Chapter 9: Machine Dose Rate Constancy

term. Dose Rate

number of x-rays coming out of the machine

Dose rate aka

output

True or False: Dose rate does not vary from day to day

False

What must the physicist check?

Baseline Calibration

Calibration in this case means:

A full and complete testing of the machine’s performance, including output (dose rate)

When is full calibration (baseline calibration) required?

1. When a machine is newly installed (commissioned)

2. There has been a change in machine operation

   1. Electronic repair

   2. timer repair

   3. flattening filter adjustment

   4. etc.

spot-check aka

constancy check

Spot-check

a basic check of the machine’s performance (as opposed to a full calibration)

can be performed by the RT(T) (as opposed to the physicist)

Frequency of Machine Dose Rate Constancy QA for:

1. Cobalt

2. Linac

1. NRC requires monthly spot-checks and yearly full calibrations

2. Weekly spot-checks and monthly full calibrations

Machine Dose Rate Constancy: Sources of malfunction for Cobalt

1. failure of the source to come in the ON position

2. Timer malfunction

3. Incorrect decay factor (dosimetrist doesn’t account for the 1.1% delay for cobalt)

Timer error:

the travel time for the source to arrive in the ON position

Typical timer error:

0.02 minutes

Machine Dose Rate Constancy: Sources of malfunction for Linac

1. Change in E of e- striking the target

2. Changes in MU measuring system (MUs 1 &2)

3. Opening of “sealed” monitor chamber to air

Materials for:

1. full calibrations

2. spot-checks

1. Ion chambers are required for full calibrations

   BUT other dosimeters can be used for spot checks

   1. Ion chambers

   2. Monitrex device

2. Baseline reading value established by physicist

3. Therapist compares baseline reading value with the readings they get

Technique of Spot-Check

• Varies significantly with:

  • dosimeter used

  • tx machine

• Each institution has its own technique established by the physicist

• But all techniques involve: using a dosimeter to measure output

Action Guidelines:

Dose rate constancy tolerance (output):

1. Spot checks:

2. Monthly calibrations:

1. ± 3%

2. ± 2%

Physicist gives you baseline reading. What formulas do you use to calculate the range that represents tolerance for:

Upper limit

Lower limit

Upper limit: baseline reading x 1.03

Lower limit: basline reading x 0.97

Eg. Baseline limit is .777

What is the tolerance range for the values?

.777 × 1.03 = highest number

.777 × 0.97 = lowest number

The number you get MUST be a number IN BETWEEN these two numbers