Dmax
Photon Dmax (cm)
  • Co-60 0.5
  • 4MV 1.0
  • 6MV 1.5
  • 10MV 2.5
  • 15MV 3.0
  • 20MV 3.5
  • 25MV 4.0

In our centre we do have 6, 10 and 18MV

  • 6MV : 1.5cm
  • 10MV : 2.5cm
  • 18MV : 3.5
Photon attenuation
  • Co-60 ~4.0% per 1 cm depth
  • 6MV ~3.5% per 1 cm depth
  • 20MV ~3.0% per 1 cm depth

PDD

Percent depth dose (SSD setup): PDD = Dose at depth / Dose at dmax
Two components: patient attenuation and inverse square dose fall-off

Factors that affect PDD :

  • Energy ==> Increases
  • Field size ==> Increases
  • SSD ==> Increases
  • Depth ==>Decreases

D2 = D1 * (PDD2 / PDD1)

By energy at 100 cm SSD, 10x10 field, and depth of 10cm

  • Co-60 56%
  • 4MV 61%
  • 6MV 67%
  • 10MV 73%
  • 20MV 80%

25MV 83%