LAUSR

Trastuzumab-deruxtecan (T-DXd) is a novel antibody drug conjugate that has improved treatment outcomes in patients with ERBB2-positive cancer, including locally advanced or metastatic gastric and gastro-oesophageal junction adenocarcinoma. One of the reported side effects of this medication is drug-induced pneumonitis. We present in this case report, a diagnostic dilemma of a patient presenting with clinical and radiographical features of drug-induced pneumonitis but was found to have pneumocystis jirovecii pneumonia (PJP). Our case is the first of PJP in a patient treated with T-DXd, highlighting the increasing incidence of this opportunistic infection in patients with solid malignancy. It also highlights the clinical and radiographical similarities between the PJP and drug-induced pneumonitis.