Classically acute radiation syndrome is divided into three main presentations: hematopoietic, gastrointestinal, and neurological/vascular. These syndromes may or may not be preceded by a prodrome. The speed of onset of symptoms is related to radiation exposure, with greater doses resulting in a shorter delay in symptom onset. These presentations presume whole-body exposure and many of them are markers which are not valid if the entire body has not been exposed. Each syndrome requires that the tissue showing the syndrome itself be exposed. The hematopoietic syndrome requires exposure of the areas of bone marrow actively forming blood elements (i.e., the pelvis and sternum in adults). The neurovascular symptoms require exposure of the brain. The gastrointestinal syndrome is not seen if the stomach and intestines are not exposed to radiation. Some areas affected are:

1. Hematopoietic. This syndrome is marked by a drop in the number of blood cells, called aplastic anemia. This may result in infections due to a low amount of white blood cells, bleeding due to a lack of platelets, and anemia due to few red blood cells in the circulation. These changes can be detected by blood tests after receiving a whole-body acute dose as low as 0.25 Gy, though they might never be felt by the patient if the dose is below 1 Gy. Conventional trauma and burns resulting from a bomb blast are complicated by the poor wound healing caused by hematopoietic syndrome, increasing mortality.

2. Gastrointestinal. This syndrome often follows absorbed doses of 6–30 Gy (600–3000 rad). The signs and symptoms of this form of radiation injury include nausea, vomiting, loss of appetite, and abdominal pain. Vomiting in this time-frame is a marker for whole body exposures that are in the fatal range above 4 Gy. Without exotic treatment such as bone marrow transplant, death with this dose is common. The death is generally more due to infection than gastrointestinal dysfunction.

3. Neurovascular. This syndrome typically occurs at absorbed doses greater than 30 Gy (3000 rad), though it may occur at 10 Gy (1000 rad). It presents with neurological symptoms such as dizziness, headache, or decreased level of consciousness, occurring within minutes to a few hours, and with an absence of vomiting. It is invariably fatal.

The prodrome (early symptoms) of ARS typically includes nausea and vomiting, headaches, fatigue, fever, and a short period of skin reddening. These symptoms may occur at radiation doses as low as 0.35 Gy (35 rad). These symptoms are common to many illnesses, and may not, by themselves, indicate acute radiation sickness.

Cutaneous radiation syndrome (CRS) refers to the skin symptoms of radiation exposure. Within a few hours after irradiation, a transient and inconsistent redness (associated with itching) can occur. Then, a latent phase may occur and last from a few days up to several weeks, when intense reddening, blistering, and ulceration of the irradiated site are visible. In most cases, healing occurs by regenerative means; however, very large skin doses can cause permanent hair loss, damaged sebaceous and sweat glands, atrophy, fibrosis (mostly Keloids), decreased or increased skin pigmentation, and ulceration or necrosis of the exposed tissue. Notably, as seen at Chernobyl, when skin is irradiated with high energy beta particles, moist desquamation (peeling of skin) and similar early effects can heal, only to be followed by the collapse of the dermal vascular system after two months, resulting in the loss of the full thickness of the exposed skin. This effect had been demonstrated previously with pig skin using high energy beta sources at the Churchill Hospital Research Institute, in Oxford.

Up to 10% of invasive cancers are related to radiation exposure, including both ionizing radiation and non-ionizing radiation. Additionally, the vast majority of non-invasive cancers are non-melanoma skin cancers caused by non-ionizing ultraviolet radiation. Ultraviolet's position on the electromagnetic spectrum is on the boundary between ionizing and non-ionizing radiation. Non-ionizing radio frequency radiation from mobile phones, electric power transmission, and other similar sources have been described as a possible carcinogen by the World Health Organization's International Agency for Research on Cancer, but the link remains unproven.

Exposure to ionizing radiation is known to increase the future incidence of cancer, particularly leukemia. The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. If the linear model is correct, then natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second.

According to the prevalent model, any radiation exposure can increase the risk of cancer. Typical contributors to such risk include natural background radiation, medical procedures, occupational exposures, nuclear accidents, and many others. Some major contributors are discussed below.

Chronic radiation syndrome is a constellation of health effects that occur after months or years of chronic exposure to high amounts of ionizing radiation. Chronic radiation syndrome develops with a speed and severity proportional to the radiation dose received, i.e., it is a deterministic effect of radiation exposure, unlike radiation-induced cancer. It is distinct from acute radiation syndrome in that it occurs at dose rates low enough to permit natural repair mechanisms to compete with the radiation damage during the exposure period. Dose rates high enough to cause the acute form (> ~0.1 Gy/h) are fatal long before onset of the chronic form. The lower threshold for chronic radiation syndrome is between 0.7 and 1.5 Gy, at dose rates above 0.1 Gy/yr. This condition is primarily known from the Kyshtym disaster, where 66 cases were diagnosed, and has received little mention in Western literature. A future ICRP publication, currently in draft, may recognize the condition but with higher thresholds.

In 2013, Alexander V. Akleyev described the chronology of the clinical course or CRS while presenting at ConRad in Munich, Germany. In his presentation, he defined the latent period as being 1-5 years, and the formation coinciding with the period of maximum radiation dose. The recovery period was described as being 3-12 months after exposure ceased. He concluded that "CRS represents a systemic response of the body as a whole to the chronic total body exposure in man." In 2014, Akleyev's book "Comprehensive analysis of chronic radiation syndrome, covering epidemiology, pathogenesis, pathoanatomy, diagnosis and treatment" was published by Springer.

Radiation dermatitis (also known as radiodermatitis) is a skin disease associated with prolonged exposure to ionizing radiation. Radiation dermatitis occurs to some degree in most patients receiving radiation therapy, with or without chemotherapy.

There are three specific types of radiodermatitis: acute radiodermatitis, chronic radiodermatitis, and eosinophilic, polymorphic, and pruritic eruption associated with radiotherapy. Radiation therapy can also cause radiation cancer.

With interventional fluoroscopy, because of the high skin doses that can be generated in the course of the intervention, some procedures have resulted in early (less than two months after exposure) and/or late (two months or more after exposure) skin reactions, including necrosis in some cases.

Radiation dermatitis, in the form of intense erythema and vesiculation of the skin, may be observed in radiation ports.

As many as 95% of patients treated with radiation therapy for cancer will experience a skin reaction. Some reactions are immediate, while others may be later (e.g., months after treatment).

Acute radiodermatitis occurs when an "erythema dose" of ionizing radiation is given to the skin, after which visible erythema appears up to 24 hours after. Radiation dermatitis generally manifests within a few weeks after the start of radiotherapy. Acute radiodermatitis, while presenting as red patches, may sometimes also present with desquamation or blistering. Erythema may occur at a dose of 2 Gy radiation or greater.

Poisoning is a condition or a process in which an organism becomes chemically harmed (poisoned) by a toxic substance or venom of an animal.

Acute poisoning is exposure to a poison on one occasion or during a short period of time. Symptoms develop in close relation to the degree of exposure. Absorption of a poison is necessary for systemic poisoning (that is, in the blood throughout the body). In contrast, substances that destroy tissue but do not absorb, such as lye, are classified as corrosives rather than poisons. Furthermore, many common household medications are not labeled with skull and crossbones, although they can cause severe illness or even death. In the medical sense, toxicity and poisoning can be caused by less dangerous substances than those legally classified as a poison. Toxicology is the study and practice of the symptoms, mechanisms, diagnosis, and treatment of poisoning.

Chronic poisoning is long-term repeated or continuous exposure to a poison where symptoms do not occur immediately or after each exposure. The patient gradually becomes ill, or becomes ill after a long latent period. Chronic poisoning most commonly occurs following exposure to poisons that bioaccumulate, or are biomagnified, such as mercury, gadolinium, and lead.

Contact or absorption of poisons can cause rapid death or impairment. Agents that act on the nervous system can paralyze in seconds or less, and include both biologically derived neurotoxins and so-called nerve gases, which may be synthesized for warfare or industry.

Inhaled or ingested cyanide, used as a method of execution in gas chambers, almost instantly starves the body of energy by inhibiting the enzymes in mitochondria that make ATP. Intravenous injection of an unnaturally high concentration of potassium chloride, such as in the execution of prisoners in parts of the United States, quickly stops the heart by eliminating the cell potential necessary for muscle contraction.

Most biocides, including pesticides, are created to act as poisons to target organisms, although acute or less observable chronic poisoning can also occur in non-target organisms (secondary poisoning), including the humans who apply the biocides and other beneficial organisms. For example, the herbicide 2,4-D imitates the action of a plant hormone, which makes its lethal toxicity specific to plants. Indeed, 2,4-D is not a poison, but classified as "harmful" (EU).

Many substances regarded as poisons are toxic only indirectly, by toxication. An example is "wood alcohol" or methanol, which is not poisonous itself, but is chemically converted to toxic formaldehyde and formic acid in the liver. Many drug molecules are made toxic in the liver, and the genetic variability of certain liver enzymes makes the toxicity of many compounds differ between individuals.

Exposure to radioactive substances can produce radiation poisoning, an unrelated phenomenon.

Radiation-induced thyroiditis is a form of painful, acute thyroiditis resulting from radioactive therapy to treat hyperthyroidism or from radiation to treat head and neck cancer or lymphoma. It affects 1% of those who have received radioactive iodine (I-131) therapy for Graves' Disease, typically presenting between 5 and 10 days after the procedure. Stored T and T are released as rapid destruction of thyroid tissue occurs, resulting in pain, tenderness, and exacerbation of hyperthyroidism.

BAL has similar symptoms with other types of leukemia, but usually more serious.

Symptoms caused by bone marrow damage

Bruising, spotting: the reason is lack of platelets, it is very common in BAL patients, most of patients die due to the

Anemia: Because the decline of hematopoietic function, need blood transfusion therapy

Persistent fever, infection prolonged healing:

Diffuse hemorrhage: also called Septicemia, which is dangerous and might lead to death.

Symptoms caused by blood cancer cells infiltration into tissues:

Lymphadenopathy

Joint pain

Swelling of the gums

Hepatoslenomegaly

Headache and vomiting: blood cancer infiltration into the wear performance of the central nervous system.

Skin lumps: Because look was slightly green, also known as the "Green tumor."

Pericardial or pleural effusion

Anaplastic tumors have a high mitotic rate and lymphovascular invasion. They rapidly invade surrounding tissues (such as the trachea). The presence of regional lymphadenopathy in older patients in whom needle aspiration biopsy reveals characteristic vesicular appearance of the nuclei would support a diagnosis of anaplastic carcinoma.

It is always considered as stage IV.

Initial symptoms can be nonspecific, particularly in children. Over 50% of children with leukemia had one or more of five features: a liver one can feel (64%), a spleen one can feel (61%), pale complexion (54%), fever (53%), and bruising (52%). Additionally, recurrent infections, feeling tired, arm or leg pain, and enlarged lymph nodes can be prominent features. The B symptoms, such as fever, night sweats, and weight loss, are often present as well.

Central nervous system (CNS) symptoms such cranial neuropathies due to meningeal infiltration are identified in less than 10% of adults and less than 5% of children, particularly mature B-cell ALL (Burkitt leukemia) at presentation.

The signs and symptoms of ALL are variable and include:

- Generalized weakness and feeling tired

- Anemia

- Dizziness

- Headache, vomiting, lethargy, nuchal rigidity, or cranial nerve palsies (CNS involvement)

- Frequent or unexplained fever and infection

- Weight loss and/or loss of appetite

- Excessive and unexplained bruising

- Bone pain, joint pain (caused by the spread of "blast" cells to the surface of the bone or into the joint from the marrow cavity)

- Breathlessness

- Enlarged lymph nodes, liver and/or spleen

- Pitting edema (swelling) in the lower limbs and/or abdomen

- Petechiae, which are tiny red spots or lines in the skin due to low platelet levels

- Testicular enlargement

- Mediastinal mass

Stewart–Treves syndrome, also known as cutaneous angiosarcoma, refers to a lymphangiosarcoma, a rare complication that forms as a result of chronic, long-standing lymphedema. Although it most commonly refers to malignancies associated with chronic lymphedema resulting from mastectomy and/or radiotherapy for breast cancer, it may also describe lymphangiosarcomas that result from congenital and other causes of chronic secondary lymphedema. Lymphangiosarcoma arising from cancer-related lymphedema has become much less common with better surgical techniques, radiation therapy, and conservative treatment. The prognosis, even with wide surgical excision and subsequent radiotherapy, is poor.

Early detection is key. Untreated patients usually live 5 to 8 months after diagnosis.

The following symptoms are typical ones which lead to testing for JMML, though children with JMML may exhibit any combination of them: pallor, fever, infection, bleeding, cough, poor weight gain, a maculopapular rash (discolored but not raised, or small and raised but not containing pus), lymphadenopathy (enlarged lymph nodes), moderate hepatomegaly (enlarged liver), marked splenomegaly (enlarged spleen), leukocytosis (high white blood cell count in blood), absolute monocytosis (high monocyte count in blood), anemia (low red blood cell count in blood), and thrombocytopenia (low platelet count in blood). Most of these conditions are common, nonspecific signs and symptoms.

Children with JMML and neurofibromatosis 1 (NF1) (about 14% of children with JMML are also clinically diagnosed with NF1, though up to 30% carry the NF1 gene mutation) may also exhibit any of the following symptoms associated with NF1 (in general, only young children with NF1 are at an increased risk of developing JMML):

- 6 or more café-au-lait (flat, coffee-colored) spots on the skin

- 2 or more neurofibromas (pea-size bumps that are noncancerous tumors) on or under the skin

- Plexiform neurofibromas (larger areas on skin that appear swollen)

- Optic glioma (a tumor on the optic nerve that affects vision)

- Freckles under the arms or in the groin

- 2 or more Lisch nodules (tiny tan or brown-colored spots on the iris of the eye)

- Various bone deformations including bowing of the legs below the knee, scoliosis, or thinning of the shin bone

Noonan syndrome (NS) may predispose to the development of JMML or a myeloproliferative disorder (MPD) associated with NS (MPD/NS) which resembles JMML in the first weeks of life. However, MPD/NS may resolve without treatment. Children with JMML and Noonan's syndrome may also exhibit any of the following most-common symptoms associated with Noonan's syndrome:

- Congenital heart defects, in particular, pulmonic stenosis (a narrowing of the valve from the heart to the lungs)

- Undescended testicles in males

- Excess skin and low hair line on back of neck

- Widely set eyes

- Diamond-shaped eyebrows

- Ears that are low-set, backward-rotated, thick outer rim

- Deeply grooved philtrum (upper lip line)

- Learning delays

Anaplastic thyroid cancer (ATC) is a form of thyroid cancer which has a very poor prognosis due to its aggressive behavior and resistance to cancer treatments. Its anaplastic cells have poor differentiation, including dedifferentiation.

Initial presenting symptoms most commonly are headache, depressed mental status, focal neurological deficits, and/or seizures. The growth rate and mean interval between onset of symptoms and diagnosis is approximately 1.5–2 years but is highly variable, being intermediate between that of low-grade astrocytomas and glioblastomas. Seizures are less common among patients with anaplastic astrocytomas compared to low-grade lesions.

Patients with Hodgkin's lymphoma may present with the following symptoms:

- Lymph nodes: the most common symptom of Hodgkin's is the painless enlargement of one or more lymph nodes, or lymphadenopathy. The nodes may also feel rubbery and swollen when examined. The nodes of the neck and shoulders (cervical and supraclavicular) are most frequently involved (80–90% of the time, on average). The lymph nodes of the chest are often affected, and these may be noticed on a chest radiograph.

- Itchy skin

- Night sweats

- Unexplained weight loss

- Splenomegaly: enlargement of the spleen occurs in about 30% of people with Hodgkin's lymphoma. The enlargement, however, is seldom massive, and the size of the spleen may fluctuate during the course of treatment.

- Hepatomegaly: enlargement of the liver, due to liver involvement, is present in about 5% of cases.

- Hepatosplenomegaly: the enlargement of both the liver and spleen caused by the same disease.

- Pain following alcohol consumption: classically, involved nodes are painful after alcohol consumption, though this phenomenon is very uncommon, occurring in only two to three percent of people with Hodgkin's lymphoma, thus having a low sensitivity. On the other hand, its positive predictive value is high enough for it to be regarded as a pathognomonic sign of Hodgkin lymphoma. The pain typically has an onset within minutes after ingesting alcohol, and is usually felt as coming from the vicinity where there is an involved lymph node. The pain has been described as either sharp and stabbing or dull and aching.

- Back pain: nonspecific back pain (pain that cannot be localised or its cause determined by examination or scanning techniques) has been reported in some cases of Hodgkin's lymphoma. The lower back is most often affected.

- Red-coloured patches on the skin, easy bleeding and petechiae due to low platelet count (as a result of bone marrow infiltration, increased trapping in the spleen etc.—i.e. decreased production, increased removal)

- Systemic symptoms: about one-third of patients with Hodgkin's disease may also present with systemic symptoms, including low-grade fever; night sweats; unexplained weight loss of at least 10% of the patient's total body mass in six months or less, itchy skin (pruritus) due to increased levels of eosinophils in the bloodstream; or fatigue (lassitude). Systemic symptoms such as fever, night sweats, and weight loss are known as B symptoms; thus, presence of fever, weight loss, and night sweats indicate that the patient's stage is, for example, 2B instead of 2A.

- Cyclical fever: patients may also present with a cyclical high-grade fever known as the Pel-Ebstein fever, or more simply "P-E fever". However, there is debate as to whether the P-E fever truly exists.

- Nephrotic syndrome can occur in individuals with Hodgkin's lymphoma and is most commonly caused by minimal change disease.

The most common symptoms in children are easy bruising, pale skin, fever, and an enlarged spleen or liver.

Damage to the bone marrow, by way of displacing the normal bone marrow cells with higher numbers of immature white blood cells, results in a lack of blood platelets, which are important in the blood clotting process. This means people with leukemia may easily become bruised, bleed excessively, or develop pinprick bleeds (petechiae).

White blood cells, which are involved in fighting pathogens, may be suppressed or dysfunctional. This could cause the patient's immune system to be unable to fight off a simple infection or to start attacking other body cells. Because leukemia prevents the immune system from working normally, some patients experience frequent infection, ranging from infected tonsils, sores in the mouth, or diarrhea to life-threatening pneumonia or opportunistic infections.

Finally, the red blood cell deficiency leads to anemia, which may cause dyspnea and .

Some patients experience other symptoms, such as feeling sick, having fevers, chills, night sweats, feeling fatigued and other flu-like symptoms. Some patients experience nausea or a feeling of fullness due to an enlarged liver and spleen; this can result in unintentional weight loss. Blasts affected by the disease may come together and become swollen in the liver or in the lymph nodes causing pain and leading to nausea.

If the leukemic cells invade the central nervous system, then neurological symptoms (notably headaches) can occur. Uncommon neurological symptoms like migraines, seizures, or coma can occur as a result of brain stem pressure. All symptoms associated with leukemia can be attributed to other diseases. Consequently, leukemia is always diagnosed through medical tests.

The word "leukemia", which means 'white blood', is derived from the characteristic high white blood cell count that presents in most afflicted patients before treatment. The high number of white blood cells are apparent when a blood sample is viewed under a microscope, with the extra white blood cells frequently being immature or dysfunctional. The excessive number of cells can also interfere with the level of other cells, causing further harmful imbalance in the blood count.

Some leukemia patients do not have high white blood cell counts visible during a regular blood count. This less-common condition is called "aleukemia". The bone marrow still contains cancerous white blood cells which disrupt the normal production of blood cells, but they remain in the marrow instead of entering the bloodstream, where they would be visible in a blood test. For an aleukemic patient, the white blood cell counts in the bloodstream can be normal or low. Aleukemia can occur in any of the four major types of leukemia, and is particularly common in hairy cell leukemia.

Individuals with a basal-cell carcinoma typically present with a shiny, pearly skin nodule. However, superficial basal-cell cancer can present as a red patch similar to eczema. Infiltrative or morpheaform basal-cell cancers can present as a skin thickening or scar tissue – making diagnosis difficult without using tactile sensation and a skin biopsy. It is often difficult to visually distinguish basal-cell cancer from acne scar, actinic elastosis, and recent cryodestruction inflammation.

Acute biphenotypic leukaemia is an uncommon type of leukemia which arises in multipotent progenitor cells which have the ability differentiating into both myeloid and lymphoid lineages. It is a subtype of "leukemia of ambiguous lineage".

The direct reason lead BAL is still not clear. BAL can be de novo or secondary to previous cytotoxic therapy. Many factors, such as virus, hereditary factors, radiation, might have relationship with BAL.

BAL is hard to treat, usually the chemotherapy is chosen according to the morphology of the blast (ALL or AML). The stem cell transplantation is highly recommended.

About 5% of acute leukaemia cases are BAL. BAL could occur in all the age of the people, but more in adults than in children.

Basal-cell carcinoma (BCC), also known as basal-cell cancer, is the most common type of skin cancer. It often appears as a painless raised area of skin, that may be shiny with small blood vessels running over it or it may present as a raised area with ulceration. Basal-cell cancer grows slowly and can damage the tissue around it but is unlikely to spread to distant areas or result in death.

Risk factors include exposure to ultraviolet light, having lighter skin, radiation therapy, long term exposure to arsenic, and poor immune system function. UV light exposure during childhood is particularly harmful. Tanning beds are becoming another common source of ultraviolet radiation. Diagnosis is often based on skin examination and confirmed by tissue biopsy.

It is not clear if sunscreen affects the risk of basal-cell cancer. Treatment is typically by surgical removal. This can be by simple excision if the cancer is small, otherwise Mohs surgery is generally recommended. Other options may include application of cold, topical chemotherapy, laser surgery, or the use of imiquimod. In the rare cases in which distant spread has occurred, chemotherapy or targeted therapy may be used.

Basal cell cancer accounts for at least 32% of all cancers globally. Of skin cancers other than melanoma, about 80% are basal-cell cancers. In the United States about 35% of white males and 25% of white females are affected by BCC at some point in their life.

Most signs and symptoms of AML are caused by the replacement of normal blood cells with leukemic cells. A lack of normal white blood cell production makes people more susceptible to infections; while the leukemic cells themselves are derived from white blood cell precursors, they have no infection-fighting capacity. A drop in red blood cell count (anemia) can cause fatigue, paleness, and shortness of breath. A lack of platelets can lead to easy bruising or bleeding with minor trauma.

The early signs of AML are often vague and nonspecific, and may be similar to those of influenza or other common illnesses. Some generalized symptoms include fever, fatigue, weight loss or loss of appetite, shortness of breath, anemia, easy bruising or bleeding, petechiae (flat, pin-head sized spots under the skin caused by bleeding), bone and joint pain, and persistent or frequent infections.

Enlargement of the spleen may occur in AML, but it is typically mild and asymptomatic. Lymph node swelling is rare in AML, in contrast to acute lymphoblastic leukemia. The skin is involved about 10% of the time in the form of leukemia cutis. Rarely, Sweet's syndrome, a paraneoplastic inflammation of the skin, can occur with AML.

Some people with AML may experience swelling of the gums because of infiltration of leukemic cells into the gum tissue. Rarely, the first sign of leukemia may be the development of a solid leukemic mass or tumor outside of the bone marrow, called a chloroma. Occasionally, a person may show no symptoms, and the leukemia may be discovered incidentally during a routine blood test.

There are few early warning signs that a patient has a DSRCT. Patients are often young and healthy as the tumors grow and spread uninhibited within the abdominal cavity. These are rare tumors and symptoms are often misdiagnosed by physicians. The abdominal masses can grow to enormous size before being noticed by the patient. The tumors can be felt as hard, round masses by palpating the abdomen.

First symptoms of the disease often include abdominal distention, abdominal mass, abdominal or back pain, gastrointestinal obstruction, lack of appetite, ascites, anemia, and/or cachexia.

Other reported symptoms include unknown lumps, thyroid conditions, hormonal conditions, blood clotting, kidney or urological problems, testicle, breast, uterine, vaginal, or ovarian masses.

Radiation-induced cognitive decline describes the possible correlation between radiation therapy and mild cognitive impairment. Radiation therapy is used mainly in the treatment of cancer. Radiation therapy can be used to cure care or shrink tumors that are interfering with quality of life. Sometimes radiation therapy is used alone; other times it is used in conjunction with chemotherapy and surgery. For people with brain tumors, radiation can be an effective treatment because chemotherapy is often less effective due to the blood–brain barrier. Unfortunately for some patients, as time passes, people who received radiation therapy may begin experiencing deficits in their learning, memory, and spatial information processing abilities. The learning, memory, and spatial information processing abilities are dependent on proper hippocampus functionality. Therefore, any hippocampus dysfunction will result in deficits in learning, memory, and spatial information processing ability.

The hippocampus is one of two structures of the central nervous system where neurogenesis continues after birth. The other structure that undergoes neurogenesis is the olfactory bulb. Therefore, it has been proposed that neurogenesis plays some role in the proper functionality of the hippocampus and the olfactory bulb. To test this proposal, a group of rats with normal hippocampal neurogenesis (control) were subjected to a placement recognition exercise that required proper hippocampus function to complete. Afterwards a second group of rats (experimental) were subjected to the same exercise but in that trial their neurogenesis in the hippocampus was arrested. It was found that the experimental group was not able to distinguish between its familiar and unexplored territory. The experimental group spent more time exploring the familiar territory, while the control group spent more time exploring the new territory. The results indicate that neurogenesis in the hippocampus is important for memory and proper hippocampal functionality. Therefore, if radiation therapy inhibits neurogenesis in the hippocampus it would lead to the cognitive decline observed in patients who have received this radiation therapy.

In animal studies discussed by Monje and Palmer in "Radiation Injury and Neurogenesis", it has been proven that radiation does indeed decrease or arrest neurogenesis altogether in the hippocampus. This decrease in neurogenesis is due to apoptosis of the neurons which usually occurs after irradiation. However it has not been proven whether the apoptosis is a direct result of the radiation itself or if there are other factors that cause neuronal apoptosis, namely changes in the hippocampus micro-environment or damage to the precursor pool. Determining the exact cause of the cell apoptosis is important because then it may be possible to inhibit the apoptosis and reverse the effects of the arrested neurogenesis.