Wednesday, August 4, 2010
Wednesday, April 14, 2010
Prostate IMRT
RMH
Rectum
Dose Vol%
75 Gy 7
70 Gy 12
65 Gy 17
60 Gy 25
50 Gy 35
Bladder
Dose Vol%
80 Gy 7
75 Gy 12
70 Gy 17
65 Gy 25
Rectum
Dose Vol%
75 Gy 7
70 Gy 12
65 Gy 17
60 Gy 25
50 Gy 35
Bladder
Dose Vol%
80 Gy 7
75 Gy 12
70 Gy 17
65 Gy 25
Sunday, March 28, 2010
Wednesday, January 27, 2010
Desirable chamber characteristics
D. Desirable Chamber Characteristics
A practical ion chamber for exposure measurement should have the following characteristics:
1)
There should be minimal variation in sensitivity or exposure calibration factor over a wide range of photon energies.
2)
There should be suitable volume to allow measurements for the expected range of exposures. The sensitivity (charge measured per roentgen) is directly proportional to the chamber-sensitive volume. For example, the reading obtained for a given exposure with a 30-cm3 chamber will be approximately 50 times higher than that obtained with a 0.6-cm3 chamber. However, the ratio may not be exactly 50, because a chamber response also depends on the chamber design, as discussed previously.
3)
There should be minimal variation in sensitivity with the direction of incident radiation. Although this kind of variation can be minimized in the design of the chamber, care is taken to use the chamber in the same configuration with respect to the beam as specified under chamber calibration conditions.
4)
There should be minimal stem “leakage.” A chamber is known to have stem leakage if it records ionization produced anywhere other than its sensitive volume. The problem of stem leakage is discussed later in this chapter.
5)
The chamber should have been calibrated for exposure against a standard instrument for all radiation qualities of interest.
6)
There should be minimal ion recombination losses. If the chamber voltage is not high enough or regions of low electric field strength occur inside the chamber, such as in the vicinity of sharply concave surfaces or corners, ions may recombine before contributing to the measured charge. The problem becomes severe with high-intensity or pulsed beams.
Khan 4th ed
A practical ion chamber for exposure measurement should have the following characteristics:
1)
There should be minimal variation in sensitivity or exposure calibration factor over a wide range of photon energies.
2)
There should be suitable volume to allow measurements for the expected range of exposures. The sensitivity (charge measured per roentgen) is directly proportional to the chamber-sensitive volume. For example, the reading obtained for a given exposure with a 30-cm3 chamber will be approximately 50 times higher than that obtained with a 0.6-cm3 chamber. However, the ratio may not be exactly 50, because a chamber response also depends on the chamber design, as discussed previously.
3)
There should be minimal variation in sensitivity with the direction of incident radiation. Although this kind of variation can be minimized in the design of the chamber, care is taken to use the chamber in the same configuration with respect to the beam as specified under chamber calibration conditions.
4)
There should be minimal stem “leakage.” A chamber is known to have stem leakage if it records ionization produced anywhere other than its sensitive volume. The problem of stem leakage is discussed later in this chapter.
5)
The chamber should have been calibrated for exposure against a standard instrument for all radiation qualities of interest.
6)
There should be minimal ion recombination losses. If the chamber voltage is not high enough or regions of low electric field strength occur inside the chamber, such as in the vicinity of sharply concave surfaces or corners, ions may recombine before contributing to the measured charge. The problem becomes severe with high-intensity or pulsed beams.
Khan 4th ed
Tuesday, January 26, 2010
Wednesday, January 20, 2010
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