Patient dose in nuclear medicine

Question 1

What factors influence patient dose in nuclear medicine?

Answer 1

• Administered activity.
• Radionuclide characteristics (emission and energy).
• Radiopharmaceutical characteristics (uptake and excretion).
• Patient specific factors.

Question 2

Who decides how much activity is administered to a patient?

Answer 2

• Administration of radioactive substances advisory committee (ARSAC) license holder.
• ARSAC provide guidance (notes for guidance) for good clinical practice but this is not mandatory.

Question 3

When may a different activity from the DRL be administered?

Answer 3

• Patients of significantly different weight to average.
• Paediatric patients.
• Pregnant patients.

Question 4

How is DRL variation for paediatrics determined?

Answer 4

• ARSAC notes for guidance provides the fraction of adult administered activity given the weight of the child. It also provides minimum activities to gain adequate image quality.
• Other ways include the European association of nuclear medicine’s recommendation to use body surface area.

Question 5

What is the procedure for pregnant patients?

Answer 5

• Ask to sign to confirm not pregnant.
• Exam may be delayed.
• Justification (i.e. benefits outweigh risks) and optimisation required if exams to go ahead.

Question 6

What level of risk does a dose of 1 mGy to a foetus correspond to when considering background and adverse effects?

Answer 6

• Comparable to variations in natural background radiation.
• 1 in 17 000 chance of adverse effect (e.g. childhood cancer).

Question 7

What steps are taken with regards to pregnancy for radionuclide therapy patients?

Answer 7

• Pregnancy is contraindicative for therapy.
• ARSAC notes for guidance provides the time required for the patient to avoid pregnancy post-therapy.

Question 8

What steps are taken with regards to breastfeeding for nuclear medicine patients?

Answer 8

• Instructions to interrupt breastfeeding for certain amounts of time (see ARSAC notes for guidance).
• Advised to bank breast milk prior to treatment and throw away radioactive breast milk.

Question 9

What are some other dose reduction techniques in nuclear medicine?

Answer 9

• Keep patient is well hydrated to ensure kidneys are working optimally and patient will excrete radioactive material quicker.
• Organ uptake blocking (e.g. thyroid blocking agent for I-123).

Question 10

What do we need to know to determine the radiation dose to the patient?

Answer 10

• Number of disintegrations.
• Average energy emitted per disintegration.
• Fraction absorbed in critical tissue.
• A method of combining the information (Medical Internal Radiation Dose (MIRD) committee guidance tables).

Question 11

What are the basic concepts of the MIRD dose calculation method?

Answer 11

• Separate into source organs (i.e. those with significant uptake of the isotope) and target organs (i.e. those that are receiving a radiation dose from the source organ).
• Work out activity taken up by source organ and the time it remains there (uptake and clearance).
• Work out total amount of radiation energy emitted by radiopharmaceutical during its residence in the source organ.
• Fraction of energy emitted by source organ which can be absorbed by target organ.
• Find or estimate mass of target organ.
• Dose to target = Energy absorbed per unit mass.

Question 12

How is the mean energy per disintegration determined?

Answer 12

• This is a characteristic of the radionuclide used and is available in textbooks.
• Some decay schemes can be complex.
• Penetrating and non-penetrating radiation need to be considered separately.
• Total energy is the sum of each component.

Question 13

What is the fraction of energy absorbed by the target organ if it is also the source organ and emits beta radiation?

Answer 13

1 as all the radiation emitted by the source organ will be absorbed by the target organ.

Question 14

How is the absorbed fraction determined?

Answer 14

• Monte Carlo modelling of distributions from specific radiopharmaceuticals in a range of human models (adults and children of various ages) to produce lookup tables.
• Assumes standard organ sizes and masses.

Question 15

What is the S-value? How does it relate to the absorbed dose? Where can S-values be found?

Answer 15

• S = (delta.phi)/m_t where delta is the mean energy emitted per decay (J), phi is the fraction of energy absorbed by the target organ and m_t is the mass of the target organ.
• It is the mean absorbed energy per disintegration (Gy/(Bq s)) such that the absorbed dose equals D = A_tilde.S where A_tilde is the is the total number of decays.
• MIRD pamphlets have tables which provide S-values (absorbed dose per cumulated activity) for selected radionuclides and organs.

Question 16

How is the total number of decays (A_tilde) determined for mono-exponential decay? How is this modified for a dual phase exponential?

Answer 16

• Total number of disintegrations over the time that the activity remains in the source organ is the integral of the exponential curve.
• A_tilde = A_0/decay constant where the decay constant can be estimated as 1/0.69.T_eff where T_eff is the effective half-life => A_tilde = 1.44.A_0.T_eff.
• For dual phase exponentials, the cumulated activity can be summed, minusing the area of overlap: A_tilde = 1.44.[T_eff1.A_1 + T_eff2.A2 - T_eff1.A2].

Question 17

What is the general equation for the absorbed dose? What is the simplified equation?

Answer 17

• D = (delta.phi).A_tilde/m_t where delta is the mean energy emitted per decay (J), phi is the fraction of energy absorbed by the target organ, A_tilde is the total number of decays and m_t is the mass of the target organ.
• D = 1.44.A_0.T_eff.S where A_0 is the activity in the source organ at T = 0, T_eff is the effective half-life and S is the S-value (Gy/(Bq s)).