Critical Machine Parameters
Linear accelerators are very sensitive and any mishandling or abnormal readings of the critical parameters such as temperature and pressure may lead to common errors that can interrupt treatment sessions. Hence need for routine check-up, Quality Assurance and Quality Control procedures to prevent systematic errors as well as reducing the random errors. Other critical parameters that can influence the accuracy of linear accelerators and radiation dosimetry include radiation field size (FS) and source-to-surface distance (SSD).
The density of atmospheric air depends on temperature, pressure, moisture content and composition of the air. A change in temperature of 10°C causes a change in air density due to its moisture content of only 0.5%. Therefore, atmospheric air density inversely depends on absolute temperature. Variation of temperature by ±3°C that corresponds to air density change would result in an error of ±1%. Increase in temperature leads to decrease in density and decrease in dosimeter reading.
Density of atmospheric air varies directly with pressure. Variation of atmospheric pressure of ±10mm Hg will introduce an error of about ±%. Meaning increase in pressure leads to increase in density and increase in linear accelerator output reading or dosimeter reading. This increases radiation absorption hence higher dose reading.
Therefore it is recommended that ionization should be measured in air at a temperature of 0° and a pressure of 760mm mercury.
Correction of temperature and pressure is only done when the ionometric measurements of radiation beam outputs from linear accelerators are beyond the established tolerance levels. The tolerance levels of maximum deviation of ionometric measurements as recommended by the American Association of Physicist in Medicine (AAPM) Task Group No.13 is ±3%.
Correction of density at 0°C (273K), absolute temperature, and 760mm mercury may be calculated from the density dt at temperature t°C (273+t) and pressure p using the following equations:
where t is temperature and p is pressure.
d0 = dt
1) Mohammed Ahmed Ali Omer. 2017. Instrumental Quality Control of Therapeutic Linear Accelerator Performance. American Journal of Physics and Applications. Vol 5.
2) Saw CB. 1997. Local Temperature and Pressure variations and its effect on daily ionometric measurements of radiation beam outputs from linear accelerators.
Retrieved from www.ncbi.nlm.nih.gov on 24/1/2019.
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