87 / 1082
S74
ESTRO 36 2017
_______________________________________________________________________________________________
which introduces errors in the SECT derived RSPs. The
DECT method determines the effective atomic number
and relative electron density and on basis of these physical
parameters enables a more accurate estimate of the RSP.
Conclusion
The developed DECT method is more accurate in
prediction of relative proton stopping powers than the
SECT calibration method for a wide range of materials and
tissues and can be of benefit to proton therapy treatment
planning.
OC-0152 Innovative solid state microdosimeters for
Radiobiological effect evaluation in particle therapy
T.L. Tran
1
, L. Chartier
1
, D. Bolst
1
, D. Prokopovich
2
, A.
Pogossov
1
, M. Lerch
1
, S. Guatelli
1
, A. Kok
3
, M. Povoli
3
, A.
Summanwar
3
, M. Reinhard
2
, M. Petesecca
1
, V.
Perevertaylo
4
, A. Rozenfeld
1
1
University of Wollongong, Centre for Medical Radiation
Physics, Wollongong, Australia
2
Australian Nuclear Science and Technology
Organisation, Engineering Material Institute, Lucas
Heights, Australia
3
SINTEF, Microsystems and Nanotechnology, Oslo,
Norway
4
SPA-BIT, SPA-BIT, Kiev, Ukraine
Purpose or Objective
Particle therapy has many advantages over conventional
photon therapy, particularly for treating deep-seated solid
tumors due to its greater conformal energy deposition
achieved in the form of the Bragg peak (BP). Successful
treatment with protons and heavy ions depends largely on
knowledge of the relative biological effectiveness (RBE) of
the radiation produced by primary and secondary charged
particles. Different methods and approaches are used for
calculation of the RBE-weighted absorbed dose in
treatment planning system (TPS) for protons and heavy ion
therapy. The RBE derived based on microdosimetric
approach using the tissue equivalent proportional counter
(TEPC) measurements in
12
C therapy has been reported,
however large size of commercial TEPC is averaging RBE
which dramatically changes close to and in a distal part of
the BP that may have clinical impact. Moreover, the TEPC
cannot be used in current particle therapy technique using
pencil beam scanning (PBS) delivery due to pile up
problems associated with high dose rate in PBS.
Material and Methods
The Centre for Medical Radiation Physics (CMRP),
University of Wollongong, has developed new silicon-on-
insulator (SOI) microdosimeter with 3D sensitive volumes
(SVs) similar to biological cells, known as the “Bridge” and
“Mushroom” microdosimeters, to address the
shortcomings of the TEPC. The silicon microdosimeter
provides extremely high spatial resolution and can be used
for in-field and out-of-field measurements in both passive
scattering and PBS deliveries. The response of the
microdosimeter was studied in passive and scanning
proton and carbon therapy beam at Massachusetts General
Hospital (MGH), USA, Heavy Ion Medical Accelerator in
Chiba (HIMAC) and Gunma University Heavy Ion Medical
Center (GHMC), Japan, respectively.
Results
Fig 1a shows the dose mean lineal energy, and frequency
mean lineal energy, measured using the SOI
microdosimeter irradiated by the 131.08 MeV pencil
proton beam as a function of depth in water. The value
was around 2 keV/µm in the plateau region, then
approximately 3 to 5 keV/µm in the proximal part of the
BP, and increasing dramatically to 9 to 10 keV/µm at the
end of the BP. Fig 1b shows derived RBE along the BP for
2Gy dose delivered in a peak. Fig 2 shows the distribution
with depth for the 290 MeV/u
12
C ion pencil beam at
GHMC. The inset graph in the left corner of Fig. 2 shows a
detailed view of the distribution at the BP measured with
submillimetre spatial resolution. It can be seen that
the distribution at the peak illustrates the effect of ripple
filter used in this facility which is impossible to observe
with any TEPC based microdosimeters. RBE values and
dose equivalent obtained near the target volume are also
derived using the SOI microdosimeters and the results will
be presented in a full paper.