warrant several considerations. In the absence of clear guidelines on how to manage SDHMs, management decisions depend on several factors including disease aggressiveness, age, frailty, and comorbidities.