Microphthalmia-associated transcription factor (MiTF): Promiscuous staining patterns in fibrohistiocytic lesions is a potential pitfall

Top 10 Differential Diagnoses for Desmoplastic Melanoma

BACKGROUND

Desmoplastic melanoma is a rare subtype of melanoma mainly appearing on sun-exposed skin. Clinically, it is many times non-pigmented and therefore the diagnosis is often not suspected.

METHODS

Review article.

RESULTS

In this paper we review the main histopathological, immunohistochemical, and molecular features of desmoplastic melanoma, as well as the top 10 morphologic differential diagnoses which should be considered in most cases. The histopathological pattern can be many times deceptive, mimicking a scar, a fibrous reaction, a fibrohistiocytic tumor such as a dermatofibroma, a vascular tumor such as angiosarcoma, a smooth muscle tumor such as leiomyosarcoma, or a neural tumor. Although an overlying atypical junctional melanocytic proliferation may be seen in most cases, it is absent in a significant percentage (up to 30%) of cases, making the diagnosis even more difficult in those instances. The range of diagnostic pitfalls is wide, which may present disastrous prognostic consequences.

CONCLUSION

Desmoplastic melanoma is often a difficult diagnosis to make, as it frequently shows nonspecific clinical findings and overlapping histologic features with many other tumors. However, the potential clinical and prognostic consequences of misdiagnosis as another entity are great. Therefore, this diagnosis must always be kept in mind when encountering spindle cell tumors affecting the head and neck area.

Two Cases of STAT6-Positive, Primary Conjunctival Giant Cell-Rich Solitary Fibrous Tumour (Giant Cell Angiofibroma) with Some Unusual Histological Features

An 84-year-old male presented with a history of a slowly growing lesion beneath the right lower lid. Examination showed a right-sided mass in the inferior bulbar conjunctiva, measuring approximately 10 × 6 mm. The lesion was excised and showed a giant cell-rich solitary fibrous tumour (GCR-SFT) with pseudo-vascular spaces with benign cytology, with STAT6, expressed in the spindle, ovoid, and tumour giant cells. A second case, a 26-year-old white female, presented with a 12-month history of a gradually expanding lesion in left inferior bulbar conjunctiva. Examination showed a well-defined, dome-shaped pink fleshy mass of 10 × 7 × 2 mm, on the inferior bulbar conjunctiva and abutting the inferior fornix. Excision followed by histology showed a similar appearance to the first case except that the multinucleate giant cells closely resembled Touton giant cells and did not show the rather hyperchromatic crowded nuclei of the giant cells observed in the first case. No pseudo-vascular spaces were identified. The tumour cells, including the Touton-like multinucleate giant cells, displayed nuclear positivity for STAT6. An unexpected feature was the presence of a Melan A- and Sox10-positive population amongst the GCR-SFT, postulated to be a subepithelial melanocytic nevus colliding with the GCR-SFT.

An Algorithmic Immunohistochemical Approach to Define Tumor Type and Assign Site of Origin

Immunohistochemistry represents an indispensable complement to an epidemiology and morphology-driven approach to tumor diagnosis and site of origin assignment. This review reflects the state of my current practice, based on 15-years' experience in Pathology and a deep-dive into the literature, always striving to be better equipped to answer the age old questions, "What is it, and where is it from?" The tables and figures in this manuscript are the ones I "pull up on the computer" when I am teaching at the microscope and turn to myself when I am (frequently) stuck. This field is so exciting because I firmly believe that, through the application of next-generation immunohistochemistry, we can provide better answers than ever before. Specific topics covered in this review include (1) broad tumor classification and associated screening markers; (2) the role of cancer epidemiology in determining pretest probability; (3) broad-spectrum epithelial markers; (4) noncanonical expression of broad tumor class screening markers; (5) a morphologic pattern-based approach to poorly to undifferentiated malignant neoplasms; (6) a morphologic and immunohistochemical approach to define 4 main carcinoma types; (7) CK7/CK20 coordinate expression; (8) added value of semiquantitative immunohistochemical stain assessment; algorithmic immunohistochemical approaches to (9) "garden variety" adenocarcinomas presenting in the liver, (10) large polygonal cell adenocarcinomas, (11) the distinction of primary surface ovarian epithelial tumors with mucinous features from metastasis, (12) tumors presenting at alternative anatomic sites, (13) squamous cell carcinoma versus urothelial carcinoma, and neuroendocrine neoplasms, including (14) the distinction of pheochromocytoma/paraganglioma from well-differentiated neuroendocrine tumor, site of origin assignment in (15) well-differentiated neuroendocrine tumor and (16) poorly differentiated neuroendocrine carcinoma, and (17) the distinction of well-differentiated neuroendocrine tumor G3 from poorly differentiated neuroendocrine carcinoma; it concludes with (18) a discussion of diagnostic considerations in the broad-spectrum keratin/CD45/S-100-"triple-negative" neoplasm.

Meningioma-like Tumor of the Skin Revisited: A Distinct CD34+ Dermal Tumor With an Expanded Histologic Spectrum

The term meningioma-like tumor of the skin (MLTS) was coined in 1993 to designate a particular whorled spindle cell superficial cutaneous tumor. No additional confirmed cases of this entity have been reported to date. Some authors have speculated that these cases might be cellular neurothekeomas. In order to delineate the histologic spectrum and the immunophenotype of this unusual tumor, we studied 5 cases, 2 previously unreported and the 3 original cases. The immunohistochemical findings of case 5, however, were limited to those from the original study. Clinically, the tumor presented as a reddish papule, plaque, or nodule, located in the extremities or trunk. The patient often referred to a recent growth of a longstanding lesion. Histologically, the characteristic whorled spindle and stellate dendritic cell population, commonly in a perivascular arrangement, and variable myxoid component, were consistently found in all cases. A prominent microvasculature was also a constant finding. The presence of large deciduoid cells was conspicuous in one case. A reticular pattern of multivacuolated cells giving a chordoma-like appearance was evident in another case. Tumor cells were diffusely positive for CD34 in all 4 cases studied, and negative for S-100, EMA, NKI-C3, CD68, and smooth muscle markers. No complete loss of retinoblastoma protein was found. No brachyury immunostaining was found in the case with chordoid features. No EWSR1 or NAB2-STAT6 gene fusions were found. From these findings, we demonstrate that MLTS is a distinct CD34 spindle cell benign dermal tumor, unrelated to cellular neurothekeoma, and exhibiting myxoid, deciduoid, or chordoma-like features.