RESUMEN
Background: Pneumocystis jirovecii is a fungal pathogen that can present as an opportunistic cause of pneumonia and can occur in individuals with various causes of immunosuppression, including malignancy and treatments for malignancy that confer increased risk. Although the guidelines for use of Pneumocystis prophylaxis in certain populations are clear, the rapid development of novel cancer therapies elicits the need to accurately assess the degree of immunosuppression conferred by these regimens and to determine if patients receiving these therapies warrant Pneumocystis prophylaxis. Case Series: We present 2 cases of Pneumocystis jirovecii pneumonia in patients with invasive ductal carcinoma of the breast treated with a dose-dense chemotherapy regimen consisting of doxorubicin, cyclophosphamide, and paclitaxel. Conclusion: The use of a dose-dense regimen, in which the interval between doses is shortened compared to a standard regimen, has become a common therapy for patients diagnosed with early breast cancer. Although this approach leads to improved disease-free and overall survival, it has also been associated with an increased risk of developing Pneumocystis jirovecii pneumonia. Further research involving patients receiving dose-dense chemotherapy regimens is needed to determine their risk of developing opportunistic infections and whether that risk warrants changes in clinical management.