Category: Dosimetry

Why Co-60 machine has 80cm SAD?

Thirumurugan2 Comments

The reasons are Geometric penumbra and Source Strength

Geometric penumbra

  • It is due to the finite size of the source

The width of the geometric penumbra (Pd) at any depth (d) from the surface of the patient is given by

  • s is the source size
  • SSD is Source to Surface Distance 
  • SDD is Source to Diaphragm Distance

Source Size is varied while SSD and SDD are kept constant

From the above image its clear that as source size increases the Geometric Penumbra increases
In case of cobalt 60 the source size are between 1.5 cm diameter to 2 cm diameter

SDD is Varied

As the SDD increases the geometric penumbra decreases, but as the diaphragm gets near the skin the scattered radiation from the diaphragm results in more skin dose, hence there must be at-least 15 cm gap between skin and diaphragm.

As the SSD increases,due to the effect of inverse square law the geometric penumbra increases.

Source Strength

The Source strength influences the dose rate, the Dose date decreases as the SSD increases due to inverse square law effect

As the PDD determines how much dose can be delivered relative to Dmax, the SSD need to be as large as possible, but as mentioned earlier the dose rate decreases due to distance.

Considering all this factors the SAD is kept at an optimal distance of 80 cm.

In new machine100 cm SSD is available due to more specific activity of source, without changing the source size, it is possible to go for higher SSD not by changing the geometric penumbra.

why temperature and pressure correction factor used while calculating dose using ion chambers?

ThirumuruganLeave a comment

The ion chambers used in measuring the adsorbed dose are generally vented ion chambers. Thus the mass of the air  inside the sensitive volume of the chamber varies with respect to the density(𝝆) and volume(V) i.e. m = 𝝆V,  As the volume of the chamber remains constant, the density varies with respect to pressure and temperature

As the pressure increases the density increases(i.e.more air molecules) {Boyel’s law} , hence more interaction occurs. This results in more number of charge collection. The dose measured in this case is overestimated from that measured in standard condition

If the temperature increases the density decreases{Charle’s Law}. In this case, the number of interactions are less and the dose measured is under estimated from the measured dose under standard conditions.

To compensate this , the temperature-pressure correction factor(KTP) was introduced

Po and To are reference values of pressure(KPa) and temperature(℃) according to SSDL. P and T are air pressure and temperature at the time of measurement

The dosimeters what we use in clinics are relative dosimeters, i.e. they are calibrated under laboratory conditions. These laboratory conditions may change between SSDL’s, one might have did the calibration with temperature as 20 ℃  and other might have done with 21℃ , so always its a best practice to refer calibration certificate for reference temperature and pressure.