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PAS positive pseudofungi: a diagnostic pitfall

Published:November 18, 2020DOI:https://doi.org/10.1016/j.pathol.2020.08.016
      Sir,
      Accurate diagnosis of pseudofungi in histology specimens is imperative for avoiding unnecessary treatment. Special stains are key to separating true fungi from their mimics. Periodic acid-Schiff (PAS) stain is a common stain used in the diagnosis of fungi, typically highlighting fungal cell walls, but also staining other polysaccharides and mucins. We report a case of striking and extensive pseudofungi incidentally detected in perigastric lymph nodes.
      A 68-year-old woman presented for routine imaging surveillance of a high risk gastric epithelioid gastrointestinal stromal tumour, resected 5 years prior and treated with adjuvant imatinib. Computed tomography (CT) and positron emission tomography (PET) imaging showed evidence of recurrent disease adjacent to the gastric anastomosis, with no evidence of nodal disease. She was systemically well and underwent a subtotal gastrectomy with intraoperative findings of an exophytic lesion at the greater curve of the distal stomach, with adhesions to the anterior abdominal wall and liver.
      Grossly, the specimen comprised a 130 mm, multilobed, fleshy and haemorrhagic tumour arising from the serosal surface of the stomach. Three lymph nodes were present in the perigastric fat and were grossly unremarkable, less than 6 mm in maximal dimension. Histological examination of the tumour showed a partially necrotic epithelioid gastrointestinal stromal tumour, which was strongly positive for DOG-1 immunohistochemistry.
      Examination of the lymph nodes showed no evidence of metastatic disease. Follicular architecture was preserved. In the subcapsular sinuses, there were abundant hyphae-like structures with mild variation in thickness, demonstrating septa and acute to right angle branching (Fig. 1). There were scattered haemosiderin laden macrophages in the background. An initial panel of stains was performed, with the structures positive for PAS and negative for Grocott's methenamine silver (GMS). Perls' stain was strongly positive in the hyphae-like structures, as well as in the haemosiderin laden macrophages. Supplementary stains showed positivity for Alizarin red, orcein, reticulin, Sirius red, and Van Gieson stains (Fig. 2).
      Fig. 1
      Fig. 1Pseudofungi presenting as septate, acute to right angled branching structures seen within a lymph node subcapsular sinus (H&E).
      Fig. 2
      Fig. 2Special stains used for diagnosis of pseudofungi. (A) Grocott's methenamine silver negative, (B) periodic acid-Schiff positive, (C) Perls' positive, (D) Alizarin red positive, (E) reticulin positive, (F) Sirius red positive.
      This pattern of staining suggests the pseudofungi consist of collagen fibres with deposition of iron and calcium. Similar structures have been observed within lymph nodes in rare case reports, also typically demonstrating a PAS positive, Perls' positive, and GMS negative staining pattern.
      • Connelly J.
      • Ro J.Y.
      • Cartwright Jr., J.
      Pseudofungi in a lymph node. A case report with energy dispersive X-ray elemental analysis.
      • Song D.E.
      • Kahn A.G.
      • Khang S.K.
      • Ro J.Y.
      Pseudofungi in pericolic lymph nodes.
      • Teague M.D.
      • Tham K.T.
      Hyphalike pseudofungus in a lymph node.
      • Singh C.
      • Manivel C.
      • Pambuccian S.E.
      Pseudofungi: coral shapes and bamboo sticks in lymph node sinuses.
      Analysis by energy dispersive radiographic microanalysis of two cases
      • Connelly J.
      • Ro J.Y.
      • Cartwright Jr., J.
      Pseudofungi in a lymph node. A case report with energy dispersive X-ray elemental analysis.
      ,
      • Song D.E.
      • Kahn A.G.
      • Khang S.K.
      • Ro J.Y.
      Pseudofungi in pericolic lymph nodes.
      showed the pseudofungi contained phosphorus, sulphur, potassium, calcium, and iron, with ultrastructural examination showing that septation was a result of fracturing of the calcified structures. Interestingly, one of these cases also involved nodes associated with resection of a large haemorrhagic gastrointestinal stromal tumour
      • Song D.E.
      • Kahn A.G.
      • Khang S.K.
      • Ro J.Y.
      Pseudofungi in pericolic lymph nodes.
      ; other cases have been associated with malignant melanoma
      • Connelly J.
      • Ro J.Y.
      • Cartwright Jr., J.
      Pseudofungi in a lymph node. A case report with energy dispersive X-ray elemental analysis.
      ,
      • Singh C.
      • Manivel C.
      • Pambuccian S.E.
      Pseudofungi: coral shapes and bamboo sticks in lymph node sinuses.
      and large cell lung carcinoma.
      • Teague M.D.
      • Tham K.T.
      Hyphalike pseudofungus in a lymph node.
      It is theorised that haemorrhage associated with such tumours may lead to haemosiderin deposition in the draining nodes, with eventual degradation leading to iron deposition on collagen or reticulin fibres.
      • Song D.E.
      • Kahn A.G.
      • Khang S.K.
      • Ro J.Y.
      Pseudofungi in pericolic lymph nodes.
      ,
      • Singh C.
      • Manivel C.
      • Pambuccian S.E.
      Pseudofungi: coral shapes and bamboo sticks in lymph node sinuses.
      This is supported by the presence of background Perls' positive haemosiderin laden macrophages observed in our case.
      Awareness of fungal mimics is important to avoid misdiagnosis of a fungal infection. In addition to histological examination, a panel of special stains should be employed to assist to differentiate fungal organisms from pseudofungi. Our case highlights the strong resemblance of pseudofungi to true fungi, and the utility of special stains for accurate diagnosis.

      Conflicts of interest and sources of funding

      The authors state that there are no conflicts of interest to disclose.

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