Radiation Dosimetry

Types of dosimetry:  Relative – measure changes in dose rather actual doses (PDD, TMR) 

Absolute: Direct measurement of dose  (e.g. calorimetry)

Reference: standard set up to use as reference to calibrate other measuring devices 

Beam quality index – A measure used to compare the penetrating effect of different radiotherapy beams. For a linac, each beam of a given energy might have a different energy spectrum. An 8MV beam has a maximum energy of 8MV, however the spectrum of energies in the beam are variable and the average energy of ALL particles is actually close to 1/3rd the maximum energy. Beam energy depends on linac, site, day —> lots of factor Not reliable. Therefore BEAM QUALITY INDICES ARE USED TO DESCRIBE THE BEAM 

Measures of beam quality: 

for PHOTONS 

HVL: thickness of material that attenuates photon beam to 50% of its original value. (USED FOR kV BEAMS). A narrow beam should be used so as to reduce scatter. Scatter with artificially increase HVL as more dose to a point with be delivered due to scatter, and more depth will be presumed to be needed to attenuate beam by 50% 

D10: Percentage Depth Dose at 10cm for a 10×10 field 

PDD ratios (TPR 20,10)

for ELECTRONS 

R50,D : depth at which measured dose is 50% of d max (USUALLY USED FOR ELECTRONS). Usually 4E in mm. 

DOSE MEASUREMENT or DOSIMETRY: What is the standard, what equipment, what measurements needed and why

Calorimetry is the best technique for absolute and direct measurement of absorbed dose. It is based on the measurement of the temperature rise of a given volume element of this instrument when it is exposed to a source of radiation. This temperature rise is expressed and calibrated as absorbed dose to water. 

A free air chamber measures air kerma. KERMA is an acronym for Kinetic Energy Released Per Unit Mass in Air. Photons interact with matter via PE and Compton effect to release free electrons. This kinetic energy of electrons released in air is the Air Kerma. These electrons deposit most of their dose in the medium though electron-matter interactions, however a small fractions is lost as radiative energy or bremstrauhlung. The energy deposited in the medium is the absorbed dose. Thus the primary reference standard free air chamber measures Air Kerma and this reading too is then calibrated according to absorbed dose to water in the NPL.

A calibration factor/coefficient is produced by the NPL that helps calibrate the readings obtained from secondary standards, according to the reference primary standard. And similarly the field instruments are calibrated against the secondary standard using an inter comparison ratio.

Farmers chambers/Thimble ion chambers are used here interchangeably. Both Farmers chambers and parallel plate chambers are ionisation chambers. These are small and can be inserted into water or perspex that act as a tissue equivalent substance. These instruments are gas filled instruments that measure the ion pairs produced as a result of incident radiation (when placed in water/perspex) at the site of the chamber. The production of ion pairs results in an ionisation current that is read by an electrometer. 

The number the electrometer reveals is the final OUTPUT or reading of the chamber in coulombs or C

The calibration factor is expressed as Gy/C. The product of the reading from field instruments in Coulombs (C)  multiplied by appropriate calibration coefficients and adjusted for correction factors (such as temperature/pressure/ion recombination), gives us the final reading in terms of dose absorbed to water in Gy. This essentially is the aim of the process of dosimetry – ie, to calculate dose.

Field instrument reading (C) x calibration coefficients x correction factor = final reading as absorbed dose to water (Gy)

Electrometers are also sent to NPL and have their own standards.

Daily Output measurements from Linacs are done with field instruments and we need an accuracy of 2% 

CONSTANCY CHECK – completed every few months. Reading from a instrument compared against previous reading under standard conditions and should be within 1%. Sr -90 used as a reference comparison standard (Beta emitter) – radioactive element with constant and reproducible energies emitted. Long half life of 30 yrs so can used for a long time in the department too.  

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s