Case Report: Fatal <i>Pneumocystis jirovecii</i> Infection in an Elderly Man Receiving Adjuvant Paclitaxel and Trastuzumab Therapy for HER2‐Positive Breast Cancer

<b>Background:</b> <i>Pneumocystis jirovecii</i> pneumonia (PJP) is a well-recognized opportunistic infection in immunocompromised patients, particularly those with hematological malignancies or HIV infection. However, its occurrence in patients with solid tumors undergoing chemotherapy remains poorly characterized. <b>Case Presentation:</b> We report the case of an 84-year-old male patient with HER2-positive breast cancer who developed severe PJP following adjuvant chemotherapy with paclitaxel and trastuzumab. The patient had no known immunosuppressive conditions and did not present chemotherapy-induced lymphopenia prior to the onset of symptoms. He was admitted with fever and dyspnea, 9 days after discontinuation of chemotherapy due to Grade 3 asthenia. Chest computed tomography (CT) revealed diffuse ground-glass opacities, and bronchoalveolar lavage confirmed the presence of <i>Pneumocystis jirovecii</i> DNA by PCR. Despite prompt initiation of sulfamethoxazole/trimethoprim and corticosteroids, the patient developed refractory acute respiratory distress syndrome (ARDS) and died after ICU admission. <b>Discussion:</b> This case highlights the potential risk of PJP in elderly patients receiving adjuvant chemotherapy, even in the absence of evident immunosuppression. Given the increasing use of chemotherapy in older populations, a thorough risk-benefit assessment should be considered, especially when the expected survival benefit is limited. Current guidelines do not recommend systematic PJP prophylaxis in patients with solid tumors, yet emerging evidence suggests that chemotherapy-related lymphopenia may increase susceptibility to opportunistic infections. <b>Conclusion:</b> Clinicians should maintain a high index of suspicion for opportunistic infections such as PJP in elderly patients undergoing chemotherapy, regardless of their immune status. This case underscores the importance of individualized risk stratification and vigilant monitoring to prevent and manage life-threatening complications.