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ESTRO 36 2017
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study is transition from 2D QA to 3D dose reconstruction in
a patient CT scan which could be achieved using dose
reconstruction method from 2D detector array in the
Compass system. The first step in the clinical introduction
of this system, instead of currently used 2D QA in OmniPro
system, is to test reliability of dose reconstructions. In this
work we investigated the validation of the method with
OmniPro results as a reference. We check whether the
Compass QA measurements of VMAT plans fulfill the QA
requirements.
Material and Methods
50 different treatments according to VMAT plans were
selected from our database; 20 prostate, 20 gynecology
and 10 brain. The QA results were divided based on the
mean gamma index and the 3%/3mm and 1%/1mm criteria.
Results from OmiPro were compared with Compass and
TPS. Additionally, recalculation plan from TPS (Monte
Carlo) in Compass system based on the different algorithm
(Collapse Cone Convolution) were performed. MLC tests
(3ABUT, 7SegA, FOURL plan) were implemented before
each set of measurements for evaluation of interleaf
leakage, tongue and groove effect.
Results
Mann-Whitney test showed good agreement between
Compass 3D-reconstructed dose and OmniPro results
(mean gamma 0.23 ±0.03 for 3%/3mm and 0.53±0.06 for
1%/1mm criteria). Scatter plot of results from TPS vs.
Compass against TPS vs. OmniPro showed small
differences in the region of gamma between 0.2 and 0.4.
Comparison TPS vs. Compass mean dose in PTV and OAR
did not reveal significant differences for prostate 50.04
Gy±0.4, 50.35Gy±0.33, bladder 32.04 Gy±0.41, 32.45
Gy±0.23; gynecology 45.07 Gy±0.34, 45.02 Gy±0.25,
bladder 35.04 Gy±0.74, 35.75 Gy±0.49; brain 60.07
Gy±0.53, 60.02 Gy±0.71, brain stem d
max
40.04 Gy±0.83,
39.08 Gy±0.33 respectively.
Conclusion
Agreement between results obtained from Compass and
OmniPro was reached. 3D dose reconstructions in CT
patient allowed to evaluate the dosimetric errors and
their clinical relevance. Compass reconstruction offers
good opportunities to examine dynamic plans and check
characteristics of MLC.
EP-1467 IPEM Code of Practice for proton and ion
beam dosimetry: update on work in progress
S. Green
1
, R. Amos
2
, F. Van den Heuvel
3
, A. Kacperek
4
,
R.I. MacKay
5
, H. Palmans
6
, D. D'Souza
2
, R. Thomas
6
1
Hall-Edwards Radiotherapy Research Group- Queen
Elizabeth Hospital, Medical Physics, Birmingham, United
Kingdom
2
University College London Hospitals, Radiotherapy
Physics, London, United Kingdom
3
Churchill Hospital, Radiotherapy Physics, Oxford,
United Kingdom
4
Clatterbridge Cancer centre, Physics Department,
Wirral, United Kingdom
5
The Christie NHS Foundation Trust, Medical Physics,
Manchester, United Kingdom
6
National Physical Laboratory, Radiation Dosimetry
Group, London, United Kingdom
Purpose or Objective
Current standard methods for reference dosimetry of
proton and ion beams typically involve the use of an
ionization chamber calibrated in a cobalt-60 beam, with a
beam quality correction factor applied to account for the
difference between the chamber response in the proton
and the calibration beams. This approach gives rise to
uncertainties (at 68% confidence level) on the reference
dosimetry of 2.4% for proton beams and 3.4% for carbon
ion beams when using a plane-parallel ionization chamber.
This poster provides an update on the development of a
new Code of Practice for reference dosimetry of proton
and ion beams, applicable to both scanned and scattered
beam configurations. It is aimed to deliver an uncertainty
on reference dosimetry for protons of approximately 2%
and will utilise a primary standard graphite calorimeter
that is robust and portable enough to be used in the end-
user facility.
Material and Methods
This project involves a core team (authors on this
submission) plus a group of experts in the field to provide
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