ESTRO 36 Abstract Book

792 / 1082

S776 ESTRO 36 2017
_______________________________________________________________________________________________
Garonne, Toulouse, France
2 SIMAD, Haute Garonne, Toulouse, France
Purpose or Objective
The INTRABEAM® system is a miniature accelerator
producing low energy photons (50 keV maximum). The
published dosimetric characterization of the INTRABEAM
system for flat and surface applicators was based on
detectors (radiochromic films or ionization chambers) not
allowing measuring the absorbed dose in the first
millimeters of the irradiated medium, where the dose is
actually prescribed. This study aims at determining the
sensitivity of a paramagnetic gel in order to measure the
dose deposited with INTRABEAM surface applicators in the
first millimeters of irradiated medium.
Material and Methods
The determination of paramagnetic gel sensitivity was
performed with irradiations at different dose levels with
the INTRABEAM® Carl Zeiss Surgical system (Oberkochen,
Germany). The ferrous gel used in this study is a new «
sensitis» material which is described by C. Stien et al and
V. Dedieu et al. Gel irradiation in tin and capsule
containers was carried out for twelve dose levels between
2 Gy and 50 Gy at the gel surface with a 4 cm surface
applicators. The applicator was in contact of the gel
during irradiation. For the calibration curve, one batch gel
was measured without being irradiated. T2 weighted multi
echo MRI acquisitions were performed on a 1,5 T
Magnetom Aera MR scanner of Siemens with surface flex
head coil technology.
Results
The T2 signal versus echo times can be fitted with a mono-
exponential function with 95% of confidence. The first
echo time was not considered for the fit. The calibration
curve determined from experiments with tins is a linear
function (R2=0.967) with a sensitivity of 1.04*10-4 s-1.Gy-1.
Gels Sensitivity with capsules are of 3.67*10 -4 s -1.Gy-1
(R2 =0.979) and 2.54*10-4 s -1.Gy-1 (R2=0.944). The
calibration curve was applied to the irradiation of a
surface applicator to obtain the 3D dose distribution in the
gel.
Conclusion
The dose distribution obtained after irradiation at low
energies with an INTRABEAM® miniature accelerator can
be measured for the first millimeters thanks to ferrous
gels. The determination of gel sensitivity was possible with
MRI measurements. Results are relevant but must be
confirmed with more irradiations with different dose
levels at the surface and different surface and flat
applicator diameters.
EP-1471 Comparison of the integral dose of IMRT,
RapidArc and helical tomotherapy prostate treatments
J. Martinez Ortega1, P. Castro Tejero2, M. Pinto
Monedero1, M. Roch Gonzalez2, L. Perez Gonzalez2
1 Hospital Universitario Puerta de Hierro, Servicio de
Radiofísica y PR, Majadahonda - Madrid, Spain
2 Hospital Universitario de la Princesa, Servicio de
Radioterapia, Madrid, Spain
Purpose or Objective
Comparison of integral dose (ID) and normal tissue integral
dose (NTID) for Helical Tomotherapy (HT), RapidArc and
static fields IMRT.
Material and Methods
A cohort of ten prostate patients were selected whose
prescription was 78 Gy mean dose to the Planning Target
Volume (PTV). Seven different plans for every patient
were computed. One sliding-window IMRT with XiO
planning system and Varian Clinac 21EX, equipped with
MLC Millennium 80. Four Intensity-Modulated Radiation
Therapy (IMRT) plans were calculated with Varian Eclipse
planning system, two step-and-shoot and sliding-window
IMRT for a Varian Clinac 2100 C/D with Millennium 80 and
two analogous plans for a Varian Clinac 2300iX with a
Millennium 120. For this last machine, a RapidArc plan was
also calculated. A HT treatment for Tomotherapy Hi-Art
was also planned for every patient.
Results
ID and NTID are 27% and 33%, respectively, larger for HT
compared to 6MV-IMRT Eclipse treatments. Statistically no
difference has been found for ID and NTID values between
RapidArc and IMRT treatments.
For IMRT treatments, no influence has been observed on
the size of MLC, the delivery technique (step-and-shoot or
sliding-window) and the number of fields. However, an ID
and NTID increments of 8% and 10%, respectively, are
reported when moving a plan from Eclipse to XiO. (Table
1).
The mean DVHs in Fig 1 show some differences depending
on the isodose evaluated. Higher values calculated below
20 Gy are compensated by the region from 20 Gy to 30 Gy,
where this technique minimizes the volume encompassed
by these isodose curves. For HT, there is no compensation,
as the volumes below 20 Gy are much higher than for the
other techniques. From 20 Gy to 30 Gy, the values are
comparable to IMRT, showing no advantage in terms of ID.
NORMAL TISSUE INTEGRAL DOSE (NTID) (·107 cGy·g)
Patie
nt
PTV
Volu
me
(cm3
)
IMR
T
XIO
SW
80
IMRT
ECLI
PSE
SW80
IMRT
ECLI
PSE
SW12
0
IMRT
ECLI
PSE
SS80
IMRT
ECLI
PSE
SS12
0
RAPIDA
RC HT
1 181.2
0
1.2
9 1.26 1.23 1.25 1.23 1.23
1.6
4
2 140.9
4
0.9
8 0.92 0.90 0.91 0.90 0.91
1.2
7
3
228.9
6
1.4
4 1.39 1.36 1.39 1.35 1.35
1.7
3
4
180.4
2
1.3
1 1.28 1.26 1.28 1.25 1.24
1.6
3
5 234.2
2
1.3
6 1.33 1.29 1.33 1.28 1.29
1.7
2
6 204.1
4
1.6
3
1.43 1.39 1.43 1.39 1.40 1.8
5
7
175.3
8
1.5
2
1.31 1.27 1.31 1.27 1.27
1.7
2
8 276.0
4
1.8
3 1.63 1.60 1.62 1.59 1.65 2.1
1
9
209.7
8
1.4
0
1.23 1.22 1.23 1.21 1.21
1.5
4
10
256.6
0
1.9
4 1.76 1.73 1.75 1.72 1.75
2.2
7
Aver
age
208.
77
1.4
7 1.35 1.33 1.35 1.32 1.33 1.
75
SD 41.06
0.2
8 0.23 0.23 0.23 0.22 0.24
0.2
8
Typi
cal
error
(k=2)
0.1
8 0.14 0.14 0.14 0.14 0.15 0.
18
Table 1. NTID calculated from the dose volume
histograms, for every treatment plan, IMRT, RAPIDARC or
HT. For IMRT treatments, both delivering technique (SW
for sliding-window and SS for step-and-shoot) and MLC