Critical organs

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Radionuclides can enter the body by ingestion, inhalation, or injection. Once taken into the body, their radiation effects depend on their anatomic distribution, duration of retention in the body, and rate of radioactive decay, as well as on the energies of their emitted radiations. An internally deposited radioactive element may concentrate in, and thus irradiate, certain organs more than others. Radio iodine, for example, collects in the thyroid gland, whereas radium and strontium accumulate chiefly in the bones. Different radio elements also vary in their rates of removal. Radio iodine  for instance, is normally eliminated from the thyroid rapidly enough so that its concentration is halved within days. Strontium-90, on the other hand, is retained in high concentrations in the skeleton for years.

Critical organs are significant one to spare during the treatment because in the human being had some significant amount radio nuclides traces as discussed above. In earlier days by using the conventional therapy many of the normal structures are being irradiated. Sometimes because of that also patient reactions and severe acute effects had come. After the development of multimodality techniques such as 3DCRT, IMRT, IGRT, SRS, SRT, SBRT, etc. sparing of normal structures quite possible and people achieved the desired levels of dose to the particular organ.

Site wise critical organs:

Head & Neck:

A. Nasopharynx,

B. Oropharynx

C. Hypopharynx,

D. Tongue,

E. Tonsils,

F. Base of tongue

G. Parotid glands

H. Teeth

I. Sub mandibular glands

J. Juglar nodes

K. Spinal cord

L. Brain    
M. Brainstem                

N. Pituitary Gland
O. Eyes
P. OpticNerve 
Q. Optic chiasma
R. Larynx
S. Temporomandibular joint & Mandible   
T. Salivary glands
U. Thyroid
V. Middle ear


1. Bladder
2. Rectum
3. Uterus
4. Vagina
5. Testes
6. Ovaries       
7. Femoral head


1. Lung
2. Heart
3. Oesophagus
4. Spinal cord
5. Lymph nodes   
6. Rib cage


1. Liver
2. Stomach
3. Kidney
4. Small intestine
5. Colon
6. Adrenal glands
7. Pancreas   

Other sites:

1. Skin
2. Peripheral Nerves

3. Bone marrows

4. RBC and WBC


1. Name any four critical organs in head & neck?

2. Which gland is most sensitive to iodine -131,125  
a) Salivary glands

b) Thyroid glands

c) Sub-mandibular glands

d) All


1. Please refer the text above for answer

2. b) thyroid gland






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