Patient Dose Calculation

Question 1

What factors influence patient dose?

Answer 1

• Administered Activity
• Radionuclide Characteristics
• Radiopharmaceutical Characteristics
• Patient Specific Factors

Question 2

Who decides what the administered activity is?

Answer 2

ARSAC Licence Holder

Question 3

What is ARSAC?

Answer 3

Administration of Radioactive Substances Advisory
Committee Guidance
* Not mandatory
* A guide to good clinical practice

Question 4

In what cases is the ARSAC License holder allowed to make changes to the DRL?

Answer 4

• Patients significantly greater than 70 kg
• Patients significantly less than 70 kg
• Paediatric patients
• Pregnant Patients

Question 5

What is the risk of an adverse effect from a dose of 1 mGy to the uterus?

Answer 5

1 in 17,000

Question 6

What is considered a ‘safe dose’ to the uterus?

Answer 6

A dose up to 1 mGy corresponds to a level of risk
comparable to that due to variations in natural
background radiation

Question 7

What techniques are used for dose reduction?

Answer 7

• Dose optimisation
• Ensure the patient is well hydrated
• Organ uptake blocking e.g. thyroid for iodine based compounds

Question 8

What is radiation dose measured as?

Answer 8

Absorbed energy per unit mass

Question 9

What is required for the calculation of the radiation dose?

Answer 9

• Number of disintegrations
• The average energy emitted per disintegration
• The fraction absorbed in the critical tissue
• A method of combining the information

Question 10

Who has calculated the method for combining the information?

Answer 10

Medical Internal Radiation Dose (MIRD) Committee

Question 11

How do MIRD define:
(a) Source Organs
(b) Target Organs

Answer 11

(a) Organs with significant uptake of isotope
(b) Organs that are receiving radiation

Question 12

For the radionuclide
I-131 what are the source and target organs?

Answer 12

The source organ is the thyroid but it is also a target as some of teh beta particles will be absorbed by the thyroid its self. Organs surrounding the thyroid are target organs as they are exposed to the 360 keV photons from the I-131.

Question 13

What is the basic MIRD methodology?

Answer 13

1. Determine the activity taken up by the source organ (uptake an dclearance) and the time it remians there (effective half life)
2. Determine the total amount of radiation energy emitted by the radiopharmaceutical (during its residence in the source organ)
3. Work out the fraction of energy emitted by the source organ that can then be absorbed by the target organ
4. Find/estimate the mass of the target organ
5. Dose to the target organ = Energy absorbed per unit mass

Question 14

What is the general equation for the absorbed dose?

Answer 14

D= ( LAMBDA . THETA ) Ã / mT

Write this out on paper

Question 15

Define the parameters in the general equation for absorbed dose

Answer 15

LAMBDA - mean energy emitted per decay (J)
THETA - fraction of energy absrobed by the target organ
mT - mass of the target organ (kg)
à - total number of decays (Bq.s)

Question 16

What are the key characteristics of LAMBDA?

Answer 16

• Characteristic of the radionuclide
• Easily available
• Can be complex depending on the different decay schemes
• Penetrating and non-penetrating radiation must be considered separately

Question 17

What is THETA for beta radiation?

Answer 17

The source = target organ therefore THETA = 1 e.g. thyroid irradiating itself with I-131

Question 18

How is the absorbed fraction determined?

Answer 18

Monte Carlo modelling of distributions of radiation from specific radiopharmaceuticals in a range of human models (children of various ages and adult) to produce tables.

Assumes organ sizes and masses, organ uptake % and relative distribution within the body (i.e. may not apply if your patient has diff mass, size or physiology)

Question 19

What is
LAMBDA .THETA?

Answer 19

The absorbed energy per disintegration (for any source -target pair)

CONSIDER PENETRATING AND NON-PENETRATING RADIATION SPEARATELY

Question 20

What is LAMBDA .THETA / mT ?

Answer 20

This quantity is called the S-value and is defined as the absorbed dose per disintergration (for any source - target pair).

Question 21

What is the simplified equation for the radiation dose and describe each component including units?

Answer 21

D = Ã . S

à = total number of decays or cumulated activity (Bq.s)
S = Absorbed dose per disintegration Gy/(Bq.s)

Question 22

What is the equation for à for single exponential behaviour?

Answer 22

à = A 0 / Decay constant
à = A 0 x 1.44 . T ½

The area under the Time Activity Curve (TAC).

Question 23

What is the decay contant equation?

Answer 23

λ = ln(2) / T ½
=1/(1.44 . T ½)

Question 24

What is the equation for à for dual phase exponential behaviour?

Answer 24

Cumulated activity now sum of two single exponentials, minus the area of overlap

à = 1.44 . [T eff 1. A 1 + T eff 2. A 2 - T eff 1. A 2 ]

Question 25

(a) What is the simplified general equation for calculating radiation dose for a single exponential clearance ?

(b) Define each compenent of the equation

Answer 25

(a)
**D **= 1.44 . A 0. T eff½ . S

(b)
A 0 = activity in source organ at t=0 (Bq)
T eff½ = effective half-life (s)
S= S-value Gy/(Bq.s)

Question 26

What reference material would you use to find the S-value for selected radionuclides and organs?

Answer 26

MIRD Pamphlet No. 11 give the “S - value” in absorbed Dose per unit cumulated activity for selected radionuclides and organs.

Question 27

What reference materials would you use to find the radiation dose to patients from different radiopharmaceuticals?

Answer 27

ICRP 53 (1987)
ICRP 80 (1998) ( Second addendum)

Question 28

What is in the ICRP 53 and addendum?

Answer 28

• Tabulates organ specific absorbed dose and effective dose for specific radiopharmaceuticals for each MBq administered.
• Uses models of pharmaceutical distribution.
• Good for “normal” cases.

Question 29

What reference material shoudl be used to look for the assessment of effective dose?

Answer 29

**ARSAC Guidance Notes **
Uses ICRP80 to list dose to the uterus (foetus) and effective dose for each radiopharmaceutical assuming the DRL has been administered.