An EGS4 Monte Carlo examination of general cavity theory

Abstract

EGS4 Monte Carlo simulations have been performed to examine general cavity theory for a number of TLD cavity materials irradiated in megavoltage photon and electron beams. The TLD materials were LiF, Li2B4O7, CaF2 and CaSO4 irradiated in Perspex, water, Al, Cu and Pb phantoms. For megavoltage photon beams, this has been done by determining the dose component (1-d) resulting from photon interactions in the cavity compared with the dose component resulting from photon interactions in the phantom material (d) by Monte Carlo simulations and analytical techniques. The results indicate that the Burlin exponential attenuation technique can overestimate the dose contribution from photon interactions in a 1 mm thick LiF cavity by up to 100% compared with the Monte Carlo results for LiF TLDs irradiated in a water or Perspex phantom. However, there is agreement to within 1% in the quality dependence factor, determined from Burlin's cavity theory, Monte Carlo simulations and experimental measurements for LiF and Li2B4O7 TLDs irradiated in a Perspex or a water phantom. The agreement was within 3% for CaF2 TLDs. However there was disagreement between Monte Carlo simulations and Burlin's theory of 6 and 12% for LiF TLDs irradiated in copper and lead phantoms respectively. The adaptation of Burlin's photon cavity theory and other modifications to his photon general cavity theory for electrons have been shown to be seriously flawed.

Evaluation and comparison of absorbed dose for electron beams by LiF and diamond dosimeters

“…[8] while this study indicates it as 1.003 7 0.011 for 5 MeV electron beam, a difference of 2.7% due to small difference in depths of measurements.…”

“…[8] while this study indicates 0.982 7 0.011; a difference of 6.8% also due to slightly different d max values and thicknesses of the dosimeters. 4.5.…”

A Monte Carlo comparison of the response of the PTW-diamond and the TL-diamond detectors in megavoltage photon beams

“…10 For the PTW-diamond detector to behave as a Bragg-Gray cavity, not only should the ranges of the electrons incident on the sensitive volume of the PTW-diamond detector ͑cavity͒ be greater than the size of the cavity, but also photon interactions in the cavity should be negligible. 6,[18][19][20] The Monte Carlo method allows analysis of the source of the electrons that deposit energy in the sensitive diamond volume. 20,21 Let the percentage of the total absorbed dose resulting from electrons generated by photon interactions in the cavity region ͑sensitive volume͒ be ␣ c , from the wall of the PTW-diamond detector be ␣ w , and from the surrounding medium be ␣ m .…”

“…6,[18][19][20] The Monte Carlo method allows analysis of the source of the electrons that deposit energy in the sensitive diamond volume. 20,21 Let the percentage of the total absorbed dose resulting from electrons generated by photon interactions in the cavity region ͑sensitive volume͒ be ␣ c , from the wall of the PTW-diamond detector be ␣ w , and from the surrounding medium be ␣ m . Thus we categorize the origin of the dose in the sensitive volume of the PTW-diamond detector according to the region in which the electron depositing energy in the sensitive detector volume is first produced from photon interactions.…”

“…The results show that ␣ c ranges from 42% for 60 Co ␥ rays to 8% for 25 MV x rays with the value of ␣ c decreasing with photon quality, as expected. 20 Values of ␣ w range from 35% for 60 Co ␥ rays to 8% for 25 MV x rays. This means that the PTW-diamond detector does not behave as a Bragg-Gray cavity in megavoltage photon beams since the contribution to the total dose from photon interactions in the cavity (␣ c ) is greater than 8%.…”