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

Abstract
Background: A 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.
Purpose: The 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.
Methods: A consistency check is formulated, which makes use of the energy
balance and the reciprocity theorem. The measurement data comprise centralaxis
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.
Results: The 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%.
This