Application of Dose Area Product and DAP
Ratio to Dosimetry in IMRT and Small Field
External Beam Radiotherapy
S Duane, F Graber, RAS Thomas
Background
The aim of this work is to improve photon dosimetry in composite fields
such as are used in delivering IMRT. In a typical IMRT treatment, the
total dose at a point in the PTV arises from an appropriately weighted
combination of in-field, out-of-field and penumbral contributions.
Measurement with an ion chamber needs to take account of its varying
sensitivity to these contributions and the possible lack of electron
equilibrium. Effective electronic equilibrium may only be achieved in the
combined treatment.
There are two aspects: absolute measurement of the dose itself is
addressed in the related poster[1], and a proposed beam quality parameter
to support the transfer of absorbed dose calibration from one composite
field to another is considered here.
Beam Quality
Quality-dependence of DAPR20/10
Definition of DAPR20/10
The proposed beam quality parameter is the ratio of DAP at depths 20 cm
and 10 cm, for fixed source to detector distance and using the same surface
S, shown in Figure 2.
As a beam quality parameter, DAPR20/10 has a similar variation with energy
to TPR20/10, Figure 5. The quality-dependent correction kQ, measured at NPL for
an ion chamber of type NE2611, is replotted in Figure 6 as a function of the
proposed beam quality parameter DAPR.
DAPR20/10 = DAP20 cm /DAP10 cm
In Monte Carlo simulations one can take S to be infinite, but the
measurements reported here were made with a PTW 786 transmission
monitor chamber, whose sensitive volume has a diameter of 155 mm,
in beams from an Elekta Synergy linac.
The quality of a photon beam normally refers to its penetrating power, but
the beam quality parameter is also used to express the variation of an ion
chamber’s absorbed dose calibration coefficient.
The penetrating power of the radiation, which is a function of photon
attenuation and scatter cross-sections and so depends on the photon
energy spectrum, determines the distribution of dose with depth within
the phantom. Accurate modelling of percentage depth dose distributions is
essential to good treatment planning.
The response of an ion chamber is a function of the effective stopping
power ratio, water to air, which is determined by the secondary electron
fluence spectrum at the position of (and as perturbed by) the chamber.
To express the quality-dependence of calibration coefficients, one needs
a beam quality parameter which is related to this stopping power ratio,
which could in principle be quite different from the penetrating power. The
transfer of absorbed dose calibration from reference conditions to actual
measurement conditions depends on this beam quality parameter.
Conventional beam quality specifiers such as TPR20/10 and %dd(10)X, are
directly related to penetrating power but only indirectly related to stopping
power ratio.
Figure 5: Beam quality parameters TPR20/10 and DAPR20/10 as a function of nominal beam
energy for an Elekta Synergy linac. DAPR20/10 has not yet been measured in the 15 MV beam.
Figure 2: The definition of Dose Area Product Ratio, DAPR20/10.
Field size dependence
Primary and scattered photon fluence vary with depth in quite different
ways. A larger field produces more phantom scatter and the beam has
a greater effective penetrating power. This causes a significant field-size
dependence in depth dose data, which is reflected directly in those beam
quality specifiers.
In contrast, the effective water to air stopping power ratio at depth in a
phantom, at a point on the central axis, varies only weakly with field size
and hardly at all with depth. This is why in ion chamber measurements of
central axis depth dose data for photon beams (unlike electron beams), we
don’t worry about the conversion from ionisation to dose: the absorbed
dose calibration coefficient of an ion chamber is essentially constant.
Field-independence of DAPR20/10
Provided the surface S contains the whole beam at both depths, the ratio
DAPR20/10 turns out to be essentially independent of the field size, Figure 3.
For comparison, the field size dependence of TPR20/10 is shown in Figure 4,
taken from Sauer [3].
Figure 6: Experimentally determined quality-dependent correction factor, kQ, plotted as a
function of DAPR20/10.
DAPR20/10, defined below, is a potential beam quality parameter. It measures
penetrating power in a very similar way to TPR20/10, and has the advantage
that it is essentially independent of field size (and shape, and focal
distance). However, it involves an integral over (and beyond) the area of the
primary beam and so the quality it measures must be some sort of average
over the whole beam. When quality varies across the beam, as is the case
with a conventional flattening filter, then DAPR20/10 cannot be expected to
represent that variation.
Application of DAPR20/10 to IMRT dosimetry
Provided that an IMRT treatment uses fields delivered with a single energy, the
field size- and shape-independence of DAPR20/10 means that the measured beam
quality parameter would take the same value for all fields. It may therefore be
regarded as a beam quality parameter for the IMRT treatment itself, and this
parameter used to obtain a value for kQ from Figure 6. This would provide one
route by which absorbed dose in an IMRT treatment may be made traceable to
standard reference dosimetry.
This prompts a key question: if Q = DAPR20/10 is to be the beam quality
parameter in an expression for the quality dependence of ion chamber’s
calibration coefficient, ND,w(Q), what would be the conditions under which
that absorbed dose is measured?
This question is addressed in the related poster[1], where it is suggested
that the absorbed dose should be an average, taken over a volume in
which the dose is essentially constant. Probably, that volume should be the
same size as the sensitive volume of the detector being calibrated, and it
could be almost as large as the PTV itself.
Figure 3: DAPR20/10 measured in an Elekta Synergy 6MV beam for a range of square fields
using a PTW 786 ion chamber in a solid water equivalent phantom.
DAPR20/10 for dynamic and rotational therapy
Dose Area Product, DAP
DAPR20/10 requires the combination of sequential measurements of DAP in two
different setups. This approach becomes more problematic when the number of
fields is large, or the beam varies continuously, as in rotational therapy. It would
be preferable to make the two measurements simultaneously and combine
them in real time. The PTW transmission chamber used in the measurements
reported here presents a minimal perturbation to the beam, and so it would
be possible to set up two such chambers in one phantom, take the ratio of
ionisation currents from the chambers and measure essentially the same beam
quality parameter in real time.
For a beam normally incident on a phantom, Dose Area Product, DAP, is the
integral of dose over an specified surface S, usually normal to the beam
axis and larger than the beam area. The use of DAP for photon dosimetry
in small fields has been proposed by Djouguela et al. [2]. The variation of
DAP with depth in phantom is shown in Figure 1.
References
[1]DUANE, S., et al., Development of an Absorbed Dose
Calorimeter for use in IMRT and Small Field External Beam
Radiotherapy, poster CN182-221, IDOS Symposium, Vienna, 9-12
November 2010.
Figure 4: Field size dependence of TPR20/10, reproduced from Sauer [3], with permission.
The values of 6MV DAPR20/10 from Figure 3 are contained in the solid rectangle.
[2]DJOUGUELA, A., et al., The dose-area product, a new
parameter for the dosimetry of narrow photon beams, Z. Med.
Phys 16 (2006) 217-227.
[3]SAUER, O.A., Determination of the quality index (Q) for photon
beams at arbitrary field sizes, Med. Phys 36 (2009) 4168-4172.
Figure 1: Percentage depth DAP, measured at 6MV for a fixed source detector distance
(analogous to a TMR curve).
Like TPR20/10, the proposed parameter DAPR20/10 is also essentially
independent of the actual source to detector distance.
National Physical Laboratory, Hampton Road, Teddington,
Middlesex, United Kingdom, TW11 0LW
[email protected]