A comprehensive review of the "tigroid" background cytological concept: what, when, where and why?

Abstract

The concept of "tigroid" background is used in cytology to describe a peculiar smear background characterized by the presence of a relatively granular, reticulated material that was described as "foamy, lazy, tiger-striped or astrakhan". It was used to describe the background seen in smears obtained from seminoma. In addition to seminoma, we now know that it can be present in different tumours, mostly carcinomas and round cell sarcomas. These share with seminoma a cytoplasm with high glycogen content and many times clear cell morphology. The "tigroid" background is seen when smears are air-dried and Romanowsky-based stains are used (May-Grunwald-Giemsa and Diff-Quik stains). It is only seen in fine needle aspiration or intraoperative squashing or scrapping samples, but not in specimens obtained from effusions or liquid-based cytology. Wet-fixed cytologic samples with alcohol or with formaldehyde tend to dissolve the background so it is not usually present in Papanicolaou stained smears. In this review, we discuss tumours in which the "tigroid" background is observed and its potential diagnostic utility and aetiology. It is interesting to remark that except for parathyroid adenoma and adenomatoid tumour all the neoplasms in which this background has been observed are malignant.

Keywords: cytology; fine needle aspiration; seminoma; “Tigroid” background.

Figure 1.

(A) A typical case of seminoma showing the foamy, lazy background that resembles astrakhan (Diff-Quik, x400). (B) Higher magnification reveals the typical alternating interwoven stripes that resemble those of a tiger. Seminoma tumour cells have cytoplasmic fragility and smears show numerous naked nuclei (Diff-Quik, x600)

Figure 2.

(A, B) Intraoperative frozen section assessment of a seminoma showing large cells with abundant cytoplasm, admixed lymphocytes and fibrous bands together with evident “tigroid” material (Haematoxylin-Eosin, x400). (C) The corresponding permanent section shows the same histopathological features except for the “tigroid” substance which is lost (Haematoxylin-Eosin, x400).

Figure 3.

(A) A peculiar case of seminoma almost devoid of neoplastic cells. Only a single naked tumoural nucleus accompanied by numerous lymphocytes and occasional macrophages is visible. In this setting, the characteristic background results very helpful for diagnosis (Diff-Quik, x600). (B) Foetal adenocarcinoma showing a similar background (Diff-Quik, x600). (C) The “tigroid” background is also present in a case of parathyroid adenoma composed of chief cells (Diff-Quik, x600). (D) A case of papillary tumour the pineal region showing the characteristic background. Note that in all four cases numerous naked nuclei are present.

Figure 4.

(A) Smears from Ewing’s sarcoma shown “tigroid” material (Diff-Quik, x600). (B) Rhabdomyosarcoma is another small round cell sarcoma that can show this peculiar background (Diff-Quik, x600).