Category Archives: IONISING RADIATIONS

RADIOSENSITIVITY OF TUMORS

Posted by on February 3, 2012 No comments

RADIOSENSITIVITY OF TUMORS

 

Tumors, like their cell of origin, differ in their susceptibility to radiation injury. In general, a poorly differentiated tumor is more radiosensitive than a well differentiated one of the same type.

1-    Radio-sensitive tumours:

Destructive dose of radiation_causes no significant damage in adjacent normal tissues.

Examples

  • Malignant lymphoma – Seminoma of testis
  • Lymphoepithelioma -Dysgerminoma of ovary
  • Ewings tumour of bone.

     

    2-    Radio-responsive tumors:

    Destructive dose of radiation could do some damage to adjacent normal tissues.

    Examples

  • Squamous and basal cell carcinoma.
  • Breast carcinoma.
  • Adenocarcinoma.

     

    Radio-resistant tumors:

    Destructive dose of radiation is equally destructive_to adjacent normal tissues.

    Examples:

  • Sarcomas
  • Malignant melanoma N.B.

    1 – Metastatic tumors are more radiosensitive.

    2- Poorly differentiated tumors are more radiosensitive than well-differentiated.

     

    Recurrence of tumours after irradiation

    Recurrent tumors after irradiation are usually radioresistant because the few resistant cells which survive transmit the property of resistance to their descendants.

RADIATION EFFECTS ON DIFFERENT TISSUES

Posted by on February 3, 2012 No comments

RADIATION EFFECTS ON DIFFERENT TISSUES

 

 

Following a substantial dose of radiation, there is a latent interval of hours or days before histological evidence of tissue injury is seen. As already explained, the damage depends on:

a- The dose

b- Type of radiation

c- The interval following exposure

d- The tissue exposed.

Skin

A) Early changes in the skin include:

  1. Dilatation of blood vessels, congestion of dermal vessels and other signs of acute inflammation and these reflect acute tissue injury.
  2. With a single dose, the mitotic activity of the basal cells is arrested with subsequent loss of the epidermis and epilation (loss of hair).
  1. The walls of the dermal vessels are infiltrated with fibrin.

    B- Late changes: The dermal vessels show the picture of endarteritis obliterans followed by replacement with hyaline collagen and this will hinder regeneration of epidermis.

    2- With repreated exposure to radiation; large bizarre fibrocytes are present in the dermal connective tissue, the dermal collagen becomes very dense and there is a tendency to become necrotic even years after exposure.

    3- Persistent melanin pigmentation is noted.

    Lymphoid and hemopoeitic tissue (are very sensitive to radiation).

    Necrosis of lymphoid and hemopoeitic tissue is manifested by changes in the peripheral blood cells which appear at different times depending on the length of life of different cells.

    1. The lymphocytes: are easily degenerated and necrosed with marked decrease in their number in the peripheral blood.
    2. The granulocytes: many show an initial rise followed by marked drop by the end of the second to the fourth weeks. This leads to diminished resistance with subsequent infection.
    3. The platelets: show an initial rise followed within few days by marked drop leading to haemorrhagic tendency.
    4. The red blood cells: are lately dcreased and anemia shows up after several weeks (6 weeks) due to their relative long life span.
    5. Leukemia may develop after many years from exposure to ionizing radiation due to mutation in lymphoid tissue.

    Intestinal Epithelium

    Degeneration and necrosis occur after two days and are accompanied by severe diarrhea. Disturbances in fluid and elecholyte balance may cause death.

    Healing of intestinal lesions may be followed by stensosis.

    Ovary

    Grauulosa cells are affected than the ovum so Graffian follicles appear small, and anovular.

    Testis

    1-    Destruction of spermatogonia prevents further
    production of spermatorytes and spermatozoa.

    2-    Interstitial cells are not affected.
    Blood vessels

    1. Degeneration and necrosis of endothelium followed by acute inflammation of the wall and thrombosis.
  2. Healing is followed by vascular occlusion.

IONISING RADIATIONS

Posted by on February 3, 2012 No comments

IONISING RADIATIONS

 

 

Types of ionizing radiation:

  1. Electromagnetic radiation: X-rays and Gamma rays.
  1. Particulate radiation:

    alpha particles – beta particles – protons – electrons and neutrons.

    - All of these radiations can be administered to animals and human beings: Externally or Internally.

    1. External sources include X-ray machines, electron accelerators.
    2. Internal irradiation arises from the ingestion of any of the hundreds of known radionucleotides (radioactive isotopes) many of which are used for medical diagnostic or therapeutic purposes.

    Mode of action of ionizing radiation

    A) Direct action:

    Radiation causes its effects by transferring energy to the substance through which it passes.

    This energy can produce two changes:

  2. Ionization: Loss or gain of an electron.
    1. Excitation: Movement of an electron into an outer orbit. Occurs in some DNA molecules (genes) and this results in a change in their chemical structure (mutation).

    (B) In direct action:

    Diffuse ionization of water in the cell leads to the formation of oxidizing agents such as H202 which inactivate cellular enzymes.

    Forms of Radiation injury of cells:

  3. Cloudy swelling, hydropic degeneration or necrosis.
  4. Temporary or permanent arrest of mitosis results from: a) Interference with synthesis of DNA.

    b) Injury of mitotic spindle.

    1. Fragmentation of chromosomes with failure of correct reconstruction results in necrosis of daughter cells.
    2. Mutation: Mutation may be lead to neoplastic transformation as follows:

     

  • If mutations are in the germ cells, the children of irradiated parents can be affected.
  • If the mutations are in somatic cells malignant tumors can be induced by radiation. The commonest radiation-induced tumors are leukemia and tumors of thyroid, breast, bone and lung.

Radiosensitivty of cells or tissues:

Two general rules govern the variable susceptibility of different tissues to the effects of ionizing radiations:

1- Embryonic and undifferentiated cells are more susceptible to radiation injury than differentiated cells.

2- Rapidly proliferating cells are more easily damaged by ionizing radiation during interphase or early phase of mitosis.

Human tissues are classified into three groups:

1-    Radio-sensitive Tissues:

a- Lymphoid tissue b- Hemopoietic tissue
c- Germ cells    d- Intestinal epithelium.

2-    Radio-responsive tissues (intermediate):

a- Skin    b- Vascular endothelium

c- Conjunctiva, cornea and lens.

3-    Radio-resistant tissues

a- Bones    b- lungs    c- Nerve cells

d- Skeletal muscles e- kidneys, f- Endocrine glands.