Nodular histiocytic/mesothelial hyperplasia as consequence of chronic mesothelium irritation by sub-phrenic abscess.

Nodular histiocytic/mesothelial hyperplasia (NHMH) is a benign localized alteration, firstdescribed in 1975 by Rosai in the hernia sac [1]. Few pulmonary cases have been reported in literature[2–6]. Sometimes it has been reported in the pericardium [7,8] or presenting as an inguinalmass [9]. The ‘mesothelial/monocytic incidental cardiac excrescence’, first described by Weinot et al.in 1994 [10] is now considered a similar lesion to NHMH [11].It consists of a reactive proliferation of histiocytes and mesothelium secondary to chronic irritationand it has been observed in pleura-damaging processes, such as pneumothorax [5], or as consequenceof cardiac catheterization, inflammation, mechanical or tumor stimulation [11].The rarity of NHMH and the moderate cytological atypia often present, make this lesion difficultto diagnose. It can be easily confused with primary mesothelial lesions and neoplasms suchas adenocarcinomas, granulosa cell tumors or Langerhans’ histiocytosis.We report a case of pleural NHMH in a patient with a subphrenic abscess, in which no pulmonarypathogenic noxa was evident. We hypothesize a transdiaphragmatic chronic irritation as apathogenetic mechanism underlying NHMH.