Khan, The Physics of Radiation Therapy, Ch7 Quality of X-ray Beams Khan, The Physics of Radiation Therapy, Ch7

Outlines What’s the quality of x-ray beam Half-value layer Filter Effective energy Megavoltage Beam Energy

The quality of the radiation The penetrating ability of the radiation is often described as the quality of the radiation Ideal : spectral spectrum Practical : HVL (half-value layer)

Spectral Distribution of x-rays

Half-value layer (HVL) The thickness of an absorber of specified composition required to attenuate the intensity of the beam to half its original value

Half-value layer (HVL)

Photon beam attenuation I(x) = I0e-μx μ : linear attenuation coefficient (cm-1) μ / ρ : mass attenuation coefficient (cm2/g)

Half-value layer (HVL) I (x) = I0e-μx X = HVL , I / I0 = ½ μx = ln ½ X= ln ½ / - μ = 0.693 / μ HVL = 0.693 / μ

Half-value layer (HVL)

The quality description of photon beams X-ray (1) low energy x-ray (≤ MV) peak energy or HVL (or both) (2) high energy x-ray (≥ MV) peak energy r-ray energy or nuclide, 60Co source  1.17 MeV and 1.33 MeV (average= 1.25 MeV)  cobalt -60 beam

Filters of x-ray beams Diagnosis & superficial x-ray -- 50~150kV -- 1~6 mm Al filter Orthovoltage x-ray -- 150~500 kV -- combination filter Megavoltage x-ray -- no additional filtration is required to improve the beam quality -- Flattening filter

Schematic graph showing changes in spectral distribution of 200 kVp x-ray beam with various filters. Curve A : for Al Curve B : for Sn + Al (combination filter) Curve C : for Sn + Cu + Al (combination filter)

Inherent filtration of x-ray tube Including : the glass envelop of the x-ray tube, the surrounding oil, and the exit window of the tube housing . This Inherent filtration is equivalent to approximately 1 mm Al in most x-ray tube.

Flattening filter is to make the beam intensity uniform in cross-section .

Effective Energy The effective (or equivalent) energy of an x-ray beam the energy of photons in a monoenergetic beam which is attenuated at the same rate as the radiation in question.

Megavoltage Beam Energy The most practical method is by measuring percent depth dose (PDD) distribution tissue air ratio (TAR) tissue-maximum ratios (TMR)

PDD curve

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