Part 4: ↑ Distance (SSD)= ↑PDD

SSD = PDD

SSD = Dose

↑SSD =↑PDD

↑SSD =↓Dose

As we increase the distance, we decrease the dose (intensity)

Dose (intensity) dependance on distance (SSD) is greater at (closer/further) distances

Closer

e.g. There is a BIG change in Dose Rate from 60 to 80 cm, but hardly any drop off from 160 to 180 cm/

SSD = PDD

SSD = Dose

↑SSD =↑PDD

↑SSD =↓Dose

As we increase the distance, we decrease the dose (intensity)

Dose (intensity) dependance on distance (SSD) is greater at (closer/further) distances

Closer

e.g. There is a BIG change in Dose Rate from 60 to 80 cm, but hardly any drop off from 160 to 180 cm/

As we increase the distance, we decrease the dose (intensity). This decrease is more dramatic at distances closer to the surface.

This means that the PDD, decreases more rapidly (near /further away from ) the source than (near /further away from ) the source.

This means that the PDD, decreases more rapidly (near /further away from ) the source than (near /further away from ) the source.

i.e. Smaller SSD = Smaller PDD

Why does the PDD increase as Distance increases? (rationale for this effect)

1. ↑SSD = ↑ molecules in the way = ↑ filtration = ↑ E_avg of the beam

2. ↑ SSD = alters the geometry of the beam = makes the beam ↑ concentrated = ↑PDD

If there is a change in SSD, there will be a change in PDD.

We will need to figure out the new PDD.

1. List the most common method used to figure out a △PPD with a △SSD.

2. List another method

3. List the most accurate method

1. Mayneord F Factor

2. Bentel method

3. TAR Method

What is Mayneord F Factor used for?

Used to calculate a △PPD with a △Distance

(△Distance = △PDD)

Weakness of using Mayneord F Factor?

It uses strict application of I² WITHOUT considering changes in scatter as distance changes

Strict application of I², DOES NOT consider △ scatter as distance △

Mayneord F Factor Formula

1. What does the Mayneord F Factor formula give you?

2. What does it not give you?

1. F Factor

2. PDD itself

1. The percent depth dose for a 15x15 field, 10cm depth, 80cm SSD is 58.4% for a Cobalt beam. Find the percent depth dose for the same field size and depth for an SSD of 100cm.

2. Dmax = 1.5 cm

3. Dmax = 3.5 cm

↑ SSD = ↑ PDD

1. Why was this property used for Cobalt?

2. Why not set SSD at 200 or more?

1. During that time, linacs weren’t available, and every little we could do to help obtain a deeper penetrating beam was sought.

2. ↑ SSD = ↓ Intensity(dose) = ↑ treatment time to get to desired dose