Chronic Mycobacterium kansasii Pleural Infection Mimicking Metastatic Breast Cancer: A Seven-Year Diagnostic Odyssey and the Critical Role of Metagenomic Sequencing.

INTRODUCTION: Nontuberculous mycobacteria such as Mycobacterium kansasii can mimic malignancy on imaging and pathology, leading to prolonged diagnostic uncertainty and inappropriate anticancer therapy.

CASE REPORT: A 76-year-old woman with remote right breast carcinoma (mastectomy and adjuvant therapy in 1996) had a persistent right chest-wall lesion with rib changes and encapsulated pleural effusion repeatedly interpreted as metastatic disease from 2017 to 2023, despite multiple biopsies showing only fibrous hyperplasia. In August 2024, fever and cough prompted re-evaluation. PET-CT demonstrated a hypermetabolic pleura-adjacent lesion (SUVmax 10.8) without distant metastases. Plasma metagenomic next-generation sequencing (mNGS) yielded a low-level M. kansasii signal; pleural fluid mNGS identified 146 reads (94% relative abundance), later confirmed by culture. Targeted anti-NTM therapy stabilized the infection; however, the patient developed severe varicella-zoster virus infection and cardiac complications and subsequently died. The death was attributed to these complications rather than the progression of the M. kansasii infection.

CONCLUSION: Chronic M. kansasii pleural infection can masquerade as metastatic breast cancer for years. PET-CT alone is insufficient to distinguish infection from malignancy; careful imaging review combined with unbiased mNGS can establish the diagnosis and avert unnecessary anticancer therapy. Multidisciplinary collaboration is essential for timely recognition and management.