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Manage Conversion Factors

David Kirkaldie
  • Updated

Topics covered in this article include:

Equipment Conversion Factors

Introduction

Conversion factors are used for the monthly output tests and for TG-51. There are separate factors for each. For the monthly output tests only, a combined solid water factor may be used that includes both the electrometer and the chamber.

Updating Equipment Calibration Factors

Updating equipment calibration factors can only be done on existing measurement devices. Instructions on adding a new measurement device can be found in this article:  Adding a New Measurement Device.

To update equipment calibration factors on an existing measurement device, navigate to Manage from the main tool bar, Measurement Devices on the manage schedule page, and then click the Calibration link for the desired measurement device.

 

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Click the Update Equipment Calibration button to bring up the Update Calibration Factors Page:

 

 

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For Ion Chamber measurement devices fill in the " CF8.png ", "Calibration Date", and "Expiration Date" fields. Optionally a calibration document can be uploaded using the Attach a Document button. Next click the Sign and Update calibration button to sign off to save the calibration factors.

 

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For electrometer measurement devices fill in the "Settings", "P elec", "Calibration Date", and "Expiration Date" fields. Optionally a calibration document can be uploaded using the Attach a Document button. Next click the Sign and Update calibration button to sign off to save the calibration factors. 

 

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Viewing Previous Equipment Calibration Factors

Navigate to Manage from the main tool bar, Measurement Devices on the manage schedule page, and then click the Calibration link for the desired measurement device.

Next click the View Older Equipment Calibration button.

 

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Clicking this button will display previous calibration settings and the data and time that they were superseded.  To return to the current calibration click the View Current Calibration button.

 

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Machine Conversion Factors

Introduction

If you are using a daily QA device that does not convert electrometer readings to dose or an ion chamber and electrometer combination you will have to establish a set of conversion factors to convert raw readings to dose for output tests. Note that some daily QA devices, such as the Standard Imaging QA BeamChecker+ and the Sun Nuclear Daily QA 3, perform this conversion internally. In these cases, the dose data is retrieved from the respective databases and no further conversion is required.

These conversion factors will have to be established for each unique measurement system where a measurement system is defined as a unique combination of:

  • Linac 
  • Energy 
  • Chamber (or chamber system e.g., Fluke) 
  • Electrometer 
  • Measurement Setup (Build up, Depth, etc.)

These conversion factors should not be confused with the traceable calibration factors established for the individual components by ADCL calibrations. 

The conversion factors at the various energies will be saved with the individual QA equipment record. You will have to ensure that your measurement setup is consistent from measurement to measurement. 

When you establish your QA protocol, you will select the QA equipment that you will use to perform the QA and the energies at which you will perform the QA. 

As you perform the QA you will enter the raw electrometer readings along with the current temperature and pressure conditions for each energy you configured in your setup. The raw readings will then be converted to dose and compared against your baseline measurements. 

Your baseline target for output can be set to either dose or dose per MU (typically 100 cGy or 1 cGy/MU). Your baseline will generally remain constant as this baseline is what your TPS is assuming when creating treatment plans.

Establishing Initial Conversion Factors

The most common way to establish an initial set of conversion factors is from annual TG51 or TRS-398 measurement data as you have a water tank reference for absolute dose at various energies. Typically, several ion chamber measurements are made for each energy with the same setup used for the monthly output tests and averaged to establish the conversion factor. 

Examples

This example demonstrates working backward from your final output. Here are the givens:

  • Three measurements are recorded 15.5, 15.6 and 15.4 nC with a particular chamber at 6X. 
  • The output as measured by the water tank is 100.2 cGy. 
  • The output test is conducted using 100 MU. 
  • The atmospheric conditions are 23℃ and 1015 hPa. 
  • The final output is measured in cGy/MU
  • The schedule is using a combined electrometer and ion chamber factor.
CTP = (273.2 + 23) ÷ (273.2 + 22) × 1013.3 ÷ 1015 
    = 1.0017

Conversion Factor: 
CF = Output/MU × Beam Monitor Units ÷ (Mean electrometer reading) × CTP 
   = 1.002 cGy/MU × 100 MU ÷ (15.5 nC × 1.0017) 
   = 6.4535 cGy/nC.

Another scenario would be to pull the individual factors from an existing spreadsheet and eliminate the factors that are already used for the schedule. If you are using a combination factor for your electrometer, the CTP needs to be removed. If you are using individual electrometer and ion chamber factors, CTP and Pelec will need to be removed. Here is an example given the following:

  • Three measurements are recorded and average 14.36 nC for a particular ion chamber and energy. 
  • The atmospheric conditions are 22℃ and 1008.3 hPa for a CTP of 1.0050. 
  • Pelec = 0.9990
  • Ppol = 0.9986
  • Pion = 1.0042
  • N60CoD,w = 5.4070
  • Kq = 0.992
  • TMR = 0.777
  • The final output is measured in cGy
  • The schedule is using a combined electrometer and ion chamber factor.
Output = ((Mean electrometer reading) × CTP × Pelec × Ppol × Pion × N60CoD,w × Kq) ÷ TMR    
 99.80 = 14.36 × 1.0050 × 0.9990 × 0.9986 × 1.0042 × 5.4070 × 0.992) ÷ 0.777
    
Conversion Factor: 
CF = (Pelec × Ppol × Pion × N60CoD,w × Kq) ÷ TMR 
   = (0.9990 × 0.9986 × 1.0042 × 5.4070 × 0.992) ÷ 0.777 
   = 6.9155 cGy/nC.

This scenario is the same as the last but using individual electrometer and ion chamber factors. Here is an example given the following:

  • Three measurements are recorded and average 14.36 nC for a particular ion chamber and energy. 
  • The atmospheric conditions are 22℃ and 1008.3 hPa for a CTP of 1.0050. 
  • Pelec = 0.9990
  • Ppol = 0.9986
  • Pion = 1.0042
  • N60CoD,w = 5.4070
  • Kq = 0.992
  • TMR = 0.777
  • The final output is measured in cGy
  • The schedule is using individual electrometer and ion chamber factors.
Output = ((Mean electrometer reading) × CTP × Pelec × Ppol × Pion × N60CoD,w × Kq) ÷ TMR    
 99.80 = 14.36 × 1.0050 × 0.9990 × 0.9986 × 1.0042 × 5.4070 × 0.992) ÷ 0.777
    
Conversion Factor: 
CF = (Ppol × Pion × N60CoD,w × Kq) ÷ TMR 
   = (0.9986 × 1.0042 × 5.4070 × 0.992) ÷ 0.777 
   = 6.9224 cGy/nC.

This process must be repeated for each energy that will be measured on the equipment. It may also be beneficial to establish the conversion factors for a second measurement device to aid in troubleshooting variations detected in daily and monthly tests. 

NOTE: The examples above assume that the electrometer is configured to display in nC and that radiation is measured in cGy. You may need to calculate a conversion factor to produce the final dose appropriately for the units displayed by your device. Also the result is displayed either in cGy or cGy/MU depending on the QA settings for the schedule.

 

Adding a New Measurement Device 

To add a new measurement device, navigate to Manage from the main tool bar, Measurement Devices on the manage schedule page, and then click the + Add device button in the right hand corner of the equipment page. 

 

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The Add Measurement Device page will open.  Remember that each measurement device represents the whole measurement system. The label and the description should reflect this. In the example below the combination of the ion chamber and the electrometer is noted. It may also be helpful to note relevant details of the device and setup in the device description. 

 

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Once you have entered the details of the device click the Sign and Update device button. The new device will be added to your list of devices.

Adding and Updating Machine Calibration Factors

To add the calibration factors to convert the raw readings to dose navigate to Manage from the main tool bar, Measurement Devices on the manage schedule page, and then click the Calibration link for the desired measurement device. The Calibration Factors for Measurement Device page will display.

Under the Machine Calibration Settings section, Click the Add Solid Water Calibration Factors button to bring up the Add Calibration Factors Page.

 

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Select a Site and Machine combination and enter the applicable calibration factors. It Is only necessary to enter the calibration factors for the energy levels clinically used.  You can set an expiration date for the calibration and you will receive an email reminder before the calibration is due. You may optionally attach a calibration document for the device to aid in your machine management. To save the calibration factors you must provide a digital signature by clicking the Sign and Update calibration button. 

 

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Updating Machine Calibration Factors

To update machine calibration factors navigate to Manage from the main tool bar, Measurement Devices on the manage schedule page, and then click the Calibration link for the desired measurement device.  The Calibration Factors for Measurement Device page will display.   

Under the Machine Calibration Settings Section, locate the Site and Machine combination and click the Update Solid Water Calibration Factors button to bring up Update Calibration Factors page.

 

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Enter the calibration factors for the energy levels you wish to update.  You may optionally update Expiration dates and attached documents for the device to aid in your machine management. To save the updated calibration factors you must provide a digital signature by clicking the Sign and Update calibration button. 

 

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Viewing Previous Machine Calibration Factors

If the device has had multiple calibration factors established over time for a given Site, a button will appear labeled View Older Calibration. 

 

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Clicking this button will display previous calibration factors and the data and time that they were superseded.  To return to the current calibration click the View Current Calibration button.

 

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Integrating Conversion Factors into QA Schedules

Once a template has been scheduled for a machine, the measurement device can be assigned to the schedule using the QA settings link from the "Manage Schedule Configurations" page. Click the sprocket that appears when hovering over the appropriate QA schedule to open the QA settings page. 

Select the specific measurement devices that will be associated with the QA and preset as many of the other settings as you wish for the QA schedule. You will also select the units for the output (cGy or cGy/MU). If you are using a direct connection to a supported measurement device you will have to install the Equipment Hub to connect to it.

Setting Baselines for Output

Generally you will want to set baselines and tolerances for your daily and monthly output measurements. Baseline and tolerances can be accessed and set from the Manage page. 

If you set your units in the QA settings to cGy you will want to set your baseline to an absolute dose amount (typically 100 cGy). If you set your units to be cGy/MU you will want to set your baseline to the dose per MU expected by your TPS (typically 1 cGy/MU).

Entering or Retrieving Data

After you have configured your templates and conversion factors you can use your scheduled template to enter data. New data entry will be done from the Schedule page. 

If you are entering data from a directly connected daily QA device a button labeled Get results will be present to retrieve the data. If the device required conversion factors these will be applied to the raw data.

If you are entering output data manually from an ion chamber array you can enter and average up to 3 readings. The application of the conversion factor when calculating the output varies depending on whether the units are cGy or cGy/MU.

cGy: Output = Cf x CTP x Raw Reading
cGy/MU : Output = (Cf x CTP x Raw Reading)/MU Delivered 

The readings will automatically be converted and compared to your established baselines. Failures against baselines will automatically generate email alerts on the failures.

Ongoing Management of Conversion Factors

  • Once conversion factors and baselines are established it is important to maintain a consistent protocol for trends and comparisons over time to be meaningful. 
  • If daily or monthly measurements show significant deviations from baseline or previous measurements investigation into possible changes in the measurement system should be done before considering machine adjustments. 
  • Adjustments to conversion factors should only be performed if you have a reason to believe that the measurement system has changed. Although TG-51 and TRS-398 measurements may reveal machine output or measurement system changes they do not by themselves constitute a reason to adjust conversion factors. Adjusting conversion factors without having identified an underlying cause can mask a deteriorating measurement system.

 

Video Example of Setting Up Monthly Output

Below is a video showing how to set up your monthly x-ray and electron output constancy tests:

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