Cobalt-60 Calculation Factors

ONCOL 355 - Tx Planning and Dosimetry I. University of Alberta

What is the calibration condition for Cobalt-60 calculations?

1. 10 × 10 field

2. measured at 80 cm

3. measured along the beam’s central axis

Do Co-60 machines have calibration factors (CF)?

no, there is no CF because this machine uses a cobalt pellet as it’s RT source

What is used instead of a calibration factor for Co-60?

Output factor

• the dose rate to a small mass of tissue at the isocentre for a 10×10 cm field

What is the output of a Co-60 treatment machine related to?

1. the source’s activity

2. source’s dose rate at different points in time due to decay

Why does the Co-60 machine’s output change over time?

due to the decaying source

half-life

time for the radioactive source to decay to half of it’s original intensity

what is Co-60 half-life?

5.26 years

• decays ~1% per month

Why is the mid-point date of treatment used to calculate the source’s output?

the output needs to reflect the average dose rate over the course of the patient’s treatment

What is the decay factor formula

in treatment timer calculations, the dose rates in cobalt tables need to be adjusted to reflect…

the average dose rate over the patient’s treatment course

what are the three steps to finding the midpoint date

1. take the # of fractions from the dose prescription and count out the actual treatment days for the patient starting from the first treatment to the last treatment (don’t count weekends or stat holidays)

2.  count the total days within the treatment course range including weekends and holidays and divide this number by two to get half of the treatment course length

3. starting on the patient’s start date, count out this half-way point number on the calendar

How does collimater scatter factor (S_c) differ between Cobalt and LINAC?

the concept is the same, but the measurement is at 80 cm along the central axis and not 100 cm

Is phantom scatter factor (S_p) used in Co-60 machines?

No!, TAR @ dmax will account for scatter at dmax depth

What is TAR

the ratio of absorbed dose at depth in a tissue to the absorbed dose at the same point in air within a build-up cap

what does TAR take into account

scatter in the patient at the depth of treatment

• in Cobalt-60 unit calcs only

what is a build up cap?

a probe that is covered with enough material to ensure the establishment of EE in the reference measurement

three factors that affect TAR

1. field size

2. depth

3. energy

is TAR independent of SSD?

yes

list 5 other factors that may be used alongside TAR in Co-60 calculations

1. ISL

2. PDD

3. Physical Wedge Factors

4. Perspex Tray Factors (to hold shielding blocks)

5. physical compensator factors

what is the formula for dose rate in a cobalt machine?

DR = midpoint output x ISL x Sc x TAR x other factors

What is a Co-60 timer setting

a measure of how long the beam is on for each treatment field, measured in minutes

• used because the source decays over time

What is the formula for Tx time

what is a time correction (tc)?

a factor that accounts for when the treatment time starts relative to the movement of the cobalt source from the source drawer to the ‘On’ position at the collimator opening

Describe a positive timer correction

the timer starts once the source starts moving, but the source is not fully in the ‘on’ position yet. Thus the full dose won’t reach the patient by the time the time stops

• time must be added to account for this shortage!

  • typically +0.01-0.02 minutes

Describe a negative time correction

the timer starts once the source is in the fully ‘on’ position, however some dose will exit the collimators and reach the patient before the timer starts. Thus time must be subtracted to account for dose overage

• typically -0.01 - 0.02 minutes