In the 2020 5th edition of the World Health Organization classification of soft tissue and bone tumours a major reorganization of Undifferentiated Small Round Cell Sarcomas (USRCS) took place based on the underlying molecular features. The classification now recognizes Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions, CIC-rearranged sarcoma and sarcoma with BCOR alterations. The focus on these genetic alterations highlights the importance of molecular techniques in the diagnosis of these entities. Knowledge of these features can drastically reduce the time to diagnosis and avoid potential misdiagnosis. Molecular diagnostic capabilities should not be limited to an overall small number of centres worldwide as is reflected by the WHO's recognition of 'essential' and 'desirable' diagnostic criteria. A good knowledge of the usual histomorphology, uncommon variants and diagnostic pitfalls remains essential even in centres with access to a full molecular testing arsenal. This review aims to give an overview of the current classification of USRCS not by going over each entity, but instead going over the molecular, morphological, immunophenotypic and clinical features step by step to allow easy comparison of these features between the separate entities.

Deposit diseases are a group of heterogenous diseases characterized by the pathological accumulation of deposits in cells and/or in the extracellular environment. The pathophysiology of these deposits is polymorphic and numerous substances can accumulate: endogenous substances, exogenous substances. Their consequences will vary, depending on the tissue in which the accumulation takes place. The role of the pathologist is essential for the diagnosis of a deposit disease and (s)he has numerous tools at her/his disposal for the diagnosis and typing of these deposits. Special stains can be used, but also immunohistochemistry, immunofluorescence, electron microscopy and more innovative techniques such as proteomics.

Hepatocellular carcinoma (HCC) is the most common primary malignant liver tumour, with a poor prognosis, ranking third for cancer mortality worldwide. HCC is a morphologically and molecularly heterogeneous tumour. This update aims to address this heterogeneity by describing the different histological and molecular subtypes of HCC. Morphologically, eight subtypes have been described according to the WHO classification: steatohepatitic, macrotrabecular massive (MTM), clear cell, chromophobe, scirrhous, fibrolamellar, lymphocyte-rich and neutrophil-rich. Other HCCs are classified as non-specific (not otherwise specified or NOS). These subtypes may be associated with a different prognosis, particularly the MTM, which displays a poorer survival than the other subtypes. Genomically, most HCCs present mutations in the TERT promoter, while other mutations occured later in carcinogenesis, such as TP53 and CTNNB1. TP53 mutated HCCs are associated with a poor prognosis and the MTM subtype. From a transcriptomic standpoint, two classifications are particularly noteworthy, as they are associated with both prognosis (proliferative vs. non-proliferative classification) and clinical, morphological and genomic tumour characteristics (G1-G6 classification). In conclusion, the morphological heterogeneity of HCC, directly linked to molecular heterogeneity, is associated with prognosis. This strongly supports the specification of the different HCC subtypes in our reports.

Amyloid angiopathy represents 20% of causes of intraparenchymal hematoma. The histopathological examination must carefully look for characteristic vascular abnormalities and the immunohistochemistry for the βA4 peptide must be carried out systematically. In practice, the evacuation products of an intracerebral hematoma must be fully included, because the lesions may be focal. The existence of a neurosurgical history may suggest an iatrogenic etiology when patients are less than 55 years-old.

Thesaurismosis or storage diseases are rare genetic disorders due to an abnormal accumulation of an organic compound or its metabolite within cells. These conditions are either secondary to a defect in catabolism caused by enzymatic dysfunction or to a deficiency in transport proteins. They encompass lysosomal storage diseases, lipid storage diseases or dyslipidemias, and glycogen storage disorders or glycogenoses. Diagnosis is typically based on clinical and biological anomalies but may be made or suggested by the pathologist when symptoms are atypical or when biochemical or genetic tests are challenging to interpret. For accurate diagnosis, it is crucial to freeze a portion of the samples. Special staining and electronic microscopy can also aid in the diagnostic process. As the diagnosis is multidisciplinary, collaboration with clinicians, biochemists and geneticists is essential.

Iron is essential for functioning of cells and of the body as a whole. Perls staining allows the histopathological identification of iron deposits. By classifying hepatic siderosis as parenchymal, mesenchymal or mixed, it may guide the search for its etiology. HFE1 hemochromatosis is the most common siderosis. Its diagnosis is currently based on genetic analysis. Its expressivity being variable and its penetrance incomplete, the demonstration of hepatic siderosis may represent a mode of discovery.

As in other organs, the diagnosis of endogenous cutaneous overload diseases is based on histopathological analysis of the lesions using special stainings, even if the clinical appearance is sometimes very suggestive. The lesions are sometimes very subtle and can be included in the group of "invisible" dermatoses, such as primary macular cutaneous amyloidosis or calciphylaxis. Superficial dermal melanosis or pigmentary incontinence generally reflects the post-inflammatory stage of a chronic or recurrent interface dermatitis. Section levels should be systematically performed to look for active lesions of diagnostic interest: Alcian blue staining to identify dermal mucinosis (connectivitis) and pan-T markers (fixed pigmented erythema, lichenoid mycosis fungoides, and vitiligo). Some pathologies have a prognostic impact, either because they reflect an underlying disease, monoclonal gammopathies, in particular myeloma, being one of the most common conditions in this context (AL amyloidosis, xanthoma and xanthogranuloma, scleromyxedema), or because they can be associated with visceral damage (AL amyloidosis, scleromyxedema). The clinical-pathological comparison is mandatory to rule out differential diagnoses, especially for life-threatening diseases: nodular amyloidosis and primary cutaneous amyloidosis versus systemic AL amyloidosis, papular mucinosis versus scleromyxedema and calcific panniculitis versus calciphylaxis.

Steatosis is defined by hepatocyte accumulation of lipids. Different types of steatosis are described (macro-, medio- and microvacuolar). Macrovacuolar steatosis is a common lesion, mainly observed during metabolic syndrome and excessive alcohol consumption. Steatohepatitis combines steatosis, the presence of ballooned hepatocytes and lobular inflammatory foci. Liver fibrosis is the main consequence of steatohepatitis. Liver biopsy is the gold standard diagnostic test.

Rhabdomyosarcomas form a heterogeneous group of malignant soft tissue tumors characterized by immature striated muscle differentiation. Epithelioid and spindle cell rhabdomyosarcoma is a recently described entity, mainly localized intraosseously and predominantly found in young patients. Its late diagnosis and high aggressiveness confer a grim prognosis to this tumor, highlighting the importance of early recognition and appropriate management. We present herein the clinical, histopathological, immunohistochemical, and molecular aspects of this entity through a case of misleading presentation.

Extraosseous calcifications correspond to ubiquitous deposits of intra-tissue calcium salts leading to dysfunction of the affected tissue or organ. There are two types: metastatic calcifications and dystrophic calcifications. Their formation mechanism is by mimicking the physiological mineralization process with an "osteoblast-like" cell. The cause of extra-osseous calcification is variable and depends on risk factors. If the subject is young, you will have to think about a genetic syndrome!

Any product injected to fill wrinkles can behave like a foreign body and cause unsightly granulomatous reactions. The subject is constantly evolving, subject to the vagaries of the market. Hyaluronic acid is the most injected product because it is resorbable and probably the least "toxic". In the event of sarcoidosis or an immune disorder, and in the event of vaccination against Covid, granulomas can develop even after very old injections.

The three most common varieties of systemic amyloidosis are transthyretin amyloidosis (ATTR), immunoglobulin amyloidosis (AL) and inflammatory amyloidosis (AA). There are two forms of transthyretin amyloidosis: the wild type, the most common, represents approximately 15% of heart diseases and the genetic, or "mutated" form, which is a rare disease and manifests mainly by peripheral neuropathy and heart disease. Major therapeutic advances have been made in recent years thanks to molecules that stabilize transthyretin and/or prevent its translation by destroying messenger RNA. Immunoglobulin amyloidosis (AL) is a hematological disease whose severity is due to the toxicity of immunoglobulin light chains forming amyloid deposits that are toxic to tissues, particularly the heart and kidneys. Treatments for immunoglobulin amyloidosis are increasingly effective, and target the plasma cell, leading to an overall improvement in the prognosis, with cardiac involvement being the most worrying condition. Inflammatory amyloidosis (AA) complicates chronic inflammatory diseases less often due to the effectiveness of anti-inflammatory biotherapies in inflammatory rheumatism, chronic inflammatory bowel diseases and genetic auto-inflammatory diseases. The causes of inflammatory amyloidosis are now more diverse with an increase in cases of unknown cause associated or not with obesity.

Monoclonal immunoglobulins can form deposits other than amyloidosis in various tissues. Immunofluorescence is the key analysis for the identification of monoclonal immunoglobulin deposits. Pulmonary light chain deposition disease is a diagnosis to be considered when dealing with diffuse cystic lung disease.

Exogenous lipid pneumonia is an underdiagnosed pathology due to acute or chronic inhalation of lipid substances (paraffin oil, nasal drops, etc.). The clinical and CT appearance is polymorphic, ranging from fibrosing interstitial lung disease to a pseudo-tumoral nodule. The key to CT diagnosis is fatty hypodensity within condensations. The key to histopathological diagnosis is the presence of macrovacuoles in the macrophages, Oil-red-O+(on fresh/frozen material). The differential diagnosis is represented by endogenous lipid pneumonia, common and associated with bronchial obstruction, with necrotic changes, and specific entities such as alveolar proteinosis, hoarding (Niemann-Pick disease) or cordarone treatment.

Despite the advent of scintigraphic diagnosis of transthyretin amyloidosis, the role of pathologists remains central for the diagnosis and typing of amyloidosis. The anatomo-pathological diagnosis and typing of amyloidosis are hindered by pitfalls that should be known, linked to the Congo red staining (technical implementation and interpretation) and the difficulties of immunohistochemistry. The use of expert centers is recommended for difficult cases and when the type cannot be confirmed.

This review focuses on uterine mesenchymal tumors that are defined on a molecular level by a single and unique genetic alteration, that is somehow necessary and sufficient to allow tumor growth and progression. Although diverse from a clinical, morphological and immunohistochemical point of view, the different entities we are going to talk about share both a simple genomic profile with a low number of chromosomal alterations observed by CGH Array (few deletions, gains or amplifications...) and a low mutational burden observed by sequencing technics. Some of these entities are already well known and described in the literature when found outside of the uterus and gynecological tract. It remains intriguing that uterine mesenchymal pathology has been lagging behind when compared to its extrauterine counterpart. How can we explain that when it comes to inflammatory myofibroblastic tumors, abundant numbers of articles have been published since the 70's, but it was only in the early 2000s that the first relevant descriptions of this tumor in the uterus emerged? Certainly, the increased accuracy, availability, and use of molecular biology technics and in particular RNA sequencing in the area of uterine pathology can partly explain the reduction of the gap between soft tissue and uterine pathology we currently observe. Other reasons explaining this gap may be the high prevalence of smooth muscle tumors in the uterus and the abounding diversity of their morphological aspects, which may have partly eclipsed the array of differential diagnoses. Last but not least, one can hypothesize that the relative "simplicity" of hysterectomy procedures, referring to their safety and accessibility, has cured most of the lesions and partly clouded our knowledge regarding the biological potential and natural history of these newly described entities. As a consequence of this situation, our reader will often encounter the wording "uncertain malignant potential", as for some of these rare entities, evidence to establish reliable prognostic variables is still insufficient. We hope this review to be a useful tool to guide pathologists through the diversity and complexity of uterine mesenchymal tumors. As a scientific and medical community, sharing this knowledge will help us to collectively raise our vigilance and awareness by expanding the array of our differential diagnoses. We hope this will lead to more cases being accurately diagnosed, and ultimately, to a deeper knowledge regarding the biological potential and clinical evolution of these tumors. From a therapeutical point of view, the consequences of an accurate diagnosis for the patient are already appreciable through the use of targeted therapy. Examples include: ALK inhibitors in inflammatory myofibroblastic tumor, tyrosine-kinase inhibitors in COL1A::PDGFB rearranged sarcomas or mTOR inhibitors in PEComa.

Mooren's ulcer is a painless and idiopathic ulcer of the peripheral cornea related to autoimmunity against a corneal stromal antigen, calgranulin C. Corneal involvement is isolated. There are no specific histopathological features to differentiate Mooren's ulcer from pseudo-Mooren's, the latter being part of a systemic disease. Mooren ulcer is a diagnosis of elimination based on a complete etiological check-up. However, histopathological examination, when performed, could provide additional data to support the diagnosis. We report the case of a 78-year-old female patient who presented with Mooren's ulcer. The patient had complained of red eye and photophobia for two weeks. Initial visual acuity was "counting fingers". Clinical examination revealed a perforated perilimbic ulcer with an iris prolapse. Based on the peripheral ulcerative keratitis, with a negative etiological work-up and anatomopathological analysis of the cornea, the diagnosis of Mooren's ulcer was proposed. This rare case illustrates the need for a multidisciplinary approach involving ophthalmologists, pathologists, and internists, to reach a diagnosis and optimize the functional prognosis.

In 2019, the 5th edition of the WHO classification of digestive tumours has retained the terminology "goblet cell adenocarcinoma" (GCA) to designate a tumour whose amphicrine nature owed it more than ten denominations since its initial description among which the most tenacious "goblet cell carcinoid" is no longer recommended today. This rare tumour represents 15-19% of appendicular tumours. Its incidence is rising. The positive diagnosis is based on morphological examination and mandatory identification of a low-grade component of glands comprising goblet cells stained by PAS and Alcian blue. The appendix must be entirely examined. Global tumour grade (low, intermediate, high) is based on the proportions of low-grade and high-grade components. This tumour's immunohistochemical profile is particular because of expression of CK20 and often CK7 as well as neuroendocrine markers. It is often an incidental finding on a surgical specimen, among individuals aged 50 or more years, presenting with a locally advanced stage with vascular and perineural invasion. Lymph node metastases are present in a third of cases. Non-specific mutations of ARID1A and genes of the Wnt pathway may be identified. GCA is not associated with microsatellite instability or Lynch syndrome. Its prognosis is intermediate. Surgery is the reference therapy based on the stage. The main differential diagnoses are colorectal adenocarcinoma NOS, mucinous adenocarcinoma and signet ring cell adenocarcinoma. Patients are referred to the RENAPE expert network.