Malignant germ cell tumors of the mediastinum are uncommon, representing only 3 to 10% of tumors originating in the mediastinum. They are much less common than germ cell tumors arising in the testes, and account for only 1 to 5% of all germ cell neoplasms.

Syndromes associated with mediastinal germ cell tumors include Hematologic Neoplasia and Klinefelter's syndrome.

Most mammary tumors in rats are benign fibroadenomas, which are also the most common tumor in the rat. Less than 10 percent are adenocarcinomas. They occur in male and female rats. The tumors can be large and occur anywhere on the trunk. There is a good prognosis with surgery. Spayed rats have a decreased risk of developing mammary tumors.

Mammary tumors are the third most common neoplasia in cats, following lymphoid and skin cancers. The incidence of mammary tumors in cats is reduced by 91 percent in cats spayed prior to six months of age and by 86 percent in cats spayed prior to one year, according to one study. Siamese cats and Japanese breeds seem to have increased risk, and obesity also appears to be a factor in tumor development. Malignant tumors make up 80 to 96 percent of mammary tumors in cats, almost all adenocarcinomas. Male cats may also develop mammary adenocarcinoma, albeit rarely, and the clinical course is similar to female cats. As in dogs, tumor size is an important prognostic factor, although for tumors less than three centimeters the individual size is less predictive. According to one study, cats with tumors less than three cm had an average survival time of 21 months, and cats with tumors greater than three cm had an average survival of 12 months. About 10 percent of cat mammary tumors have estrogen receptors, so spaying at the time of surgery has little effect on recurrence or survival time. Metastasis tends to be to the lungs and lymph nodes, and rarely to bone. Diagnosis and treatment is similar to the dog. There is a better prognosis with bilateral radical surgery (removing the both mammary chains) than with more conservative surgery. Doxorubicin has shown some promise in treatment.

Neoplastic tumors are often heterogeneous and contain more than one type of cell, but their initiation and continued growth is usually dependent on a single population of neoplastic cells. These cells are presumed to be clonal – that is, they are derived from the same cell,

and all carry the same genetic or epigenetic anomaly – evident of clonality. For lymphoid neoplasms, e.g. lymphoma and leukemia, clonality is proven by the amplification of a single rearrangement of their immunoglobulin gene (for B cell lesions) or T cell receptor gene (for T cell lesions). The demonstration of clonality is now considered to be necessary to identify a lymphoid cell proliferation as neoplastic.

It is tempting to define neoplasms as clonal cellular proliferations but the demonstration of clonality is not always possible. Therefore, clonality is not required in the definition of neoplasia.

A neoplasm can be caused by an abnormal proliferation of tissues, which can be caused by genetic mutations. Not all types of neoplasms cause a tumorous overgrowth of tissue, however (such as leukemia or carcinoma in situ).

Recently, tumor growth has been studied using mathematics and continuum mechanics. Vascular tumors (formed from blood vessels) are thus looked at as being amalgams of a solid skeleton formed by sticky cells and an organic liquid filling the spaces in which cells can grow. Under this type of model, mechanical stresses and strains can be dealt with and their influence on the growth of the tumor and the surrounding tissue and vasculature elucidated. Recent findings from experiments that use this model show that active growth of the tumor is restricted to the outer edges of the tumor, and that stiffening of the underlying normal tissue inhibits tumor growth as well.

Benign conditions that are "not" associated with an abnormal proliferation of tissue (such as sebaceous cysts) can also present as tumors, however, but have no malignant potential. Breast cysts (as occur commonly during pregnancy and at other times) are another example, as are other encapsulated glandular swellings (thyroid, adrenal gland, pancreas).

Encapsulated hematomas, encapsulated necrotic tissue (from an insect bite, foreign body, or other noxious mechanism), keloids (discrete overgrowths of scar tissue) and granulomas may also present as tumors.

Discrete localized enlargements of normal structures (ureters, blood vessels, intrahepatic or extrahepatic biliary ducts, pulmonary inclusions, or gastrointestinal duplications) due to outflow obstructions or narrowings, or abnormal connections, may also present as a tumor. Examples are arteriovenous fistulae or aneurysms (with or without thrombosis), biliary fistulae or aneurysms, sclerosing cholangitis, cysticercosis or hydatid cysts, intestinal duplications, and pulmonary inclusions as seen with cystic fibrosis. It can be dangerous to biopsy a number of types of tumor in which the leakage of their contents would potentially be catastrophic. When such types of tumors are encountered, diagnostic modalities such as ultrasound, CT scans, MRI, angiograms, and nuclear medicine scans are employed prior to (or during) biopsy or surgical exploration/excision in an attempt to avoid such severe complications.

The nature of a tumor is determined by imaging, by surgical exploration, or by a pathologist after examination of the tissue from a biopsy or a surgical specimen.

MCACL has a much more favorable prognosis than most other forms of adenocarcinoma and most other NSCLC's. Cases have been documented of continued growth of these lesions over a period of 10 years without symptoms or metastasis. The overall mortality rate appears to be somewhere in the vicinity of 18% to 27%, depending on the criteria that are used to define this entity.

Accurate incidence statistics on MCACL are unavailable. It is a very rare tumor, with only a few dozen cases reported in the literature to date.

In the few cases described in the literature to date, the male-to-female ratio is approximately unity, and right lung lesions occurred twice as commonly as left lung lesions. Approximately 2/3 of cases have been associated with tobacco smoking. Cases have been reported in patients as young as 29.

LCIS (lobular neoplasia is considered pre-cancerous) is an indicator (marker) identifying women with an increased risk of developing invasive breast cancer. This risk extends more than 20 years. Most of the risk relates to subsequent invasive ductal carcinoma rather than to invasive lobular carcinoma.

While older studies have shown that the increased risk is equal for both breasts, a more recent study suggests that the ipsilateral (same side) breast may be at greater risk.

Some investigators suggest that this distribution arises as a consequence of abnormal migration of germ cells during embryogenesis. Others hypothesize a widespread distribution of germ cells to multiple sites during normal embryogenesis, with these cells conveying genetic information or providing regulatory functions at somatic sites.

Spermatocytic seminoma is not considered a subtype of seminoma and, unlike seminoma and most other germ cell tumours, it does not arise from "intratubular germ cell neoplasia". It has not been described as arising in locations outside the testis, and does not occur in association with other germ cell tumours.

The new WHO classification, due to be published in 2016, will rename the condition spermatocytic tumour to avoid confusion with classical seminoma as the biology and treatment are different.

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.

Although estimates vary, the annual incidence of clinically significant neuroendocrine tumors is approximately 2.5–5 per 100,000; two thirds are carcinoid tumors and one third are other NETs.

The prevalence has been estimated as 35 per 100,000, and may be considerably higher if clinically silent tumors are included. An autopsy study of the pancreas in people who died from unrelated causes discovered a remarkably high incidence of tiny asymptomatic NETs. Routine microscopic study of three random sections of the pancreas found NETs in 1.6%, and multiple sections identified NETs in 10%. As diagnostic imaging increases in sensitivity, such as endoscopic ultrasonography, very small, clinically insignificant NETs may be coincidentally discovered; being unrelated to symptoms, such neoplasms may not require surgical excision.

Neuroendocrine tumors (NETs) are neoplasms that arise from cells of the endocrine (hormonal) and nervous systems. Many are benign, while some are malignant. They most commonly occur in the intestine, where they are often called carcinoid tumors, but they are also found in the pancreas, lung and the rest of the body.

Although there are many kinds of NETs, they are treated as a group of tissue because the cells of these neoplasms share common features, such as looking similar, having special secretory granules, and often producing biogenic amines and polypeptide hormones.

Seminoma (also known as "pure seminoma" or "classical seminoma") is a germ cell tumor of the testicle or, more rarely, the mediastinum or other extra-gonadal locations. It is a malignant neoplasm and is one of the most treatable and curable cancers, with a survival rate above 95% if discovered in early stages.

Testicular seminoma originates in the germinal epithelium of the seminiferous tubules. About half of germ cell tumors of the testicles are seminomas. Treatment usually requires removal of one testicle. However, fertility usually isn't affected. All other sexual functions will remain intact.

The average age at time of EIN diagnosis is approximately 52 years, compared to approximately 61 years for carcinoma. The timeframe and likelihood of EIN progression to cancer, however, is not constant amongst all women. Some cases of EIN are first detected as residual premalignant disease in women who already have carcinoma, whereas other EIN lesions disappear entirely and never lead to cancer. For this reason, treatment benefits and risks must be individualized for each patient under the guidance of an experienced physician.

Risk factors for development of EIN and the endometrioid type of endometrial carcinoma include exposure to estrogens without opposing progestins, obesity, diabetes, and rare hereditary conditions such as hereditary nonpolyposis colorectal cancer. Protective factors include use of combined oral contraceptive pills (low dose estrogen and progestin), and prior use of a contraceptive intrauterine device.

Spermatocytic seminomas are not considered a subtype of seminoma and unlike other germ cell tumours do not arise from intratubular germ cell neoplasia.

Germ cell neoplasia in situ, abbreviated GCNIS, represents the precursor lesion for many types of testicular germ cell tumors. As the name suggests, it represents a neoplastic process of germ cells that is confined to the spermatogonial niche.

The term GCNIS was introduced with the 2016 edition of the WHO classification of urological tumours. It replaces the previous term intratubular germ cell neoplasia, abbreviated ITGCN or IGCN and also known as testicular intratubular germ cell neoplasia and intratubular germ cell neoplasia of the testis. GCNIS more accurate describes the lesion as it arises between the basement membrane and Sertoli cells (the cells that 'nurse' the developing germ cell). The common, unspecified variant of the entity was once considered to be a carcinoma in situ although the term "carcinoma "in situ"" is now largely historical as it is not an accurate description of the process.

Parathyroid cancer occurs in midlife at the same rate in men and women.

Conditions that appear to result in an increased risk of parathyroid cancer include multiple endocrine neoplasia type 1, autosomal dominant familial isolated hyperparathyroidism and hyperparathyroidism-jaw tumor syndrome (which also is hereditary). Parathyroid cancer has also been associated with external radiation exposure, but, most reports describe an association between radiation and the more common parathyroid adenoma.

LCIS may be treated with close clinical follow-up and mammographic screening, tamoxifen or related hormone controlling drugs to reduce the risk of developing cancer, or bilateral prophylactic mastectomy. Some surgeons consider bilateral prophylactic mastectomy to be overly aggressive treatment except for certain high-risk cases.

The World Health Organisation classification of testicular tumours subdivides ITGCN into (1) a more common, unspecified type (ITGCNU), and (2) other specific subtypes. The most common specific subtypes are intratubular embryonal carcinoma and intratubular seminoma.

There is increased life-time risk of secondary cancers (relative risk 3.63), with a slightly increased mortality risk (1.21) according to a 2004 Swedish study of 481 patients.

Genetic changes are very high in SCLC and LCNEC, but usually low for TC, intermediate for AC.

Without treatment, persons with MEN2B die prematurely. Details are lacking, owing to the absence of formal studies, but it is generally assumed that death in the 30s is typical unless prophylactic thyroidectomy and surveillance for pheochromocytoma are performed (see below). The range is quite variable, however: death early in childhood can occur, and it is noteworthy that a few untreated persons have been diagnosed in their 50s. Recently, a larger experience with the disease "suggests that the prognosis in an individual patient may be better than previously considered."

Thyroidectomy is the mainstay of treatment, and should be performed without delay as soon as a diagnosis of MEN2B is made, even if no malignancy is detectable in the thyroid. Without thyroidectomy, almost all patients with MEN2B develop medullary thyroid cancer, in a more aggressive form than MEN 2A. The ideal age for surgery is 4 years old or younger, since cancer may metastasize before age 10.

Pheochromocytoma - a hormone secreting tumor of the adrenal glands - is also present in 50% of cases. Affected individuals are encouraged to get yearly screenings for thyroid and adrenal cancer.

Because prophylactic thyroidectomy improves survival, blood relatives of a person with MEN2B should be evaluated for MEN2B, even if lacking the typical signs and symptoms of the disorder.The mucosal neuromas of this syndrome are asymptomatic and self-limiting, and present no problem requiring treatment. They may, however, be surgically removed for aesthetic purposes or if they are being constantly traumatized.

In medicine, desmoplasia is the growth of fibrous or connective tissue. It is also called desmoplastic reaction to emphasize that it is secondary to an insult. Desmoplasia may occur around a neoplasm, causing dense fibrosis around the tumor, or scar tissue (adhesions) within the abdomen after abdominal surgery.

Desmoplasia is usually only associated with malignant neoplasms, which can evoke a fibrosis response by invading healthy tissue. Invasive ductal carcinomas of the breast often have a scirrhous, stellate appearance caused by desmoplastic formations.

Pheochromocytoma is seen in between two and eight in 1,000,000, with approximately 1000 cases diagnosed in United States yearly. It mostly occurs in young or middle age adults, though it presents earlier in hereditary cases.

- About 10% of adrenal cases are bilateral (suggesting hereditary disease)

- About 10% of adrenal cases occur in children (also suggesting hereditary disease)

- About 15% are extra-adrenal (located in any orthosympathetic tissue): Of these 9% are in the abdomen, and 1% are located elsewhere. Some extra-adrenal pheochromocytomas are probably actually paragangliomas, but the distinction can only be drawn after surgical resection.

- About 11.1% of adrenal cases are malignant, but this rises to 30% for extra-adrenal cases

- About 15–20% are hereditary

- About 5% are caused by VHL disease

- About 3% recur after being resected

- About 14% of affected individuals do not have arterial hypertension (Campbell's Urology)