Medicare’s moment of truth on a potential blockbuster cancer therapy is rapidly approaching.

The Centers for Medicare & Medicaid Services are slated to decide by the end of May whether to cover CAR T-cell therapy for all program enrollees.

The treatment, which can take several weeks and cost up to $1 million, is a new option for people with leukemia and lymphoma. It requires extracting and then genetically engineering a patient’s immune system T-cells in a lab, multiplying the modified cells, and then putting them back into the body to attack the cancerous cells.

Medicare contractors currently decide whether to...