Case report: Reactive nodular fibrous pseudotumor: report of 4 new cases including a rare pulmonary presentation and a literature review covering 40 cases

Reactive nodular fibrous pseudotumor (RNFP) is a rare benign fibrocollagenous lesion that often presents diagnostic challenges due to nonspecific clinical and radiological features, particularly at unusual sites such as the lung. The study analyzed 4 confirmed RNFP cases including a rare pulmonary case and combined them with 40 published cases (total of 44 cases). RNFP occurs at multiple anatomical sites, most commonly in the abdomen, followed by extremities and chest, with the pulmonary lesion presenting as a solitary nodule mimicking malignancy. Histologically, all lesions showed low cellularity, abundant hyalinized collagen, and scattered bland spindle cells without cytologic atypia. Immunohistochemically, all cases were positive for vimentin, 50% expressed SMA, and 25% showed focal CD34 expression. During follow-up (6-60 months), no recurrence or progression was observed. RNFP is a benign reactive lesion with characteristic histological and immunophenotypic features.