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Detector density impacts
small-field dosimetry
A group of researchers at the
Clatterbridge Cancer Centre are
investigating how physical
characteristics of detectors
affect small-field dosimetric
measurements. The group
recently published their report
on the impact of density and
atomic composition on the
response of various detectors in
small fields.
Monte Carlo modelling was
used to examine variations of a
correction factor, F detector ,
with field size. F detector is
defined as the ratio of dose to a
water voxel and dose to the
same voxel with the density of
the detector.
In total, three detector types
were studied: PTW diamond
detector (density 3.5 g/cm 3 );
PTW 31016 Pinpoint chamber
(0.0012 g/cm 3 ) and Scanditronix
unshielded electron diode (2.3
g/cm 3 ). Monte Carlo simulations
modelled a 15 MV beam incident
upon a water phantom with
source-to-surface distance
(SSD) of 100 cm. Detector voxels
were located at 5 cm depth with
volumes roughly equal to each
detector’s active volume.
The specific impact of
detector density on response
was isolated through two sets of
calculated values for F detector .
The group first calculated
F detector using the density and
mass radiological properties of
the modelled detectors, and
then repeated the calculations
using the densities of diamond,
silicon and air, but with mass
radiological properties fixed at
those of unit density.
Simulations using field sizes
ranging from 0.25 to 10 cm
showed that F detector varied
significantly as a function of field
size for all three detector types.
The high-density diamond and
silicon detectors over-read at
small field sizes, relative to
wide-field readings, whereas
the low-density air-filled
detector under-read at small
field sizes. Similar patterns
were observed for the densitymodified
water voxels, although
values for F detector converged
to unity for large fields. This
behaviour suggests that the
variation of F detector at small
field sizes arises from
differences in detector density,
rather than atomic composition.
The researchers then
studied integral doses of slit
fields using the silicon diode
and Pinpoint detectors. It was
observed that doses measured
along a profile varied far less
than central axis doses,
suggesting that, while doses at
the centres of individual small
fields would contain errors if left
uncorrected, integral doses
calculated for VMAT or IMRT
plans (which contain many
overlapping small fields) would
be approximately correct. This
implies that correcting
measured small-field dose
distributions simply by scaling
them using central axis
F detector values may lead to
erroneous estimates of the
integral doses delivered by
techniques using multiple small
fields. As such, the authors
describe the ideal small-field
dosimeter as having a small
active volume and water-like
density.
Work is ongoing to develop a
cavity theory that describes this
density dependence.
A closer look at proton range uncertainties
The major challenge for
accurate range calculations for
therapeutic proton beams is the
uncertainty in patient stopping
power ratios (SPRs). Margins
along the beam axis, both distal
and proximal to the clinical
target volume, are used during
the treatment planning stage to
account for this uncertainty and
ensure target coverage. A value
of 3.5 per cent has been
commonly used in the design of
these margins for many years,
but recently a group of
researchers at the University of
Texas MD Anderson Cancer
Center took a closer look at this
uncertainty.
Using a combination of
previously published data and
new, measured data, the
researchers assessed five
contributions to SPR
uncertainty in three
representative tissue types:
low-density lung, intermediatedensity
soft tissue and highdensity
bone. Four of the
contributing factors correspond
to steps in the stoichiometric
calibration method, the most
commonly used method for
deriving SPRs for different
tissue types. The fifth originates
from the dose calculation
algorithm used in a treatment
planning system. Stopping
NEWS BY USMAN I. LULA AND RICHARD AMOS | SCOPE
F detector as a function of field size.
power ratios vary with proton
energy and consequently vary
along the proton beam path,
whereas common algorithms
ignore this effect.
The researchers used the
uncertainties for each tissue
type to estimate the composite
range uncertainty for each
beam in the treatment plans of
15 patients who were
undergoing proton therapy for
lung, prostate and head and
neck cancers.
Uncertainty in individual
tissue types, expressed to one
standard deviation, ranged from
1.6 per cent for soft tissue to 5.0
per cent for lung. Expressed
MORE INFORMATION
This story was first reported on Medical
Physics Web on 11th July:
http://medicalphysicsweb.org/cws/art
icle/research/50234
using the 95th percentile,
beams used to treat lung
tumours exhibited the greatest
composite uncertainty of 3.4
per cent. Beams treating
prostate and head and neck
cancer both resulted in an
uncertainty of 3.0 per cent.
This study highlighted that
the currently used value of 3.5
per cent is appropriate as a
general recommendation to
account for SPR uncertainties,
and the group have no
immediate plans to change
their clinical practice.
MORE INFORMATION
This paper was published in Phys Med
Biol 2012; 57: 4095–115.
SCOPE | SEPTEMBER 2012 | 07