Radionuclide Therapy Flashcards
What radionuclide is used for SIRT?
Y-90
What type of radionuclide emitter is used for SIRT? (gamma, alpha, beta?)
Beta
What are the administration routes for radionuclides?
IV injection or infusion, oral, intracavitary, intra-arterial.
What properties determine the absorbed dose in the patient for radionuclide therapy?
The distribution within body, patient size, patient biology, uptake & retention, along with physical radiation properties.
What is the maximum deviation aimed for from the prescribed activity in radionuclide therapy?
5%
What are the limitations of radionuclide therapy?
Potentially small workload in terms of patients, but large in terms of time & cost.
No agent is entirely selective to desired target – get uptake by other tissues, e.g. Ca. thyroid treatments
Patient-specific dose calculations are difficult due to inhomogeneous dose deposition and individual physiology affecting behaviour of agent
Radiation is not risk free - IR(ME)R requires a ‘net benefit’
Dose Limiting Factor = normal tissue toxicity e.g. myelotoxicity due to bone irradiation
What are the properties of a diagnostic radionuclide?
Short effective half life: order of study
No particulate emission: EC or IT
Mid gamma energy: 100 - 300 keV, high yield
High target / background ratio
What are the properties of a therapeutic radionuclide?
Ideally beta particles, alpha particles (charged particles). – beta or alpha decay.
Longer half life to give more dose (not too long – roughly days (diagnostic = hours)).
Price is low – funding & expense
Radionuclide & radiochemical purity – all activity in right nuclide & chemical form. (I-131 with MIBG – if some of free iodine isn’t connected to MIBG will be a radiochemical impurity)
Ideally emits gamma rays for imaging – brem, etc.
Is a high or low LET useful for therapy?
High
What is the energy of an alpha particle?
< 7.5 MeV
What is the energy of an auger electron?
< 10 keV
What is the energy of a conversion electron?
< 100 keV
What is the energy of a beta particle?
< 1000 keV
What is the range of an alpha particle?
< 0.07 mm (cell diameter)
What is the range of an auger electron?
< 0.01 mm (cell nucleus)
What is the range of a conversion electron?
< 0.14 mm (few cells)
What is the range of a beta particle?
< 4.4 mm (many cells)
On a time activity curve, what does the area under the curve represent?
Amount of dose deposited in tissue.
Longer half life = bigger area = more dose.
How many disintegrations per second is 1 MBq?
1 million per second.
What is P-32 orthophosphate used to treat?
Myeloproliferative disorders
What is used to treat bone pain palliation?
Strontium [89Sr] chloride
Samarium [153Sm] EDTMP
What is used to treat bone metastases?
Radium-223 Chloride
What is 90Y or 186Re Colloid used to treat?
Joint arthritis, inflammations & effusions.
What is used to treat thyrotoxicosis or thyroid cancer?
Radioiodine [131I] NaI
Describe P-32 therapy.
Treats myeloproliferative disease (eg polycythaemia vera) where too many blood cells produced and blood gets thick, often associated with bone marrow hyperactivity.
IV injection 111-222 MBq (chosen by clinical assessment).
Costs ~ £400. (varies as air fare from Poland is shared by hospitals using it)
Taken up in haemopoetic (blood forming) tissue and reduces cell viability & proliferation (radiation injury to cell precursors and bone marrow)
Physical Half Life = 14 days.
Pure beta (b-) particle emitter Emax = 1.71 MeV Emean = 0.7 MeV (Beta gives off range of energies as has excess of neutrons – converts to proton & gives off beta and anti-neutrino, as energy gets less the anti-neutrino is carrying away more energy. So need to know mean energy for dose calcs.)
Radiation Protection: Flush toilet twice for 2 days, Don’t donate blood for 2 weeks, given card explaining instructions.
What are the 3 mechanisms involved in stopping an electron?
Bremstrahlung (Radiative)
Excitation (Collisional)
Ionisation (Collisional)
Describe Ra-223 therapy.
Use to treat: Metastatic bone disease
Chemical form: Radium chloride
Physical half-life: 11 days
Principal radiation: alpha particles (small amount of beta and gamma (roughly 3%))
Route: i.v. 6 injections at 4 weekly intervals
Administration:
Draw up the Radium immediately prior to injection
55 kBq per kg patient weight (manufacturer informed as delivered the correct amount with a small amount of excess)
Check the cannula with 10ml saline (careful not to tissue)
Slow IV injection using syringe shield.
10ml flush of saline
Separate sharps bin - seperate radiation waste
Return in 4 weeks
Radiation Protection: Flush toilet twice for 2 days Don’t give blood for 2 days Given card wtih instructions on Tiny amount of gamma - not bothered about proximity
Describe the properties of I-131 therapy.
Chemical form: Sodium Iodide
Physical half-life: 8.02 days
Principal radiation: 606 keV beta (87%), 364 keV gamma (82%)
Made in: nuclear reactor as high atomic mass & decays by beta – as extra neutrons. Neutron bombardment, or fission product in reactor.
Administered activity: hyperthyroid ~ 400-550 MBq, ca. thyroid ~ 1– 5 GBq
Route: oral (capsule)
Uptake : Active Transport
Cost of capsule: £100 – 300
Describe I-131 for thyrotoxicosis.
Dose given to damage cells to produce less hormone.
Over-active thyroid: High levels of circulating T4 and T3 hormones
Common symptoms: Increased metabolic rate, palpitations, sweating, weight loss, nervousness, cardiac and eye complications can occur
Treat with Carbimazole / Propylthiouracil
Long-term side effects of carbimazole (cardiotoxic)
If not cured by drug therapy, may prefer alternatives – surgery (remove part or whole of thyroid) or radioiodine (reduce function)
Out-patient therapy (usually)
Careful workup to allow proper radiation risk assessment (e.g. are patients caring for children, incontinent, in a nursing home)
Clinical follow-up required to assess response. Test blood levels of thyroid hormones & assess clinical symptoms. If not low enough, may need 2nd treatment (<10% patients), if too low -> give thyroxine tablets to replace hormones (often the aim). Usual practice is to give extra to reduce function to under par, then top up with hormones.
Radiation protection:
5-6 days avoid long exposures (>1 hr) of close contact with everyone (limit dose)
Up to 3 weeks avoid close contact with children & pregnant women
No blood samples/invasive procedures for 1 month
Carry instruction card for 1 month, 3 month for travelling abroad (radiation detectors at airports)
Describe I-131 for thyroid cancer.
Purpose: Eliminate residual thyroid tissue after thyroidectomy.
Method: I-131 capsule
Patients stop taking medication or given thyrogen administration which raises Thyroid Stimulating Hormone (TSH) levels stimulating I-131 uptake and Thyroglobulin production
Radiation Protection:
In patient procedure with lead lined walls.
Avoid close prolonged contacted dependent on rate of activity dispersion (Effective half life (= physical + biological))
Given card to keep for 3 months.
Dose: 1.1 GBq – low risk, 3.7 GBq – high risk, 5.0 GBq – patients with known metastasis
Post ablation scan taken 6-9 months later.
