Nodular Histiocytic/Mesothelial Hyperplasia - not such a rare phenomenon in children with Spontaneous Pneumothorax.

<b>Introduction:</b> Primary spontaneous pneumothorax (PSP) is a relatively rare condition in children, posing significantchallenges in its management and complicating efforts to identify those at higher risk of complications. Recent studieshave drawn attention to Nodular Histiocytic/Mesothelial Hyperplasia (NHMH) as an occasionally observed histologicalchange in this patient group. Differentiating NHMH from other potential neoplastic processes has become essential toensure accurate diagnosis and appropriate treatment.<b>Aim:</b> This study aimed to evaluate the frequency and clinical relevance of NHMH in pediatric patients with spontaneous pneumothorax at the Department of Pediatric Surgery in Cracow, Poland.<b>Materials and methods:</b> We conducted a retrospective re-analysis of histological images from patients who underwentsurgery for spontaneous pneumothorax between 2012 and 2024.<b>Results:</b> Our findings revealed a higher incidence of NHMH in patients who required reoperation after initial pneumothoraxtreatment.<b>Discussion:</b> The presence of NHMH, particularly in these cases of reoperation, suggests that chronic irritation of lung tissue, possibly due to the presence of emphysematous bullae, plays a significant role in its development.<b>Conclusions:</b> Recognizing NHMH in these patients is crucial for preventing misdiagnosis as a neoplastic process, which can have serious clinical implications.<b>Significance for the field:</b> Understanding this histological feature is important for guiding future therapeutic decisions andavoiding unnecessary interventions.