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peer-review. Proposed key elements of the protocol are
the use of the NPL portable graphite calorimeter,
calibration in a composite field defined to cover what is
termed a Standard Test Volume (STV) of delivered dose
for scanned beams and calibration in a broad field spread-
out Bragg Peak to cover the STV for scattered beams.
Results
While for scattered beams the recommendations will be
largely in line with those already published, the key steps
for scanned beams are proposed to be as follows:
Step 1:
Derive the curve which defines the number of
particles per Monitor Unit (MU) for a range of incident
proton/ion energies. This is the Hartmann method and
utilises a plane-parallel ionization chamber at a shallow
depth in pristine Bragg peaks.
Step 2
: Input the curve above into the treatment planning
system (TPS) for the centre/treatment room and then use
the TPS to plan a prescribed dose to the STV in water and
deliver this treatment to the calorimeter with its core at
the centre of the STV.
Step 3:
Re-normalise the data obtained in step 1 if
necessary, to ensure that the calibration in terms of the
number or particles per MU results in the measured dose
to the STV.
Step 4.
Test against alternative STVs to quantify
uncertainties in the dose delivered.
In addition, ionisation chambers belonging to the clinical
centre will be cross-calibrated against the standard
calorimeter at the time of beam commissioning and at
regular intervals (to be defined) thereafter.
Conclusion
A proton and ion beam dosimetry protocol will be
developed which involves direct use of a primary standard
level calorimeter in clinical ion beams. This may provide
a model to be followed elsewhere, ultimately reducing
dose uncertainty for patient treatments worldwide.
The code is under development and due for completion at
the end of 2017. This will coincide with beam
commissioning at the UK centres during 2018. This poster
will describe the proposed methodology with the aim of
stimulating wider debate and comments on this approach.
EP-1468 Skin dose in radiotherapy: results of in vivo
measurements with gafchromic EBT3 films
A. Giuliano
1
, V. Ravaglia
2
1
Istituto Nazionale di Fisica Nucleare INFN, Pisa, Pisa,
Italy
2
San Luca Hospital, Medical Physics, Lucca, Italy
Purpose or Objective
Clinical side effects to skin are a major concern with
radiotherapy patients during the treatment of malignant
disease by radiation. As a consequence, it becomes
important to accurately determine the dose delivered to
a patient skin during radiotherapy owing to complications
that can arise. However, the Treatment Planning Systems
(TPS) do not accurately model skin dose. The aim of this
study is to report the results of surface dose
measurements performed during treatments in
tomotherapy, at Linac both with 3D-CRT (TPS Pinnacle)
and VMAT (TPS Monaco) and in Plesio-Röntgen therapy
using EBT3 Gafchromic films.
Material and Methods
In vivo measurements were performed with the
application of EBT3 film pieces of 2x2 cm
2
directly on the
skin of patients or in the inner side of thermoplastic mask,
if used during the treatment. The target sites included
head and neck (H&N), brain and sarcoma in tomotherapy;
breasts at Linac and skin tumors in Plesio-Röntgen
therapy. For each patient films were located in 1 to 3
reproducible points (see figure 1 and 2) and measurements
were repeated on average in three consecutive fractions.
EBT3 films were read with a flatbed scanner Epson
10000XL and images were analyzed using the red channel
calibration. In vivo dose evaluations were compared with
measurements performed on Cheese phantom both with
and without thermoplastic mask at Linac and in
Tomotherapy.
Results
A total of 117 film measurements were performed on 21
patients. The absolute value of the mean difference
between measured and TPS-calculated dose and its
standard deviation was 11.3% ± 6.5% for all treatments. A
mean absolute difference of 17.7% for Linac plans, 11.6%
in Tomotherapy and 4.6% in Plesio-Röntgen therapy were
achieved. Both at Linac and in Plesio-Röntgen therapy
there was not a clear trend of overestimation of the TPS
with respect to measurements. Instead in Tomotherapy
there was an underestimation of the TPS (-9.1%) for H&N
and brain treatments (in these case measurements were
performed with thermoplastic mask) and an
overestimation for the sarcoma (9.2%). This trend was
confirmed by the measurements made on the Cheese
phantom in Tomotherapy, where there was an
overestimation of the TPS without mask (28.6% vs -0.7%
with mask). Moreover, an improvement of the agreement
between EBT3 measurements and Pinnacle and
Tomotherapy dose estimation was shown in presence of
mask (28.6% to -0.7% in Tomotherapy and -20.7% to -16.3%
at Linac).
Conclusion
Gafchromic films are suitable detectors for skin dose
measurements in radiotherapy. In vivo surface dose
measurements with EBT3 are a useful tool for quality
assurance in radiotherapy, since the TPS does not give
accurate dose values in the first millimiters of skin.