Quality of X-ray Beams
HVL 0.693 HVL 0.693
D. MEAN ENERGY
The spectral distribution of a radiation field (particles or photons) is characterized by the distribution of fluence or energy fluence with respect to energy. Suppose Φ(E) denotes fluence Φ of photons with energy between 0 and E. The differential distribution (ΦE) of the fluence with respect to energy is given by
Ed(E) dE
The product ΦE dE is the fluence of photons with energies lying between E and E + dE. The total fluence (Φ) is given by
10EmaxEdE The mean energy (E) of a photon beam can be calculated as
E10EmaxE~dE
10EmaxE~dE
(7.2)
7.4. MEASUREMENT OF MEGAVOLTAGE BEAM ENERGY
The complete energy spectrum of a megavoltage x-ray beam can be obtained by calculation using thin target bremsstrahlung spectra (12), scintillation spectrometry (13,14), and photoactivation (15). However, for the characterization of a megavoltage x-ray beam by a single energy parameter, namely by its maximum energy, one needs to determine the energy of the electron beam before incidence on the target. Several methods for determining this energy are discussed in Chapter 14.
The most practical method of determining the megavoltage beam energy is by measuring per- cent depth dose distribution, tissue-air ratios, or tissue-maximum ratios (Chapters 9 and 10) and comparing them with the published data such as those from the Institute for Physical Sciences in Medicine (16). Although clinically relevant, the method is only approximate since depth dose distributions are relatively insensitive to small changes in the peak energy.
A sensitive method of monitoring x-ray beam spectral quality has been proposed by Nath and Schulz (17) and is referred to as the photoactivation ratio (PAR) method. The basic procedure involves irradiating a pair of foils that can be activated by the photodisintegration process (Sec- tion 2.8F). The choice of foils must be such that one of them is sensitive to higher energies than the other in the energy spectrum of the x-ray beam. After irradiation, the induced radioactivity in the foils is measured using a scintillation counter. The ratio of induced activities gives the PAR, which can be related to the peak photon energy. The PAR method provides a more sensitive method of measuring x-ray spectral quality than the conventional method of measuring HVL in water.
7.5. MEASUREMENT OF ENERGY SPECTRUM
Although the HVL is a practical parameter characterizing therapeutic beams, it is only approxi- mate and cannot be used in systems that are sensitive to spectral distribution of photons. For example, some radiation detectors show a large variation in response with different photon ener- gies (e.g., film, diodes), and even ion chambers are more or less energy dependent, depending on
30
20
10
0 10 20
Lead Water
30 40
Peak photon energy (MeV) Half-value layer (mm Pb or cm H2O)
Figure 7.5. half-value layer (hVL) as a function of peak photon energy for water and lead. Note: Since these data were calculated from thin-target Schiff (12) spectra, hVL values plotted here are slightly lower than those measured in practical radiotherapy machines. (Data from Nath r, Schulz rJ. On the choice of material for half-value-layer measurements for megavoltage x-rays.
Med Phys. 1977;4:132, with permission.)
82453_ch07_p089-096.indd 94 1/7/14 12:25 PM
ChaPter 7 Quality of X-ray Beams 95
their design. In such instances, spectral distribution is the relevant parameter of beam quality. In this and other investigative work, it is important to determine experimentally spectral distribu- tions of photon beams. There are many references dealing with spectrometry (12–15), and the interested reader is referred to those papers. Only one method, namely scintillation spectrometry, will be briefly described here.
The scintillation spectrometer consists of a crystal or phosphor, usually sodium iodide, attached to a photomultiplier tube (Fig. 7.6). When a photon beam is incident on the crystal, electrons are ejected that travel in the crystal and produce ionization and excitation of the crystal atoms. As a result, photons of energy in the optical or ultraviolet region are produced along the electron tracks.
These light photons, on striking the photosensitive surface (photocathode) of a photomultiplier tube, eject low-energy photoelectrons, which are collected and multiplied about a million times by the photomultiplier dynodes. This results in an output pulse that is proportional to the energy of the original x-ray photon entering the crystal. A multichannel pulse height analyzer is used to sort out electronically different-size pulses. Each channel corresponds to particular input photon energy and accumulates counts or number of photons with a particular energy. The spectrum is then displayed in terms of photons per unit energy interval as a function of photon energy (Fig. 7.6).
Diaphragm X-ray
source
Scintillation crystal
Photomultiplier
Lead shielding Pulse height
analyzer Scaler K lines
of tungsten
Photon energy
Photon fluence/energy interval
Figure 7.6. energy spectrum of an x-ray beam determined by a scintillation spectrometer (shown in the inset).
• Quality of x-ray beams is specified by kVp, filtration, and HVL (for diagnostic, superficial, and orthovoltage beams); and MV and percent depth dose in water (for megavoltage x-rays).
• Quality of cobalt-60 beams is designated simply as cobalt-60 because it is known that they have the same energy, namely g rays of 1.17 and 1.33 MeV.
• HVL must be measured under “good geometry” conditions: a narrow beam and a large dis- tance between absorber and detector in order to avoid measurement of scattered radiation.
• Peak voltage (kVp) applied to an x-ray generator can be measured directly (e.g., voltage divider, sphere-gap method) or indirectly (e.g., fluorescence, attenuation, or a penetrameter device such as an Adrian-Crooks cassette).
• Peak energy (MV) of a megavoltage x-ray beam can be measured directly by
scintillation spectrometry or by photoactivation of appropriate foils (e.g., PAR method).
Most commonly used methods, however, are indirect, such as comparing measured percent depth dose distribution in water with published data.
• Effective or equivalent energy of an x-ray beam is the energy of a monoenergetic photon beam that has the same HVL as the given beam.
• Energy spectrum of an x-ray beam can be measured by scintillation spectrometry.
The spectrum may be displayed in terms of photon fluence per unit energy interval as a function of photon energy.
K e Y P O I N t S
96 Part I Basic Physics
R e f e r e n c e s
1. Thoraeus R. A study of the ionization method for measuring the intensity and absorption of x-rays and of different filters used in therapy. Acta Radiol. 1932;Supplement:15.
2. International Commission on Radiation Units and Measure- ments. Physical Aspects of Irradiation. Report 10b. Wash- ington, DC: U.S. National Bureau of Standards; 1964.
3. Gilbertson JD, Fingerhut AG. Standardization of diagnostic x-ray generators. Radiology. 1969;93:1033.
4. Giarratano JC, Waggener RG, Hevezi JM, et al. Compari- son of voltage-divider, modified Ardran-Crooks cassette and Ge (Li) spectrometer methods to determine the peak kilovoltage (kVp) of diagnostic x-ray units. Med Phys.
1976;3:142.
5. Greening J. The measurement of ionization methods of the peak kilovoltage across x-ray tubes. Br J Appl Phys.
1955;6:73.
6. Morgan R. A simple method of measuring peak volt- age in diagnostic roentgen equipment. Am J Roentgenol.
1944;52:308.
7. Newell RR, Henny GC. Inferential kilovoltmeter: measur- ing x-ray kilovoltage by absorption in two filters. Radiology.
1955;64:88.
8. Glasser O, Quimby EH, Taylor LS, et al. Physical Founda- tions of Radiology. 3rd ed. New York, NY: Paul B. Hoeber;
1961:241.
9. Stanton L, Lightfoot DA, Mann S. A penetrameter method for field kV calibration of diagnostic x-ray machines. Radio- logy. 1966;87:87.
10. Ardran GM, Crooks HE. Checking diagnostic x-ray beam quality. Br J Radiol. 1968;41:193.
11. Nath R, Schulz RJ. On the choice of material for half- value-layer measurements for megavoltage x-rays. Med Phys. 1977;4:132.
12. Schiff LI. Energy-angle distribution of thin target brems- strahlung. Phys Rev. 1951;83:252.
13. Skarsgard LD, Johns HE. Spectral flux density of scattered and primary radiation generated at 250 kV. Radiat Res.
1961;14:231.
14. Epp ER, Weiss H. Experimental study of the photon energy spectrum of primary diagnostic x-rays. Phys Med Biol.
1966;11:225.
15. Nath R, Schulz RJ. Determination of high energy x-ray spectra by photoactivation. Med Phys. 1976;3:133.
16. Joint Working Party of the British Institute of Radio logy and the Institute of Physics and Engineering in Medi- cine and Biology. Central axis depth dose data for use in radiotherapy. Br J Radiol. 1996;Supplement:26.
17. Nath R, Schulz RJ. Photoactivation ratios for specifica- tion of high-energy x-ray quality: part I and II. Med Phys.
1977;4:36.
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97
8.1. DEFINITION OF ABSORBED DOSE
In Chapter 6, the quantity exposure and its unit, the roentgen (C/kg), were discussed. It was then pointed out that exposure applies only to x and g radiations, is a measure of ionization in air only, and cannot be used for photon energies above about 3 MeV. The quantity absorbed dose has been defined to describe the quantity of radiation for all types of ionizing radiation, includ- ing charged and uncharged particles; all materials; and all energies. Absorbed dose is a measure of the biologically significant effects produced by ionizing radiation.
The current definition of absorbed dose, or simply dose, is the quotient d / dm where d is the mean energy imparted by ionizing radiation to material of mass dm (1). The old unit of dose is rad (an acronym for radiation absorbed dose) and represents the absorption of 100 ergs of energy per gram of absorbing material:
1 rad = 100 ergs/g = 10−2 J/kg (8.1)
The SI unit for absorbed dose is gray (Gy) and is defined as
1 Gy = 1 J/kg (8.2)
Thus, the relationship between gray, centigray (cGy), and rad is
1 Gy = 100 rad = 100 cGy (8.3)
or
1 rad = 10−2 Gy = 1 cGy (8.4)