Figure 2 shows two beam profiles measured at two different depths in water, 1.5c

ID: 2304371 • Letter: F

Question

Figure 2 shows two beam profiles measured at two different depths in water, 1.5cm and 10cm. The profile at 1.5cm depth is less flat and exhibits what are known as dose ‘horns’, where the dose in the off-axis sections of the profile exceed the dose at the central axis. a. At what depth in water is a beam’s flatness normally defined and measured; (1 mark) b. Explain the reasons for the dose ‘horn’ effect in the dose profile at 1.5cm depth and why it is less pronounced at 10cm depth. (2 marks)

Figure 2: Beam profiles for a 20 x 20 cm2 6MV x ray beam at depth dmax (d- 1.5cm) and 10cm (d- 10cm) 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 1.5 cm depth ° 10cm depth ·10-505 10 15 Profile width x or y (cm)

Explanation / Answer

Solution:

A. Beam flatness refers to variation in the beam strength across the central part of the beam. Within the central region, ratio of the dose over 90% to that under 90% is known as the uniformity index.

The ICRU recommends that the uniformity index is kept above a value dependent on beam quality and field size . Dose in the central region should also be kept beneath 103% of the central axis dose.

The variation of dose occurs on a line perpendicular to the central beam axis at a certain depth is known as beam profile. It represents how dose is altered at points away from the central beam axis. There are typically 3 paths:

a) The central region which is flat and includes doses over 80% of the central beam axis.

b)The penumbra region where dose falls and rapidly at the beam edge, between a dose of 20%-80% of the central beam axis.

c)The umbra region where dose is minimal(under 20% of the central beam dose)

2. Most electrons liberated by 6MV photons will have an energy of around 1-2 MeV but this varies from close to zero upto the maximum energy of the photon. Electrons with this energy will travel around 1cm, explaining the buildup region of the photon beam. Once the depth of the maximum dose is reached at 1.5cm, the dose falls off gradually to 90% by 5cm. It then continues to fall by about 10% every 2.5cm, the rate of dose fall off slowing after 10cm.

Depth

0 cm

1 cm

1.5cm (Zmax)

5cm

6.5 cm

10 cm

Dose

20%

98%

100%

90%

80%

70%