5: Corrections for Contour Irregularities/ Oblique Incidence

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12 Terms

1
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Contour irregularities aka

Oblique incidence

2
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When does oblique incidence happen?

when the beam strikes a curved surface (striking an odd angle, an unconventional angle)

3
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What are the 2 ways we can fix oblique incidence?

  1. Clinically, using a beam modifying device such as wedges or bolus

  2. Dosimetrically

4
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What are the 3 dosimetric methods for treating Contour Irregularities?

  1. Effective SSD method

  2. TAR method

  3. Isodose Shift Method

5
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<p>What method <strong>slides the isodose chart down </strong>so that its surface line is at S1-S1, reading off the PDD value point A and then multiplying in an I² correction for change in SSD?</p>

What method slides the isodose chart down so that its surface line is at S1-S1, reading off the PDD value point A and then multiplying in an I² correction for change in SSD?

Effective SSD method

6
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TAR stand for

Tissue-Air Ratio

7
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<p>What method is where the SSD is kept the same as the CR, then a <strong>TAR correction factor</strong> is added to compensate for the air space.</p>

What method is where the SSD is kept the same as the CR, then a TAR correction factor is added to compensate for the air space.

TAR method

8
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What method is used for making full isodose curves rather than making a point dose calculation?

Isodose Shift Method aka 2/3rd shift method

<p>Isodose Shift Method aka 2/3rd shift method</p>
9
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Why is the 2/3rd shift method called that?

The beam doesn’t exactly follow the angle of the skin surface, depending on the Energy of the beam that you use.

E.g. If you have a 45 degree tilt in the skin surface, the angle of the isodose curves would only be 2/3rd of that

<p>The beam doesn’t exactly follow the angle of the skin surface, depending on the Energy of the beam that you use.</p><p>E.g. If you have a 45 degree tilt in the skin surface, the angle of the isodose curves would only be 2/3rd of that</p>
10
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<p>What kind of machine is this?</p>

What kind of machine is this?

KV

11
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<p>What are the lines in the box called? What method is this associated with?</p>

What are the lines in the box called? What method is this associated with?

Fan Lines

Associated with Isodose Shift Method aka 2/3rds Shift Method

12
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True or False” The 2/3rds Shift Method is used for all energy beams

False:

different beams have different amounts of shift

e.g. 6 MV it is 2/3rds shift

18 MV is ½ shift