On the definition of absorbed dose

Grusell, Erik

doi:10.1016/j.radphyschem.2014.10.008

Cited by 5 publications

(8 citation statements)

References 4 publications

(4 reference statements)

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“…Q nucl refers to the average Q-value of transmutation reactions which take place in water and start with a single primary proton. 16 The integral over the dose distribution D w,spot (x, y, z) of an individual spot is rewritten in the following form…”

Section: Consistency Of the Energy Balancementioning

confidence: 99%

“…The Q-value of reactions for the break-up of 16 O was calculated based on the scored MC results. The role of the scored proton ejectiles is ambiguous, because they might also result from elastic and inelastic nuclear reactions including those from 1 H. Hence, the relative number of secondary protons from transmutations and the relative number of 16 O atoms,which experienced break-up,was calculated from the other generated isotopes assuming conservation of the number of protons and neutrons. Q nucl was calculated as the difference of the mass excess of 16 O and the generated isotopes, which were weighted by their production rate.…”

Section: Simulation Of the Neutral Escape Neutron Dose Equivalent And...mentioning

confidence: 99%

“…The role of the scored proton ejectiles is ambiguous, because they might also result from elastic and inelastic nuclear reactions including those from 1 H. Hence, the relative number of secondary protons from transmutations and the relative number of 16 O atoms,which experienced break-up,was calculated from the other generated isotopes assuming conservation of the number of protons and neutrons. Q nucl was calculated as the difference of the mass excess of 16 O and the generated isotopes, which were weighted by their production rate.…”

Section: Simulation Of the Neutral Escape Neutron Dose Equivalent And...mentioning

confidence: 99%

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Consistency of Faraday cup and ionization chamber dosimetry of proton fields and the role of nuclear interactions

Wulff,

Paul,

Bäcker

et al. 2023

Medical Physics

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BackgroundA Faraday cup (FC) facilitates a quite clean measurement of the proton fluence emerging from clinical spot‐scanning nozzles with narrow pencil‐beams. The utilization of FCs appears to be an attractive option for high dose rate delivery modes and the source models of Monte‐Carlo (MC) dose engines. However, previous studies revealed discrepancies of 3%–6% between reference dosimetry with ionization chambers (ICs) and FC‐based dosimetry. This has prevented the widespread use of FCs for dosimetry in proton therapy.PurposeThe current study aims at bridging the gap between FC dosimetry and IC dosimetry of proton fields delivered with spot‐scanning treatment heads. Particularly, a novel method to evaluate FC measurements is introduced.MethodsA consistency check is formulated, which makes use of the energy balance and the reciprocity theorem. The measurement data comprise central‐axis depth distributions of the absorbed dose of quasi‐monochromatic fields with a width of about 28.5 cm and FC measurements of the reciprocal fields with a single spot. These data are complemented by a look‐up of energy‐range tables, the average Q‐value of transmutations, and the escape energy carried away by neutrons and photons. The latter data are computed by MC simulations, which in turn are validated with measurements of the distal dose tail and neutron out‐of‐field doses. For comparison, the conventional approach of FC evaluation is performed, which computes absorbed dose from the product of fluence and stopping power. The results from the FC measurements are compared with the standard dosimetry protocols and improved reference dosimetry methods.ResultsThe deviation between the conventional FC‐based dosimetry and the IC‐based one according to standard dosimetry protocols was −4.7 (± 3.3)% for a 100 MeV field and −3.6 (±3.5)% for 200 MeV, thereby agreeing within the reported uncertainties. The deviations could be reduced to −4.0 (± 2.9)% and −3.0 (± 3.1)% by adopting state‐of‐the‐art reference dosimetry methods. The alternative approach using the energy balance gave deviations of only −1.9% (100 MeV) and −2.6% (200 MeV) using state‐of‐the‐art dosimetry. The standard uncertainty of this novel approach was estimated to be about 2%.ConclusionsAn alternative concept has been established to determine the absorbed dose of monoenergetic proton fields with an FC. It eliminates the strong dependence of the conventional FC‐based approach on the MC simulation of the stopping‐power and of the secondary ions, which according to the study at hand is the major contributor to the underestimation of the absorbed dose. Some contributions to the uncertainty of the novel approach could potentially be reduced in future studies. This would allow for accurate consistency tests of conventional dosimetry procedures.

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Section: Consistency Of the Energy Balancementioning

confidence: 99%

Section: Simulation Of the Neutral Escape Neutron Dose Equivalent And...mentioning

confidence: 99%

Section: Simulation Of the Neutral Escape Neutron Dose Equivalent And...mentioning

confidence: 99%

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Consistency of Faraday cup and ionization chamber dosimetry of proton fields and the role of nuclear interactions

Wulff,

Paul,

Bäcker

et al. 2023

Medical Physics

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“…Assume that on the average n photons per unit area hit the water surface and that the photons are distributed at random and that the positions of different photons are statistically independent. The expected value of the dose at a point at the z -axis at depth z will then be

If we use the pencil beam formula from [ 5 ] and neglect the scatter component and let the radius of the small volume used to define absorbed dose be r 0 [ 6 ], then

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mentioning

confidence: 99%

“…If a ( z ) at 5 cm depth is taken to be a (0.05) = 659 m −1 [ 5 ] and r 0 = 2 · 10 −9 m [ 6 ] and n = 2 · 10 15 m −2 , corresponding to 2 Gy, then the relative standard deviation caused by the random variation of the photons can be calculated. The result is less than 10 −4 , which is several orders of magnitude smaller than the variations described by Wiklund et al…”

mentioning

confidence: 99%