ESTRO 36 Abstract Book

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S86 ESTRO 36
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Conclusion
Fast CBCT imaging can be safely used for ES-NSCLC tumors
with tumor movement amplitude < 1cm. In 73.7 % of the
cases there is no image quality loss and even more, in 18.8
% of the cases IQ of the fast scan is preferred compared to
the standard scan.
(1) Rit, S., et al., Comparative study of respiratory motion
correction techniques in cone-beam computed
tomography. Radiotherapy and Oncology, 2011. 100(3): p.
356-359
Symposium: Novel approaches in particle biology
SP-0167 The ESTRO initiative on biological effects of
particle therapy
B.S. Sørensen1
1 Aarhus University Hospital, Exp. Clin. Oncology, Aarhus
C, Denmark
Particle therapy as cancer treatment, with either protons
or heavier ions, provide a more favourable dose
distribution compared to x-rays. While the physical
characteristics of particle radiation have been the aim of
intense research, less focus has been on the actual
biological responses particle irradiation gives rise to. One
of the biggest challenges for the radiobiology is the RBE,
with an increasing concern that the clinical used RBE of
1.1 is an oversimplification, as RBE is a complex quantity,
depending on both biological and physical parameters, as
dose, LET, biological models and endpoints. Most of the
available RBE data is in vitro data, and there is very
limited in vivo data available, although this is a more
appropriate reflection of the complex biological response.
There is a need for a systematic, large-scale setup to
thoroughly establish the RBE in a number of different
models, in a clinical relevant fractionated scheme. The
aim of the ESTRO initiative is to form a network of the
research and therapy facilities. This would open for the
possibility of standardising radiobiological experiments,
and coordinating the research in order to deliver the
needed experimental data.
SP-0168 RBE of protons
B. Jones1
1 Jones Bleddyn, CRUK-MRC Oxford Institute- Department
of Oncology, Oxford, United Kingdom
Introduction. Increasing clinical use of proton therapy
(PT) is not simply an extension of photon radiotherapy
(RT), but requires more detailed knowledge of clinical
physics and radiobiology in order to achieve optimal
outcomes. A critical difference is that megavoltage RT has
linear energy transfer (LET) of around 0.22 -1, but
LET further increases towards and within proton Bragg
peaks. ‘Spread-out’ Bragg peaks (SOBP), depending on
their volume, normally have LET of 1-2 -1, but
higher values between 2-10 -1 can be found in
treatment plans.
Methods and Results:
Studies concluding that the mid-SOBP relative biological
effect (RBE) of protons is 1.1 for all tissues and tumours
at all doses per fraction have recently been criticised due
to their use of:
1. kilovoltage x-ray controls (which mostly provide RBE
values less than 1 and should be excluded),
2. a very limited number of cell lines,
3. a predominance of high doses per fraction (as used for
eye melanomas),
4. linear-only fitting (rather than linear quadratic),
5. animal based studies that used only acute reacting
tissues (with high α/β ratios), known to show little RBE
change with dose per fraction when using fast neutrons
(which ionise mostly by forming recoil protons).
No classical late reacting (low α/β) tissue RBEs have been
published so far: it is these tissues that will influence PT
late effects for important normal tissues within the PTV
and closely around it. Of prime concern is neurological
tissue with α/β of 2 Gy. Using a scaling model based on
the original work of Wilkens & Oelfke, but with added
saturation effects for increases in both α and β with LET,
figures 1 and 2 shows the predicted RBEs in the range of
LET normally in the SOBP -1 ) and the general
increase in RBE with LET and decrease of RBE with dose
per fraction; at higher values of LET (2-10) further
increases in RBE occur, in some cases to beyond 2 at LETs
of 6-10.
Outside the brain, other normal tissue types may carry
lesser importance so that, for example, a slightly raised
RBE in muscle may not produce enhanced late effects in a
very confined volume, as may serially organised tissues
such as lung and liver, but cardiac tissue, bowel and
kidney remain at risk depending on the volume irradiated.
One intriguing aspect is the fall of RBE with increased dose
per fraction, especially in tisses with low α/β values,
which may encourage the use of carefully estimated
hypofractionated total doses, using BED equations with
imbedded RBE limits: the RBEmax and RBEmin
(respectively reflecting the change in α and β with LET):
Figures 1 and 2 show how different α/β ratio bio-systems
may behave with the lowest α/β system crossing over to
have the lowest RBE at higher doses. Values lower than
1.1 can occur in high α/β systems, with risk of
underdosage if a 1.1 RBE is used.
Conclusions. There should be no complacency about RBE
values, even within SOBP`s: 1.1 is not be appropriate.
These higher values may explain some reported adverse
toxicities following PT, such as necrosis of the optic
chiasm and temporal lobe, and failure to cure some very
radiosensitive tumour types with high α/β (lymphomas and
many childhood cancers). Comprehensive RBE studies are
urgently indicated.
References: Jones, B in Cancers (Basle) 2015, 7, 460-480;
also, Brit J Radiol, Why RBE must be a variable and not a
constant. Published Online: May 05, 2016.
Figures 1&2