Tolerance Levels

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The proper dose of radiation depends on the type of cancer (lymphomas are sensitive to low doses and sarcomas require high doses) and the size of the cancer (large tumors require a higher dose.) Other factors such as how much surgery have been performed or if chemotherapy is being given will affect the required tumour dose.

How much is safe will depend on which normal structures are in the way of the beam (the lens of the eye is very sensitive but muscle tissue tolerates a very high dose) and what percent of the organ is being hit (if most of the organ is involved then the dose must be kept lower.) Radiation given in one dose is more damaging than if it is divided into smaller fractions

     Tolerance dose (TD) for various organs for one-third, two-thirds of target volume and one-third of target volume at the 5% complication rate 5 years after treatment.


Questions:


1. What is the TD 5/5 dose if, the 3/3rd of the volume of spinal cord is irradiated?

a) 50 Gy

b) 45 Gy

c) 47 Gy

d) 40 Gy



2. What is the tolerance dose for pituitary gland?

a) 50 Gy

b) 75 Gy

c) 45Gy

d) 30 Gy



Answers:

1. c) 47 Gy

2. c) 45 Gy


References:

1. http://www.aboutcancer.com/radiation_dose.htm

2. http://www.rooj.com

3. Radiobiology for the radiologist by E.J.Hall, Amato J. Giaccia

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