A 10-year-old boy presents with fever and an anterior mediastinal mass. What is the most likely diagnosis?

The presentation of a 10-year-old boy with fever and an anterior mediastinal mass is most consistent with non-Hodgkin's lymphoma. In pediatric patients, non-Hodgkin's lymphoma, particularly lymphoblastic lymphoma, often presents with an anterior mediastinal mass, which can lead to symptoms due to mass effect on surrounding structures. Fever can be associated with systemic symptoms from the lymphoma as well.

Additionally, while acute lymphoblastic leukemia is also a strong contender for this age group and does involve the lymphoid system, it more commonly presents with hematological manifestations, such as anemia, thrombocytopenia, or neutropenia. The prominent mediastinal mass with associated fever points more specifically towards non-Hodgkin's lymphoma in this case.

The age group and typical manifestations of an anterior mediastinal mass make non-Hodgkin's lymphoma the most likely diagnosis in this scenario.