Radioactive Isotopes Flashcards
Half-life of Ra-225
1600 years
Half-life of Rn-222
3.83 days
Half-life of Co-60
5.26 yrs
Half-life of Cs-137
30 yrs
Half-life of Ir-192
73.8 days
Half-life of Au-198 (gold)
2.7 days
Half-life of I-125
59.4 days
Half-life of Pd-103
17 days
Photon E (MeV) of Radium
0.83 avg
Photon E (MeV) of Rn-222
0.83 avg
Photon E (MeV) of Co-60
1.17; 1.33 (1.25 avg)
Photon E (MeV) of Cs-137
0.662
Photon E (MeV) of Ir-192
0.38 avg
Photon E (MeV) of Au-198
0.412
Photon E (MeV) of I-125
0.028 avg
Photon E (MeV) of Pd-103
0.021 avg
Exposure Rate Constant (Γ) of Radium
8.25 R-cm^2/mg-hr
Exposure Rate Constant (Γ) of Rn-222
10.15 R-cm^2/mCi-hr
Exposure Rate Constant (Γ) of Co-60
13.07 R-cm^2/mCi-hr
Exposure Rate Constant (Γ) of Cs-137
3.26 R-cm^2/mCi-hr
Exposure Rate Constant (Γ) of Ir-192
4.69 R-cm^2/mCi-hr
Exposure Rate Constant (Γ) of Au-198
2.38 R-cm^2/mCi-hr
Exposure Rate Constant (Γ) of I-125
1.46 R-cm^2/mCi-hr
Exposure Rate Constant (Γ) of Pd-103
1.48 R-cm^2/mCi-hr
Natural log of 2
ln2 = .693
Decay Constant formula
λ = .693/T1/2
Half Life formula
T1/2 = .693/ λ
Mean/Avg Life formula
Ta = 1.44 x T1/2
Activity formula
A = Ao x e^(- λt)
Total Dose formula
TD = Ta x output;
or
TD = 1.44 x T1/2 x Do
(Do = dose ouput)
Formula used to determine exposure by radioactive isotope:
X = Γ x (A/(r^2)) x t
r=distance
Formula used to convert source to Ra Equivalent
A x( Γ / (8.25Rcm2/mg-hr))
Γ = exposure rate constant of source
Formula used to determine dose distribution of radioactive isotope
Γ x mgRa Eq Source x (d1/d2)^2
ex. Dose rate at 9cm from 10mgRa eq source = 8.25 x 10 x (1/9)^2
**used after source has been converted to mgRa Equivalent
Decay per year (%) for Pd-103
100%
Decay per year (%) for I-125
98.5%
Decay per year (%) for Ir-192
96.7%
Decay per year (%) for Co-60
12.3%
Decay per year (%) for Cs-137
2.3%
How many half-lifes until a radionuclide is gone?
10 half-lifes
Rule of Thumb: Cs-137 decays _____?
1% per 6 months
Rule of Thumb: Co-60 decays _____?
1% per month
Rule of Thumb: Ir-192 dcays ____?
1% per day
Describe Secular Equilibrium
- half-life of parent»_space;> half-life of daughter;
- decay constant of daugher»_space;> decay constant of parent;
ex. Ra-226 (parent) and Rn-222 (daughter)
Describe Transient Equilibrium.
- half-life of parent > half-life of daughter;
- decay constant of daughter > decay constant of parent
ex. Mo-99 (parent) and Tc-99m (daughter)
HVL (mm Pb) for Ra-226
8mm
HVL (mm Pb) for Cs-137
5.5 mm
HVL (mm Pb) for Co-60
11mm
HVL (mm Pb) for Au-198
2.5mm
HVL (mm Pb) for Ir-192
2.5mm
HVL (mm Pb) for I-125
0.025mm
HVL (mm Pb) for I-103
0.008mm
What is the Air KERMA equation:
U = uGy - m^2/hr
**can be measure in Gy or distance
What isotopes are used for prostate brachy (LDR)
I-125 and Pd-103
Disadvantage of Rn-222 and what replaced it?
leakage; Cs-137
Advantages of Cs-137
- less hazardous
- less shielding
- Same penetrating power as Radium up to 10cm
Define the location of Point A for T and O
2cm superior of Os, 2cm lateral of cervical canal
What is typical dose for Point A of T and O
0.35 (tandem) + 0.19 (Ovoid) = 0.54 Gy/hr
Define the location of Point B of T and O
2cm superior of Os, and 5cm lateral from midline
What is the typical dose for Point B of T and O
Pt B dose = Pt A dose / 3
What is the physical length of a Cs-137 source
2cm
What is the active length of a Cs-137 source
1.4-1.5cm
Define Source Anisotropy
Self-Attenuation
At what distance must ISL be accounted for, for a line source
at 2x the source length
ISL must be used to account for source strength for what type a source
point source
What is Sr-90 and what is it used to treat?
Strontium-90; Pterygium
What is the E and half-life of Sr-90 compared to Y-90
Sr-90 = 540 keV B-; T1/2 = 28.5 years Y-90 = 2.2 MeV B-; T1/2 = 64.2 hours
What radionuclide is used for Thyroid Ablation, and what is the typical dosage?
I-131; 100mCi - 200mCi
What is the physical and biological half-life of I-131?
Physical = 8 days Biological = 12 days
Formula for Effective half-life:
Effective half-life = (Tb x Tp) / (Tb + Tp)
*Tb = biological half-life; Tp = physical half-life
Typical Rx for prostate seed implant (LDR):
Primary: I-125 = 144Gy; Pd-103 = 115Gy
Boost: I-125 = 120Gy; Pd-103 = 90Gy
Dose Rate of I-125
5-10 cGy/hr
Dose Rate of Pd-103
20-30 cGy/hr
Dose Rate of Au-198
64 cGy/hr
Where in the prostate are seeds implanted, why?
peripheral; to stay off urethra
What is max dose to Urethra for brachy prostate (LDR)
<150% of Rx
How many seeds are ordered for brachy prostate (LDR)
10% more than needed
ICRU-38 LDR dose rate:
0.4 Gy/hr to 2.0 Gy/hr, OR
40 rad/hr to 200 rad/hr
ICRU-38 MDR dose rate:
2.0 Gy/hr to 12 Gy/hr, OR
200 rad/hr to 1200 rad/hr
ICRU-38 HDR dose rate:
> 12 Gy/hr, OR
>1200 rad/hr
What source is used for HDR prostate, and what is the initial strength when ordered?
Ir-192; 10 Ci
Daily QA for Brachy afterloader include?
Interlocks, Source Positioning (+-1mm), Area Monitor (GM)
Quarterly QA for Brachy afterloader?
Calibration (re-entrant chamber); Source Positioning (film); Emergency Source Retraction; Adaptors
Advantages of HDR:
- Rad Protection
- Decrease Human Error
- Optimized dose
- More accurate
- decreased pt discomfort
- Outpatient
- Avoid General Anesthesia
- Scheduling
Disadvantages of HDR:
- Can’t take back dose
- Rad-bio (depending on fractionation)
- Source Cost
For HDR planes and dwell positions are separated by ___
1cm
What is the basal dose point?
the geometric center of triangles
What is the typical dose for Mammosite?
10 fx at 340cGy - TD = 34 Gy (3400cGy), BID
Where is Rx point located for Mammosite?
1cm from ballon
Symmetry of balloon for Mammosite:
<2mm symmetric
