Pneumocystis jirovecii pneumonia and prophylaxis in patients with solid tumours: a narrative review.

Pneumocystis jirovecii pneumonia (PJP) is an uncommon but potentially fatal opportunistic fungal infection in immunocompromised patients. In patients with solid tumours, the risk is not universal; it is concentrated in defined clinical contexts of impaired cell-mediated immunity, particularly prolonged corticosteroid exposure, temozolomide administered with radiotherapy for high-grade glioma, craniospinal irradiation, severe or persistent lymphopenia, and combined immunosuppressive treatment. Mortality is substantially higher in HIV-negative immunocompromised populations than in HIV-associated disease, commonly reaching 30-50% in severe cases. Evidence specific to adults with solid tumours remains limited, and no prospective randomised prophylaxis trial has focused exclusively on this population. Most oncology recommendations therefore extrapolate from haematological malignancy, transplant, paediatric oncology, and broader non-HIV immunocompromised cohorts. Current guidance is consistent in two high-risk solid-tumour scenarios: patients expected to receive at least 20 mg/day prednisone-equivalent for approximately 4 weeks or longer, and patients receiving concomitant temozolomide-radiotherapy, particularly when lymphopenia or corticosteroid exposure is present. Trimethoprim-sulfamethoxazole (TMP-SMX) remains the preferred prophylactic regimen. Recent evidence has refined the clinical approach without removing the need for prevention in clearly high-risk patients. A 2026 systematic review and meta-analysis suggested that the absolute PJP incidence during temozolomide-radiotherapy may be lower than the historically cited 3.5% prophylaxis threshold, although interpretation is limited by heterogeneous diagnostic confirmation and incomplete reporting of corticosteroid exposure and lymphopenia. Similarly, adjunctive corticosteroids remain standard in moderate-to-severe HIV-associated PJP, whereas in HIV-negative severe PJP they should be considered selectively: a recent randomised trial did not significantly reduce 28-day mortality but suggested benefit in secondary outcomes, including 90-day mortality and intubation among non-intubated patients. This narrative review summarises the epidemiology, risk factors, treatment principles, and prophylaxis recommendations for PJP in patients with solid tumours, with particular attention to corticosteroid use, brain tumours, and immune checkpoint inhibitor toxicity.