Wk08 LDR Brachytherapy Flashcards
What are isotopes?
Atoms of the same element with the same no of protons but different number of neutrons (i.e differ in mass number)
What are the difference among isotopes?
- different in number of neutrons
- atomic masses
- position in the nuclide chart
- physical properties determined by the mass of atom
Eg. Densities, mass spectrometry, radioactivity, chemical reaction rates
What are the decay mode and half life of I-125 and I-131 respectively?
I-125: electron capture ; 60days
I-131: beta decay ; 8.04 days
Both brachytherapy and teletherapy obey the ___________________.
Inverse square law
(One very close to source and one quite far away)
What is the normal dose rate for low dose rate brachytherapy?
<2Gy/h
What is the main equipment for LDR source measurement?
The well chamber
What implant source is used for prostate brachytherapy?
Iodine-125
LDR dose calibrator well chamber is________________ to increase ______________.
Pressurised ; sensitivity
Dose distribution of a point source is not a a perfect spherical, is a bit oval. This phenomenon is called ___________________.
Anisotropic
What model is used to calculate unsealed source radiation therapy?
MIRD model
Selective internal radiation therapy make use of which radionuclide?
Yttrium-90
What is the half-life and emission of Y-90?
64 hr ; pure beta emission
What are the limiting factors / consideration factors when perform SIRT?
Lung shunting and normal tumour size
What is the mechanism of SIRT?
To block nutrient supply and induce radioactive reaction
What is the another usage of Y-90 besides HCC?
Chronic synovitis and effusion of joints
Pheumatoid arthritis
Due to its good retention in the joint and less traumatic than open surgery
For Iodine-131, what is its physical half-life and effective half-life
Physical half life = 8 days
Effective half-life = 5.5days
At what dose rate and radioactivity that a patient receiving Iodine-131 treatment needed to be hospitalised?
Dose rate >25 uSv/h at 1m
Radioactivity >400MBq
What are normal prescription for I-131 thyroid cancer treatment?
30-150mCi, mainstream 80 mCi after thyroidectomy
More residual thyroid, __________ dose prescribed.
Less
What is the use of I-131 in paediatric?
MIBG scan
To diagnose neuroblastoma
For pain relief to bone metastasis, what radionuclide can be used for treatment?
Sr-89 and Ra-223
They have similar chemical properties as calcium
Selective uptake by bones.
What radionuclide can be used to treat non-Hodgkin lymphoma?
Y-90 labelled lymphoma specific antibody
What is the mechanism behind radioimmunotherapy?
- injected antibody carrying the Y-90 isotope
- the antibody target the antigen CD20 which express in lymphoma
- attachment of the antibody to the antigen induce radiation crossfiring effect and cause radiolysis
- dose delivered to kill lymphoma cells selectively
What is the importance of brachytherapy dosimetry systems?
Dosimetric systems are rules to guide the source arrangement and dose levels according to the geometry of the lesions in a brachytherapy case.
Name some common brachytherapy dosimetry systems
- Manchester system
- Paris system
- Quimby system
- memorial system
What is the principle of early Manchester system?
Half strength needles placed at the middle
Full strength needles placed at the peripheral
What are the 4 basic principles of the PARIS system ?
- The radioactive sources must be rectilinear and parallel, and locate on the same principle plane.
- The reference linear air kerma strength must be uniform and identical for all sources
- Adjacent sources must be equi-distant from each other.
- Applicable from LDR multi-sources to HDR single source brachytherapy plans.
What is RD and BD, and what is their relation?
RD = reference dose ( I.e. prescription dose)
BD = basal dose = local minimum
RD = 0.85BD
BD = 1.18RD
What is Bystander effect?
Directly irradiated cells transmit damaging signal to non-irradiated cells thereby inducing a response similar to that of irradiated cells.
Which artery are the Y-90 microspheres injected through?
Hepatic artery
What is the assumption of the partition model?
Assume all administered activity is deposited in normal liver, tumour and the lungs
In 3D dose-volume analyses in IGBT, what is the definition of D90, D98, D100, V90, V100, V200, D2cc, D1cc, D0.1cc and Vd
D90 = the minimum dose delivered to 90% of the target volume
D98 = the minimum dose delivered to 98% of the target volume
D100 = the minimum dose delivered to the target volume
V90 = the percentage coverage of the target volume by the 90% isodose level
V100 = the percentage coverage of the target volume by the prescription dose level
V200 = the percentage coverage of the target volume by the high dose zone
D2cc = minimum dose to the most irradiated 2cm3 of the OAR (to assess morbidity)
D1cc = minimum dose to the most irradiated 1cm3 of the OAR
D0.1cc = minimum dose to the most irradiated 0.1x3 of the OAR (to access morbidity of very small OARs eg urethra)
Vd = volume of a certain dose D or above in a OAR
What do MIRD stands for
Medical Internal radiation dose
What are the dose limits and dose requirement for SIRT (Y-90) therapy to HCC patients?
Dose limit to lung = 30Gy single therapy and 50Gy cumulative
Dose limit to normal liver = 80Gy to normal liver and 70Gy to cirrhotic liver
Dose requirement to tumour is 120 Gy
What are the assumptions of Y-90 SIRT dose calculations?
- No biological degradation is expected
- Assume a homogeneous distribution of micro-particles in the tumour and liver parenchyma
- Assume Tc99m particles will approximate Y-90 microsphere distribution in partition modelling
- Body surface area is an estimation of patient liver volumes
