4
ABSORBED DOSE
Question 2
Why is the concept of exposure not suitable for dosimetry of megavoltage beams?
Question 3
What is the definition of the roentgen (R)? Express this quantity in SI units.
Question 4
What is the f factor for absorbed dose?
Question 5
How is the f factor used to calculate absorbed dose?
Exposure is defined as the quotient 
, where dQ is the total charge of ions of one sign that are produced in air when all charged particles produced by photons are completely stopped, and dm is the mass of air within which the charged particles are liberated.
Answer 2
The definition of exposure requires that charged particles are completely stopped in air. For megavoltage beams, the range of charged particles in air is too large for measurement of exposure to be practical.
Answer 3
The R is the unit of exposure. The original unit was defined as the amount of radiation that liberates 1 esu (electrostatic unit) of charge per cubic centimeter of air. To express the roentgen in SI units, we use the conversion factor for esu to C (Coulombs) and the density of air (approximately 1.293 kg/m3). Since 1 esu ≈ 3.34 × 10−10 C, we have
Answer 4
The f factor is used to convert exposure, which is only defined in air, to dose in a medium. It is termed the roentgen to rad factor. For air, f is 0.876.
Answer 5
Assuming that charged particle equilibrium (CPE) exists at the point of interest; dose in the medium can be calculated using the formula:
where X is the exposure (R) and 
is the energy fluence ratio in the medium m to that of air. The f factor of water ranges from 0.0.88 to 0.97 depending on the energy of the X-rays.
Question 7
What is absorbed dose? What is its unit?
Question 8
Does absorbed dose uniquely specify the biological effect on tissues?
Question 9
What theory relates the charge collected in an ionization chamber to the dose delivered to the medium around the chamber?
Question 10
According to the Bragg–Gray cavity theory, how is the dose to the medium surrounding the chamber related to the dose to the air inside the chamber?
The f factor is defined as:
The numerator of this expression contains the average mass energy absorption coefficient for the material of interest. Different materials have different interaction cross sections and hence f is inherently medium dependent. The mass energy absorption coefficient is also energy dependent since different interaction processes dominate in different energy domains. Recall from Question 4 that 0.876 is the f factor for air.
Answer 7
Absorbed dose is the energy absorbed per unit mass. This is a physically measurable quantity. Its standard unit is the gray (Gy), which is 1 joule (J) of energy absorbed by 1 kg of mass.
Answer 8
The biological effect of radiation on tissue is not uniquely determined by the absorbed dose. It may depend on other qualities of the radiation such as the type of radiation and the rate the radiation is delivered.
Answer 9
Bragg–Gray cavity theory relates the number of ions collected in an ion chamber to the dose delivered in a medium in which the chamber is placed.
Answer 10
The mean ratio of mass stopping powers of the medium and the gas inside the chamber is the same as the ratio of dose in the medium to dose in the gas.
Question 12
How does Spencer–Attix cavity theory differ from Bragg–Gray cavity theory?
Question 13
What is charged particle equilibrium (CPE)?
Question 14
What equation is used for calibration of linear accelerators in the TG-51 protocol? Briefly describe each of the terms and its units.
First, the presence of the cavity medium does not affect the charged particle field because its thickness is small compared to the range of charged particles impinging on it. Second, all the dose in the cavity medium is deposited by the charged particles traversing it.
Answer 12
Spencer–Attix cavity theory instead uses the restricted mass stopping power with a cutoff value of Δ (typically 10–20 keV) to take into account the effect of delta rays. Spencer–Attix is therefore more accurate.
Answer 13
CPE is the concept that given a volume V, every charged particle of one sign and energy exiting V is, on average, replaced by an identical charged particle of the same energy entering V.
Answer 14
where 
is the absorbed dose to water measured at the reference point in a beam of quality Q. Its unit is the Gy.
is the calibration coefficient for converting measured charge to dose. It has units of Gy/C.
M is the fully corrected electrometer reading, in units of C.
kQ is a quality conversion factor which corrects the calibration coefficient for differences in beam quality between the beam which is being calibrated and a 60Co beam. It is unitless.
Question 16
Why is the temperature and pressure correction necessary?
Question 17
What is the result of performing the TG-51 protocol?
Question 18
What is the typical calibration geometry?
Question 19
Why does changing the polarity of the voltage applied to an ion chamber affect its response?
TG-51 defines the temperature and pressure correction factor as:
where P is the atmospheric pressure in units of mmHg. For P given in units of kPa, the conversion 1 kPa = 7.5 mmHg must be used. The temperature T in ℃ is converted to kelvin (K) by adding 273.2, and 295.2 is the temperature in K under reference conditions.
Answer 16
For unsealed ion chambers, response can change because the mass of air within the chamber varies with pressure and temperature. PT,P corrects for deviations of temperature and pressure from reference conditions (295.2 K and 760 mmHg) under which the chamber was calibrated.
Answer 17
The TG-51 protocol provides a means for calibrating the output of a linear accelerator. The result is the calibration of dose to monitor units (MUs) at one specific point in a water phantom, at one distance, for one field size.
Answer 18
Typically, the calibration geometry is a reference field size of 10 cm × 10 cm, 100 cm distance from the source to the surface of water, and at the nominal maximum depth of the energy calibrated. Determination of dose at other points can then be calculated based on measured output factors, for example, tissue-maximum ratio (TMR), collimator scatter factor (Sc), phantom scatter factor (Sp), off-axis ratio (OAR), percent depth dose (PDD), and so on.
Answer 19
The charge measured by the electrometer depends on the polarity applied to the ion chamber. Photons interact not only with the gas in the ion chamber but also with the central electrode, ejecting electrons which may increase or decrease depending on the polarity applied to the ion chamber. Additional current outside the collection cavity, known as extra-cameral current, may also occur due to poorly screened circuitry or interactions that take place in the ion chamber cable.
Question 21
In the TG-51 protocol, how is the electrometer correction factor determined?
Question 22
Suppose 1 g of water, 1 g of air, and 1 g of polystyrene each absorbs 1 J of energy. In which of these media is the absorbed dose the highest?
Question 23
Which treatment would you expect to result in more dose to the skin, 10 MV photons or 10 MeV electrons?
Question 24
Water equivalent phantoms are frequently used in radiation therapy. What is the necessary condition for a phantom to be water equivalent?
An ion chamber’s high voltage is used to separate and collect ions that form in the presence of ionizing radiation. The collection process is not 100% efficient and some ions will recombine before reaching the electrodes, thereby causing loss of signal.
Answer 21
The electrometer correction factor, Pelec, is necessary when the ion chamber and electrometer are not calibrated as a unit. This factor is provided by an Accredited Dosimetry Calibration Laboratory (ADCL), and must be rechecked by the ADCL every two years.
Answer 22
Absorbed dose is defined as the energy absorbed per unit mass. Since each medium absorbs the same amount of energy and also contains the same mass, they all receive the same dose.
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