Patient Dose calculation systems

Question 1

Purpose of independent dose calc system (x2)

Answer 1

• Independent check of treatment planning
• Used for patient treatments that don’t require full modelling in TPS

Question 2

Method for factor based dose calc

Answer 2

1. Assume patient is rectangle and hetrogen
2. Identify all differences from reference conditions to treatment conditions
3. Calculate/measure factors for each difference
4. Select appropriate factors for patient set-up

Question 3

Beam Size factors (x4)

Answer 3

1. St - total scatter factor
2. Sc - collimator scattor
3. Sp - phantom scatter
4. TPR - quantify scatter at depth for different beam sizes

Question 4

Facots for MLC shape

Answer 4

1. TPR
2. PDD
3. Physical depth
4. Radiological depth

Question 5

Factors for missing tissue

Answer 5

1. St
2. Sp
3. TPR

Question 6

Factors for Heterogeneity

Answer 6

1. TPR
2. PDD
3. Physical depth
4. Radiological depth

Question 7

Factors for wedged isodose

Answer 7

Wedge attenuation factor

Question 8

Factors for Setup type (iso vs SSD)

Answer 8

ISO - Dref,iso

SSD - Dref,SSD

Question 9

ESTRO dose calc principle to get from reference to idealised patient conditions (4 steps)

Answer 9

REFERENCE

1. Remove scattering vol:
2. Introduce head scatter for beam size:
3. Introduce patient scatter for beam size:
4. Introduce depth dose ratio:

IDEALISED PATIENT

Question 10

ESTRO removal of scattering volume

Answer 10

• 1/V
• V - Volume scatter ratio - ratio of absorbed dose in fill scatter conditions to the dose in miniphantom at same beam size c and depth z
• The traction of the additional dose to the calc point introduced by the rest of the phantom

Question 11

ESTRO in-air ouput factor

Answer 11

• Output ratio in miniphantom, O
  
  • Ratio of absorbed dose at reference depth for beam size c to dose at same depth for reference beam size cR.
  • Represents change in dose in miniphantom from LINAC when collimator is changed from reference
  • i.e. keep everything the same, chnage the collimator size, what happens to your measurement

Question 12

ESTRO Volume Scatter ratio

Answer 12

• Volume scatte ratio for colimator setting
• Fraction of additional dose at treatment collimator setting introduced by including the rest of the phantom
  *

Question 13

ESTRO TPR

Answer 13

• Ratio of dose at depth z to dose at reference depth zref for treatment collimator setting c.

Question 14

2 problems with ESTRO formalism

Answer 14

1. Volume scatter ratios used in all formulae - v is not easily measurable
2. Extensions to 100cm SSD and extended SSD have beam size defined at surface - formalism is different and more complex

Question 15

Why is it hard to measure V?

How do you measure it?

Answer 15

Problem:

• Miniphantom and full scatter water tank measurements in same session
• Same chamber and same orientation
• Direct dose measurements

Solution:

• Since V is only ever used as a raito, measre ratios of dose.
• Equivelent to Sp for unwedged beams

Question 16

Why is ESTRO formalism of defining beam size difficult?

What’s the soln

Answer 16

Problem

• Hard to extend to fixed SSD where beam size is defined at surface
• Have to use confusing set of nomenclature for both

Solution

• Always define beam size at isocentre (collimator setting)
• Measure all fixed SSD as function of beam size
• Unify isocentric and fixed SSD formalisms to remove a set of nomenclature