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Benign multicystic peritoneal mesothelioma: a diagnostic challenge

In the July 2005 issue of the Journal, Dr Samson and colleagues reported the truly first case of benign multicystic peritoneal mesothelioma (BMPM) diagnosed during hernia repair, which, they believed, also represented the first reported case of the disease diagnosed in a man over the age of 60 years.1 In the same month, the Clinicopathological Conference at our hospital discussed the case of a 72-year-old manwith the surprise finding of a multicystic tumour inside the sac of an inguinal hernia, which was reported as BMPM by our consultant pathologists. What a coincidence! Equally coincidental is the fact that the two cases closely resemble each other in the histopathology of the hernia specimen and also in the findings of their abdominal computed tomography (CT) scans. But more importantly, our case provides new information: unlike Dr Samson’s patient who opted for observation, our patient consented to laparotomy, which unveiled an underlying pathology initially overlooked by us and possibly, by Dr Samson as well. Our patient first presented in early 2004 with a 1-year history of distending discomfort in the right lower quadrant. There were no associated bowel symptoms. The haemoglobin level was 11.2 g/dL. A colonoscopy showed two small polyps in the transverse colon, which were removed and were later shown to be tubular adenoma with moderate dysplasia. A CT scan showed a 6.5 x 4–cm, thinwalled, unilocular cystic mass with fluid attenuation arising from the right side of the pelvis above the bladder. The impression then was either a bladder diverticulum or a duplication cyst. The patient defaulted follow up until April 2005, when he noticed a reducible swelling in the left groin region. At herniotomy under local anaesthesia, the hernial sac was found to be occupied by a multilocular cystic mass containing mucinous substance. Histological section of the mass showed a multicystic lesion lined by a single layer of flattened cells that were positive for mesothelial markers, calretinin and cytokeratin. Abundant basophilic mucin was seen in the lumen and stroma, whereas no epithelial elements were found throughout the specimen. Neither atypical cells nor mitoses was seen. Three weeks later, the patient was readmitted with increasing right lower quadrant pain. A tender mass was palpable in that region. An urgent CT scan depicted the same right lower quadrant cystic mass recognized 1 year earlier, which now contained air/fluid level inside. At laparotomy, an inflamed appendiceal mass was found, which was distended with mucin. There was diffuse deposition of small gelatinous nodules over the pelvic sidewall, small bowel mesentery and greater omentum. Right hemicolectomy was carried out, from which the patient made an uneventful recovery. On gross examination of the specimen, the appendix was replaced by a ruptured, necrotic cyst distended with mucin, measuring 7 cm in its widest dimension. Microscopic examination showed a mucinous adenocarcinoma consisting of dysplastic adenomatous proliferation with evidence of infiltration into the muscularis propria (pT2). Mucin was seen dissecting through the muscle layers to the subserosa, as well as around the rupture site. The pathological diagnosis was pseudomyxoma peritonei (PP). The remarkable resemblance between these two concurrent cases led us to strongly suspect that the primary pathology of Dr Samson’s case could be the same—mucinous cystic tumour of the appendix—for the following reasons. First, BMPM occurs predominantly in women in the reproductive age group frequently with a history of a pelvic operation or inflammatory disease, whereas both our patient and that of Samson and Cacala were previously healthy elderly men.1 Second, in retrospect, the CT scan of our patient taken in 2004 has already displayed tell-tale evidence of an underlying PP. Besides the main oblong cystic tumour in the right iliac fossa, hypoattenuating substance was present (albeit in so small amount as to elude our radiologist) filling the pelvis, the right paracolic gutter and the space around the liver (causing the characteristic scalloping of the liver2), as well as the left inguinal canal (Fig. 1). An almost identical picture was described by Dr Samson. In fact, the aforementioned spaces where the ‘cysts’ extended in either case are exactly the sites typically involved by the mucinous deposits in PP.2,3 Third, the absence of significant changes between CT scans taken 1 year apart, as in Dr Samson’s case, does not rule out PP. It is well known that because of the unpredictable nature of the disease, PP may, for years, progress very slowly even without chemotherapy.2 Fourth, it has often been mentioned that PP may first appear as hernias.2,3 Nonetheless, histological descriptions of the hernia specimen are sparse in the international published reports. Young et al. collected eight case reports on hernia sac involvement by PP secondary to an appendiceal mucinous lesion in 1997; they added two cases of their own.4 The hernia specimens in their two cases, similar to those from the present two cases, were devoid of associated epithelial cells, making the diagnosis of intra-abdominal epithelial tumour very difficult without laparotomy. Furthermore, in all these confirmed cases of mucinous cystic tumour of the appendix presenting as cystic mass in a hernial sac, the cysts were lined by mesothelial cells.4 Therefore, even in the presence of lining cells positive for mesothelial markers on immunohistochemical staining, clinicians should not exclude the possibility of PP. Finally, a striking histological feature of the present two cases was the presence of abundant mucin. In contrast, the usual contents of BMPM are either clear fluid or serosanguinous fluid; only rarely is mucinous or gelatinous material found. The mucin recovered from the hernial sac in our case stained strongly with periodic acid-Schiff but weakly with alcian blue, just like that within the appendicular tumour, thereby confirming its epithelial origin (cf. negative periodic acid-Schiff and positive alcian blue staining for mesothelial cell origin5). Likewise, both cases of PP reported by Young et al. showed the epithelial staining pattern.4 The pathogenesis of BMPM remains controversial, uncertainties still exist as to whether it represents a neoplastic or reactionary process. It seems that chronic peritoneal irritation has become increasingly implicated.1 The consistent coexistence of