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ESTRO 36 2017
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maximum). All irradiation modes deposit a mean dose of
1.7 Gy. For RBE determination dose response curves of
reference radiation were used.
Results
The RBE values, as determined by measuring dicentrics in
human-hamster hybrid (AL) cells, are significantly higher
when 117 protons were focused to a 0.78 µm spot within
a 5.4 × 5.4 µm
2
matrix compared to homogenous applied
protons (RBE = 1.96 ± 0.16 vs. RBE = 1.30 ± 0.16). By
doubling the spot size to 1.6 µm the RBE decreased to 1.52
± 0.16. By further increasing the spot size to 2.7 µm the
RBE was not longer different (RBE = 1.36 ± 0.14) to the
homogenous radiation.
Conclusion
Our experiments demonstrate evidence that low LET
radiation focused to sub-micrometer diameters results in
an increase in RBE for the induction of dicentrics
depending on the spot size. The local density of DSB is
increased at the irradiated spots enhancing also the
probability for the interaction of the DSB and thus raising
the probability of connecting the wrong ends. We
hypothesize that a tighter beam spot of protons might
further enhance the RBE value.
Supported by the DFG-Cluster of Excellence ‘Munich-
Centre for Advanced Photonics’, by the BMBF-project
02NUK031A and 02NUK031B “LET-Verbund”.
OC-0244 Does the RBE depend on ion type?
A. Lühr
1,2,3
, C. Von Neubeck
2,3
, M. Baumann
1,2,3,4
, M.
Krause
1,2,3,4
, W. Enghardt
1,2,3,4
1
Helmholtz-Zentrum Dresden - Rossendorf, Institute of
Radiooncology, Dresden, Germany
2
OncoRay–National Center for Radiation Research in
Oncology, Faculty of Medicine and University Hospital
Carl Gustav Carus - Technische Universität Dresden -
Helmholtz-Zentrum Dresden - Rossendorf, Dresden,
Germany
3
German Cancer Consortium DKTK, Partner Site Dresde,
Dresden, Germany
4
University Hospital Carl Gustav Carus at the Technische
Universität Dresden, Department of Radiation Oncology,
Dresden, Germany
Purpose or Objective
Currently, modeling of RBE as a simple function of linear
energy transfer (LET) receives much attention in the
proton therapy community. However, such LET-RBE
parametrizations are purely empirical and ion type
specific. Additionally, their applicability is restricted by
large uncertainties associated with the biological input
parameters from proton experiments. In contrast, long
term clinical experience on RBE modeling as well as
treatment outcome data exist for carbon ion therapy. The
aim is to establish a clinically relevant RBE modeling for
proton therapy that is directly based on available clinical
and pre-clinical experience from carbon ion therapy.
Material and Methods
The RBE dependence on the radiation field – i.e., on
physics – was mathematically derived in a convenient way
using assumptions also applied by the local effect model
(LEM) and the micro kinetic model (MKM); both used in
patient treatment. A large set of in vitro literature data
(several hundred data points, including six different ion
types) on RBE and the linear-quadratic model parameter
α
p
for particles was used to validate the derived model.
Pre-clinical RBE data of the rat spinal cord (one and two
fractions) at six different depth positions in a carbon ion
treatment field were used to demonstrate the transfer of
carbon ion RBE data to proton therapy. Physical properties
of the applied carbon treatment field as a function of
depth were obtained by Monte Carlo simulation
considering the full particle spectrum: dose, LET, beam
quality Q = Z
2
/E (Z = ion charge; E = kinetic energy).
Results
The derivation revealed for α
p
and RBE a linear increase
with beam quality Q but no dependence on ion type. These
findings were well confirmed by the experimental in vitro
data for different ions (Fig. 1). Specifically, the
independence of ion type holds true for different cell
types and irradiation under normoxic and hypoxic
conditions.
The pre-clinical spinal cord RBE data increased linearly
with Q (Fig. 2). The linear slope depends in the same way
on fractionation dose as described by the derived model.
Due to the apparent independence of RBE on ion type, the
experimentally obtained RBE for carbon ions as function
of Q could also be used to estimate the RBE in a proton
SOBP where Q can be determined at any depth (Fig. 2).
Fig.1 Beam quality dependence:
In vitro RBE and α
p
as
function of beam quality Q (Z
2
/E) for HSG and V79 cell
lines. H: proton, He: helium, C: carbon, Ne: neon.
Fig.2 Concept of RBE translation
: (A) Obtain (pre-)
clinical RBE from carbon ion therapy as (B) function of
beam quality and (C) use it to optimize dose prescription
in proton therapy. Spinal cord RBE; 1 and 2 fractions (Fx).
Conclusion
The RBE seems to depend on the beam quality Q but not
on ion type for clinically relevant treatment situations.
This opens up the possibility to directly transfer clinically
and pre-clinically obtained parameters from carbon ion to
proton therapy. Currently, RBE experiments and Monte
Carlo simulations of patient treatments are performed as
a next step to translate this approach to proton therapy.
OC-0245 Clinical evidence that end-of-range proton
RBE exceeds 1.1: lung density changes following chest
RT
T. Underwood
1,2
, C. Grassberger
1
, R. Bass
1
, R. Jimenez
1
,
N. Meyersohn
3
, B. Yeap
1
, S. MacDonald
1
, H. Paganetti
1
1
Massachusetts General Hospital & Harvard Medical
School, Department of Radiation Oncology, Boston MA,
USA
2
University College London, Department of Medical
Physics and Bioengineering, London, United Kingdom
3
Massachusetts General Hospital & Harvard Medical
School, Department of Radiology, Boston MA, USA
Purpose or Objective
Clinical practice assumes a fixed proton relative biological
effectiveness (RBE) of 1.1, but it has been postulated that
higher RBEs occur at the distal edge of proton spread out
Bragg peaks, i.e. within the lung for chest wall patients.
We performed retrospective qualitative & quantitative
analyses of late-phase lung-density changes (indicative of
asymptomatic fibrosis) for chest wall patients treated
using protons & X-rays. Our null hypothesis (H0) was that,
assuming a fixed proton RBE of 1.1, these changes would