Perivascular epithelioid cell tumour, also known as PEComa or PEC tumour, is a family of mesenchymal tumours consisting of perivascular epithelioid cells (PECs). These are rare tumours that can occur in any part of the human body.

The cell type from which these tumours originate remains unknown. Normally, no perivascular epitheloid cells exist; the name refers to the characteristics of the tumour when examined under the microscope.

Establishing the malignant potential of these tumours remains challenging although criteria have been suggested; some PEComas display malignant features whereas others can cautiously be labeled as having 'uncertain malignant potential'. The most common tumours in the PEComa family are renal angiomyolipoma and pulmonary lymphangioleiomyomatosis, both of which are more common in patients with tuberous sclerosis complex. The genes responsible for this multi-system genetic disease have also been implicated in other PEComas.

Many PEComa types shows a female predominance in the sex ratio.

In general, epithelioid sarcoma is a slow-growing and relatively painless tumor, often resulting in a lengthy period of time between presentation and diagnosis. Due to its ambiguity, it is often misdiagnosed, mistaken as a persistent wart or cyst. It most commonly presents itself in the distal limbs (fingers, hands, forearms, or feet) as a small, soft mass or a series of bumps. It is most often described as a firm-to-hard palpable mass, either in the deep soft tissue or in the dermis. Often, superficial lesions will ulcerate causing a mistaken diagnosis of a poorly healing traumatic wound or wart. About 13% of patients will present with multifocal tumors, and about 13% of patients will present with metastatic disease.

Epithelioid sarcoma is a rare soft tissue sarcoma arising from Mesenchyme tissue and characterized by epithelioid-like features. It accounts for less than 1% of all soft tissue sarcomas. It was first clearly characterized by F.M. Enzinger in 1970. It commonly presents itself in the distal limbs (fingers, hands, forearms, or feet) of young adults as a small, soft mass or a series of bumps. A proximal version has also been described, frequently occurring in the upper extremities. Rare cases have been reported in the pelvis, vulva, penis, and spine.

Histologically, epithelioid sarcoma forms nodules with central necrosis surrounded by bland, polygonal cells with eosinophilic cytoplasm and peripheral spindling. Epithelioid sarcomas typically express vimentin, cytokeratins, epithelial membrane antigen, and CD34, whereas they are usually negative for S100, desmin, and FLI-1. They typically stain positive for CA125.

Epithelioid sarcoma most commonly strikes young adults, yet no age group is immune. The disease has a tendency to develop local recurrences and metastasis thereafter to regional lymph nodes, lung, bone, brain, and other locations, including the scalp. Generally speaking, epithelioid sarcoma has a high rate of relapse after initial treatment and tends to recur locally (at or near the original tumor site). Epithelioid sarcoma also demonstrates lymphatic spread (in 22-48% of cases), and metastasis (in 21-63% of cases). These events, as well as advanced stage (progression) and grade (aggressiveness), are predictive of an overall worse outcome. The overall five-year survival rate for epithelioid sarcoma is anywhere from 25 to 78%. Importantly, the 10-year and 15-year survival rate drops off significantly. Associated with a more positive outcome are younger age, female vs. male sex, distal vs. proximal location, smaller tumor size, and negative margins upon tumor resection.

The precursor cell of PEComas is currently unknown; there is no normal counterpart "perivascular epitheloid cell". Genetically, PECs are linked to the tuberous sclerosis genes TSC1 and TSC2, although this link is stronger for angiomyolipoma and lymphangioleiomyomatosis than for other members of the PEComa family.

It presents as a slow growing mass that especially affects tendons and aponeuroses and it is deeply situated. Patients often perceive it as a lump or hard mass. It causes either pain or tenderness but only until it becomes large enough. This kind of tumor is commonly found in the extremities especially around the knee, feet and ankle. Patients diagnosed with clear cell sarcoma are usually between the ages of 20 and 40.

Large cell lung carcinoma with rhabdoid phenotype (LCLC-RP) is a rare histological form of lung cancer, currently classified as a variant of large cell lung carcinoma (LCLC). In order for a LCLC to be subclassified as the phenotype variant, at least 10% of the malignant tumor cells must contain distinctive structures composed of tangled intermediate filaments that displace the cell nucleus outward toward the cell membrane. The whorled eosinophilic inclusions in LCLC-RP cells give it a microscopic resemblance to malignant cells found in rhabdomyosarcoma (RMS), a rare neoplasm arising from transformed skeletal muscle. Despite their microscopic similarities, LCLC-RP is not associated with rhabdomyosarcoma.

Although rhabdoid variants of LCLC are sometimes referred to as "rhabdoid carcinomas", this particular term should be reserved for examples of "pure" rhabdoid neoplasms (i.e. those that do not contain cells containing other histological variants)

Solid pseudopapillary tumours are typically round, well-demarcated, measuring 2–17 cm in diameter (average 8 cm), with solid and cystic areas with hemorrhage on cut sections.

The differential diagnosis of LCLC-RP includes secondary metastatic lesions, melignant melanoma of the lung with rhabdoid phenotype, mucinous adenocarcinomas (particularly those featuring signet-ring cells), rhabdomyosarcoma, epitheloid angiosarcoma, pleural mesothelioma, and plasmacytoma.

A solid pseudopapillary tumour (also known as solid pseudopapillary neoplasm or, more formally, solid pseudopapillary tumour/neoplasm of the pancreas) is a low-grade malignant neoplasm of the pancreas of architecture that typically afflicts young women.

Synonyms for FA include well differentiated fetal adenocarcinoma, high-grade fetal adenocarcinoma, pulmonary adenocarcinoma

of fetal type, and pulmonary endodermal tumour resembling fetal lung.

GP consist of three components (1) ganglion cells, (2) epithelioid cells (neuroendocrine-like), and (3) spindle cells (schwannoma-like). The microscopic differential diagnosis includes poorly differentiated carcinoma, neuroendocrine tumour and paraganglioma.

GPs may be sporadic or arise in the context neurofibromatosis type 1.

FA can produce repeated hemoptysis, possibly related to cavitation of the tumor.

Other presenting symptoms described have included: flu-like syndrome with cough and fever,

When the tumor is large and there is presence of necrosis and local recurrence, the prognosis is poor. Presence of metastasis occurs in more than 50% cases and the common places of its occurrence are the bone, lymph node and lungs. Five-year survival rates, which are reported to be between 50-65%, can be misleading because the disease is prone to late metastasis or recurrence. Ten and twenty-year survival rates are 33% and 10%, respectively.

A gangliocytic paraganglioma, abbreviated GP, is a rare tumour that is typically found in the duodenum and consists of three components: (1) ganglion cells, (2) epithelioid cells (paraganglioma-like) and, (3) spindle cells (schwannoma-like).

Congenital mesoblastic nephroma typically (76% of cases) presents as an abdominal mass which is detected prenatally (16% of cases) by ultrasound or by clinical inspection (84% of cases) either at birth or by 3.8 years of age (median age ~1 month). The neoplasm shows a slight male preference. Concurrent findings include hypertension (19% of cases), polyhydramnios (i.e. excess of amniotic fluid in the amniotic sac) (15%), hematuria (11%), hypercalcemia (4%), and elevated serum levels of the kidney-secreted, hypertension-inducing enzyme, renin (1%). Congenital anomalies have been reported in 11 patients: 6 with genitourinary anomalies, 2 with gastrointestinal anomalies, 1 with hydrocephalus, and 1 with the Beckwith–Wiedemann syndrome. The vast majority of patients present with localized (i.e. non-metastatic) disease. Most patients' disease is classified at presentation as stage I or II (i.e. localized), few patients present with stage III (i.e. locally advanced/infiltrating), and virtually no patients present with stage IV (metastases present or V (i.e. tumors in both kidneys) disease (see staging of renal cancer).

Spermatocytic seminoma is a rare tumour, making up only one to two percent of all testicular germ cell tumours. Men presenting with this tumour are generally 50 to 60 years old, and its occurrence is rare in men under 30 years old. Most present with slow, painless testicular enlargement, which may involve both testes.

Squamous epithelial cells are not found in normal thyroid, thus the origin of SCTC is not clear. However, it might be a derived from the embryonic remnants such as thyroglossal duct or branchial clefts. Often SCTC is diagnosed in one of the thyroid lobes, but not in the pyramidal lobe. Another possible way of SCTC development can be through the squamous metaplasia of cells. However, that theory is also controversial, since the Hashimoto's thyroiditis and chronic lymphocytic thyroiditis (neoplasms to be showed squamous metaplasia) are not associated with SCTC. Primary STCT is usually diagnosed in both lobes of thyroid gland. The histopathology of STCT shows a squamous differentiation of tumor cells.

Benign and borderline variants of this neoplasm are rare, and most cases are malignant.

These tumors may have a worse prognosis than serous tumors.

Hemangioendothelioma is used to describe a group of vascular neoplasms that may be considered benign as well as malignant, depending on the specific group member's activity.

Squamous-cell thyroid carcinoma (SCTC) is rare malignant neoplasm of thyroid gland which shows tumor cells with distinct squamous differentiation. The incidence of SCTC is less than 1% out of thyroid malignancies.

Diagnosis of mesoblastic nephroma and its particular type (i.e. classic, mixed, or cellular) is made by histological examination of tissues obtained at surgery. Besides its histological appearance, various features of this disease aid in making a differential diagnosis that distinguish it from the following childhood neoplasms:

- Wilm's tumor is the most common childhood kidney neoplasm, representing some 85% of cases. Unlike mesoblastic nephroma, 3 years of age. Bilateral kidney tumors, concurrent birth defects, and/or metastatic disease at presentation favor a diagnosis of Wilm's tumor.

- congenital infantile sarcoma is a rare aggressive sarcoma typically presenting in the lower extremities, head, or neck of infants during their first year of life. The histology, association with the "ETV6-NRTK3" fusion gene along with certain chromosome trisomies, and the distribution of markers for cell type (i.e. cyclin D1 and Beta-catenin) within this tumor are the same as those found in cellular mesoblastic nephroma. Mesoblastic nephroma and congenital infantile sarcoma appear to be the same diseases with mesoblastic lymphoma originating in the kidney and congenital infantile sarcoma originating in non-renal tissues.

- Rhabdoid tumor, which accounts for 5-510% of childhood kidney neoplasms, occurs predominantly in children from 1 to 2 years of age. Unlike mesoblastic nephroma, rhabdoid tumors may present with tumors in other tissues including in ~13% of cases, the brain. Rhabdoid tumors have a distinctive histology and abnormalities (i.e. loss of heterozygosity, single nucleotide polymorphism, and deletions) in chromosome 22.

- Clear cell sarcoma of the kidney, which is responsible for 5-10% of childhood pediatric tumors, occurs predominantly in children from 2 to 3 years of age. Unlike meoblastic nephorma, clear cell sarcoma of the kidney presents with metastasis, particularly to bone, in 5-6% of cases; it histology is diverse and has been mistaken for mesoblastic nephroma. One chromosomal translocations t,(10;17)(q22;p13), has been repeatedly reported to be associated with clear cell sarcoma of the kidney.

- Infantile myofibromatosis is a fibrous tumor of infancy and childhood most commonly presenting during the first 2 years of life as a single subcutaneous nodule of the head and neck region or less commonly as multiple lesions of skin, muscle, bone, and in ~33% of these latter cases, visceral organs. All of these lesions have an excellent prognosis and can regress spontaneously except for those in which there is visceral involvement where the prognosis is poor. While infantile myofibromatosis and classic mesoblastic nephroma have been suggested to be the same diseases because of their very similar histology, studies on the distribution of cell-type markers (i.e. cyclin D1 and Beta-catenin) indicate that they have different cellular origins.

Spermatocytic seminoma is a neoplasm of the testis ("i.e." a tumour of the testis), and classified as a germ cell tumour.

The name of the tumour comes from the similarity (under the microscope) between the small cells of the tumour and secondary spermatocytes.

There is a wide range of symptoms that patients show. Symptoms can lie dormant, but come about due to Obstructive hydrocephalus. These symptoms include:

- Intracranial pressure

- Headache

- Papilledema

- Vomiting

- Light headedness

- Impaired mental activity

- Gait instability

In rare and extreme cases, more severe symptoms can be observed:

- Memory disturbance

- Dementia

- Hemiparesis

- Seizures

- Hemorrhage

- Psychosis

Patients typically present with a non-productive cough and weight loss.

Typically, they are cystic neoplasms with polypoid masses that protrude into the cyst. On microscopic pathological examination, they are composed of cells with clear cytoplasm (that contains glycogen) and "hob nail" cells (from which the glycogen has been secreted). The pattern may be glandular, papillary or solid.