Radiation Therapy

1.    Distinguish between the mass attenuation coefficient (?/?), the mass energy transfer coefficient (?tr/?), and the mass energy absorption coefficient (?en/?).  Rank them from largest to smallest.  Why do we divide each of these by the density of the material (?)?

2.    The mass attenuation coefficient for a particular x-ray beam and material is 0.04 cm2/g.  The material has a density of 2.5 g/cm3.  Calculate the percentage of photons that are transmitted through layers of the material of 1 mm, 5 mm, and 1 cm.  Also calculate the half-value-layer (HVL) for this material.

3.    An exposure meter is placed in a cobalt-60 beam and records 90 R/min.  Calculate the rate at which energy is absorbed from the beam by the air.  Now, if a small mass of water is placed at this location, calculate the dose rate to the water.

4.    The percentage depth dose for 60Co for depth 15 cm and SSD 50 cm is 33.2 for a 10×10 field.  Find the Mayneord F-factor to convert the depth dose to SSD 100 cm and estimate the depth dose.

5.    Define TAR.  What is the advantage of TAR over percentage depth dose?

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