L 12: TP III, Field shaping, skin dose and field separation

Question 1

Blocks used is

Answer 1

Cerobend block
Lead blocks are not used as lead is toxic.
But cerobend is always compared to lead in terms of thickness

Question 2

Thickness of lead required to give 5% of beam transmission is

Answer 2

4.3 HVLs

Question 3

6 MV
10MV
what thickness of lead is needed

Answer 3

6MV = 6.5 cms of Pb
10MV = 7.0 cms of Pb

Question 4

Facts about cerrobend

Answer 4

• It is 9.4 gms/cc, 83% of Pb density
• Non-Toxic at room temp
• It is composed of 50% BIsmuth, others Pb, tin, cadmium
• It is 1.21 times thicker than Pb
• Melting point is 70 deg C; Pb is 370 deg C

Question 5

MLCs

Answer 5

• Made of Tungsten alloy.
• 6-7.5 cms thick along the beam direction.
• This thickness is enough to provide primary transmission of < 2%
• Inter leaf transmission is < 3%
• Physical pnumbra with MLC is wider than collimator blocks penumbra.

Question 6

Skin Dose

Answer 6

• Skin dose for incident photon beam is due to electron contamination.
• This surface dose is best measured by extrapolation chamber or plane parallel chamber.
• Surface dose increase with increase in angle of obliquity
• Surface dose depends on
  1. Dose
  2. Beam energy
  3. Field Size
  4. SSD
  5. TRay to surface distance

Question 7

Electron Filters

Answer 7

• They are medium atomic number absorbers (Z~50)
• They reduce the surface dose by scaterring contaminant electrons than generating them.

Question 8

Separation of adjacent fields

Answer 8

Needed to reduce the hot and cold spots
Important formula to remember
* Easy formula in Dr. Ahmads notes

Question 9

Orthogonal field junctions

Answer 9

• It is an arrangment where the central axis of the adjacent fields are perpendicular.
• Used for treatment of CSI (medulloblastoma)
• Look at the formula