A 48-year-old woman from Mumbai with newly diagnosed multiple myeloma (stage IIIA, Durie–Salmon) is started on induction chemotherapy with bortezomib, dexamethasone, and an alkylating agent. After 2 cycles, she develops a dry cough, progressive dyspnea, and chest tightness. Chest X-ray shows bilateral interstitial infiltrates. Pulmonary function tests reveal a restrictive pattern with reduced DLCO. Bronchoalveolar lavage is non-diagnostic for infection. The oncologist suspects drug-induced pulmonary fibrosis. Which alkylating agent is MOST likely the culprit, given its well-known association with cumulative pulmonary toxicity?

Explanation

## Clinical Context The patient has multiple myeloma and is on induction chemotherapy. After 2 cycles, she develops progressive dyspnea, restrictive lung disease pattern, and reduced DLCO—classic features of drug-induced pulmonary fibrosis. The question asks which alkylating agent is most notorious for cumulative pulmonary toxicity. ## Alkylating Agent Pulmonary Toxicity Profile | Agent | Pulmonary Toxicity | Onset | Cumulative Dose Risk | Other Notable Toxicities | Clinical Use | |-------|-------------------|-------|----------------------|--------------------------|---------------| | **Busulfan** | **Pulmonary fibrosis (classic)** | **2–6 months** | **Yes, dose-dependent** | Skin hyperpigmentation, seizures | Chronic myeloid leukaemia, conditioning | | Melphalan | Rare pulmonary toxicity | Late | No | Secondary malignancy, neuropathy | Multiple myeloma, lymphoma | | Cyclophosphamide | Pulmonary fibrosis (less common) | Variable | Possible | Hemorrhagic cystitis, cardiomyopathy | Breast, ovarian, lymphomas | | Ifosfamide | Rare pulmonary toxicity | Variable | No | Encephalopathy, hemorrhagic cystitis | Germ cell tumours, sarcomas | ## Key Point: **Busulfan** is the alkylating agent MOST strongly associated with cumulative dose-dependent pulmonary fibrosis. This is a classic teaching point in medical oncology and pharmacology. ## High-Yield: Busulfan-induced pulmonary fibrosis (BPF) is an idiosyncratic reaction that develops insidiously, typically 2–6 months after initiation, and is characterized by: - Progressive dyspnea and cough - Restrictive lung disease pattern on PFTs - Bilateral interstitial infiltrates on imaging - Reduced DLCO - Cumulative dose dependency (risk increases with total dose > 500 mg) ## Clinical Pearl: Busulfan is used as a conditioning agent before stem cell transplantation and in chronic myeloid leukaemia. Patients on busulfan require baseline and periodic pulmonary function testing and chest imaging. Early detection and discontinuation can prevent progression to irreversible fibrosis. ## Mnemonic: **BUSULFAN = Pulmonary Fibrosis** (remember: BU-sul-FAN → lung FAN-like fibrosis pattern). Busulfan is the alkylating agent most feared for pulmonary toxicity. ## Warning: Do not confuse busulfan's pulmonary fibrosis with cyclophosphamide's hemorrhagic cystitis or ifosfamide's encephalopathy. While cyclophosphamide can rarely cause pulmonary fibrosis, busulfan is the classic and most common culprit.