547 / 1096
S532
ESTRO 36
_______________________________________________________________________________________________
at different depths on their way through the irradiated
volume.
Material and Methods
A total of six different minibeam sizes were applied to the
ear of Balb/c mice using 20 MeV protons. The average dose
of 60 Gy was distributed in 4x4 minibeams with beam sizes
of σ = 0.09, 0.2, 0.31, 0.45, 0.56 and 0.9 mm and a beam-
to-beam distance of 1.8 mm. Inflammatory response, i.e.
ear swelling and skin reactions, were observed for 90 days
after irradiation.
Results
The results show a link between the applied beam sizes
and the dimension of acute side effects after irradiation.
The largest beam sizes lead to significant inflammatory
reactions such as ear swelling, erythema and
desquamation within 3-4 weeks after irradiation. The
maximum skin reactions were reduced with decreasing
beam sizes until almost no ear swelling or other visible
skin reactions to the irradiation could be detected.
Conclusion
Our results show that the tissue sparing effect of proton
minibeams is highest for the smallest beam sizes as
occurring in the superficial layers of an irradiated volume.
The positive effect decreases with increasing beam size
and is therefore smallest for the biggest beam size which
is equivalent to a homogeneous dose as desired in the
target volume. However, since all minibeams have
significantly reduced acute side effects compared to
broad beam irradiation, proton minibeam radiotherapy
may offer various possibilities for new approaches in
clinical proton radiotherapy.
Supported by the DFG Cluster of Excellence: Munich-
Centre for Advanced Photonics.
PO-0963 RBE variations along the Bragg curve of a 200
MeV proton beam
C. Vandevoorde
1
, A. Baeyens
2
, A. Vral
2
, J. Slabbert
3
1
iThemba LABS, Radiation Biophysics, Cape Town, South
Africa
2
Ghent University, Basic Medical Sciences, Ghent, Belgium
3
iThemba LABS, Medical Directorate, Cape Town, South
Africa
Purpose or Objective
A lack of strong radiobiological datasets has resulted in
the clinical adoption of a fixed, generic relative biological
effectiveness (RBE) of 1.1 in current proton therapy (PT).
However, in the distal area of the spread-out Bragg peak
(SOBP), the RBE is certainly higher than 1.1 due to the
rapid decrease in proton energy, resulting in an increased
linear energy transfer (LET). Therefore, the RBE was
quantified at different positions of the depth-dose profile
for the 200 MeV clinical proton beam at iThemba LABS.
Material and Methods
V-79 fibroblasts were irradiated as monolayers at the
plateau, proximal, middle and distal positions, as well as
in the distal edge (32% of the maximum dose) of a 7 cm
SOBP. At the same time, V-79 cells were also irradiated
with
60
Co γ-rays as reference radiation. α and β values
were determined from the cell survival curves and the 95%
confidence ellipses of these covariant parameters were
compared in the analysis. Mean inactivation dose (MID)
values were calculated and used for the RBE calculations.
Results
A large overlap in the 95% confidence ellipses was
observed for proton plateau and
60
Co γ-rays, so there is no
statistical significant difference in radiation quality. The
MID decreases with depth from 3.65 Gy at the entrance
plateau, to 3.52 Gy, 3.40 Gy and 3.15 Gy for the proximal,
middle and distal position along the SOBP respectively.
Since the entrance plateau results were not significantly
different from
60
Co γ-rays, RBE was calculated based on
the plateau MID as a reference. This resulted in RBE values
of 1.04, 1.07 and 1.16 for the proximal, middle and distal
positions respectively. Furthermore, a clear separation
was observed between the 95% confidence ellipses for the
three positions in the SOBP (see Fig 1). An RBE increase up
to 1.46 was determined for the distal fall-off position (see
Fig 2).
Conclusion
The results obtained in the current study with V-79
fibroblast cells confirm the expected increase in RBE along
the proton Bragg curve.
Poster: Radiobiology track: Radiobiology of head and
neck cancer
PO-0964 Biomarkers in wound drainage fluids affect
response to radiations of head and neck cancer cells
M. Mangoni
1
, M. Sottili
1
, T. Gualtieri
2
, A. Javarone
2
, M.
Loi
1
, I. Meattini
1
, P. Bonomo
1
, I. Desideri
1
, A. Deganello
2
,
L. Livi
1
1
University of Florence, Experimental and Clinical
Biomedical Sciences, Firenze, Italy
2
University of Florence, Academic Clinic of
Otorynolaryngology and Head and Neck Surgery, Firenze,
Italy
Purpose or Objective
In recent years in head and neck oncology many efforts
have been made in order to characterize molecular
biomarkers with potential prognostic and therapeutic
value. The detection of significant features in the early
perioperative setting could possibly lead to a refinement
of current adjuvant treatments in high-risk patients. The
purpose of our study is to report the feasibility and
preliminary results of a pilot prospective study on wound
drainage fluids (WDF) analysis in head and neck squamous
cells carcinoma (HNSCC) and to evaluate effect of WDF
microenvironment on HNSCC response to radiation.