Posted by DAM on February 3, 2012 in Disturbance of growth and neoplasia | Short Link
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The actual cause of tumors remains obscure up till now. However many pre-disposing factors known as carcinogenic agents, appear to play a certain role in the development of certain tumor.

These factors are:

1) Chemical carcinogens:

  1. The hydrocarbons: the development of carcinoma of the scrotum in chimney sweepers was observed very early. Experimentally carcinoma of the skin of the ear of the rabbit can be induced by painting tar daily for 6 months.
  2. The aromatic amines: as aniline, benzidine. Carcinoma of urinary bladder is observed in laborers working with aniline dyes.
  3. Heavy cigarette smoking has direct relation with development of carcinoma of lung and urinary bladder.
  4. High incidence of mesothelioma (a malignant tumor of pleura) is found in laborers working in contact with asbestos.


2- Physical agents:

a) Ionizing radiation: usually leads to the development of carcinoma of the skin, leukemia, and osteogenic sarcoma.


  1. The continuous exposure of the skin to the ultraviolet rays of sunshine may lead to basal cell carcinoma and squamou cell carcinoma.
  2. Heat: it was shown that an increased incidence of carcinoma of the lip occurs in pipe smokers, but the action of tobacco may be added to the effect of heat.


3-    Hormones: THE sex hormones are considered as
carcinogenic agents and carcinoma of the breast in
mice can be produced by administration of large
doses of estrogen. There are some tumors which
depend in their growth on hormones and are known
as hormone dependant tumors such as:

b- Carcinoma of the prostate: is stimulated by androgen and its growth can be arrested by administration of estrogen.

c- Carcinoma of the breast: depends on its growth estrogen and prolactin. Removal of both ovaries may lead to arrest of growth of this tumor.


  1. Benign tumors: may be followed by malignant transformation such as villous papilloma of the urinary bladder and duct papilloma of the breast.


  1. Chronic inflammation:

    d- Urinary bilharziasis: usually predisposes to carcinoma of the urinary bladder (squamous metaplasia).

    e- Squamous cell carcinoma may develop in the edge of a chronic ulcer of the skin.

    f- Chronic gastric ulcers are precancers lesions.




    6- Viruses and tumors: virus infection as an etiological factor in tumor formation is now becoming more and more popular. (It may induce certain changes in the genetic material of the affected cells rendering them capable of division and proliferation). Recently a true virus (the plyoma virus) was isolated from leukaemic mice, which in turn produced leukemia when injected into healthy mice.

    Three classes of genes are target for genetic change which result in the formation of tumors:

    1. Growth promoting genes (proto-oncogenes) which when change to oncogenes become tumorogenic.
  2. Cancer suppressor genes
  3. Genes regulating apoptosis.

    Any disturbance in the balance between these genes may result in tumorogenesis.

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