Neutrophilic Leukocytosis due to Infection or Inflammation: Given the recent chemotherapy, the patient is at high risk for infections due to immunosuppression. The elevated WBC count with a predominance of neutrophils suggests a bacterial infection or an inflammatory response, which is a common complication post-chemotherapy.

Chemotherapy-Induced Leukocytosis: Some chemotherapy agents can cause an increase in white blood cell count, including neutrophils, as part of the body's response to the treatment.
Reaction to Blood Transfusion: If the patient received a blood transfusion during or after chemotherapy, a transfusion reaction could lead to an elevated WBC count.
Tumor Lysis Syndrome: Although less common in solid tumors, this condition can occur after chemotherapy, leading to the release of intracellular contents into the bloodstream, potentially causing an inflammatory response and elevated WBC count.

Sepsis: A life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. Given the patient's immunosuppressed state, sepsis is a critical diagnosis not to miss.
Neutropenic Fever: Despite the elevated neutrophil count, if the patient had a significantly low neutrophil count before the current lab result, neutropenic fever should be considered, especially if the patient is showing signs of infection.
Hemorrhagic Cystitis: A condition associated with certain chemotherapy agents, characterized by bleeding from the bladder mucosa, which could potentially lead to an inflammatory response and elevated WBC count.

Leukemoid Reaction: A rare condition characterized by an extremely high WBC count that mimics leukemia but is actually a response to a non-cancerous condition, such as a severe infection or inflammatory process.
Paraneoplastic Syndromes: Rare disorders that are triggered by an altered immune system response to a neoplasm, which could potentially cause an elevated WBC count among other symptoms.