ESTRO 36 Abstract Book

533 / 1082

S518 ESTRO 36 2017
_______________________________________________________________________________________________
Cardenal Carro2, J.T. Anchuelo Latorre2, M. Ferri Molina 2,
A. Kannemann2, D. Guirado3, P.J. Prada2
1 Hospital Universitario Marqués de Valdecilla,
Radiophysics, Santander, Spain
2 Hospital Universitario Marqués de Valdecilla, Radiation
Oncology, Santander, Spain
3 Hospital Universitario San Cecilio, Radiophysics,
Granada, Spain
Purpose or Objective
In vivo dosimetry (IVD) applied to HDR BT treatments
allows to monitor real dose delivered to clinically relevant
areas. MOSFET detectors are the most suitable devices for
this task because of their tiny dimensions, which enables
their introduction into identical needles to those used in
treatments. However, these type of detectors show
responses depending on source-to-detector angle and
distance. Mathematical models describing these
dependences can be obtained from a correct detector
characterization. Applying these models on the
measurements should minimize the impact of those
dependences, improving precision and accuracy. The
purpose of this study was to evaluate clinical data of IVD
applied to HDR prostate BT using MOSFET TN-502RDM from
Best Medical Canada with the Ir-192 Vr2 source contained
in Flexitron aferloader (Nucletron-Elekta) and
mathematical models describing those dependences
obtained in a previous characterization work.
Material and Methods
Clinical data were taken from five patients suffering from
prostate cancer. One to three measuring points were
taken for each patient, where the MOSFET were
positioned. Anatomical areas measured were
neurovascular bundles, bulbourethral area and
periurethral area. Nine measuring points were taken and
evaluated.
Real time ultrasound image guided technique was used to
implant the treatment needles. An additional needle was
needed for each measuring point. Oncentra Prostate
4.2.2.4. was used to calculate the treatment planning
following a standard procedure. Subsequently,
coordinates of measuring points and dwell positions were
taken as well as dose contribution of each dwell position
to each measuring point.
After irradiation, mathematical models were applied on
measured dose. Table 1 shows the three models
considered, their parameters and the goodness of fit. TPS
dose, direct MOSFET measured dose and calculated dose
after applying the mathematical models on direct MOSFET
dose were evaluated.
Results
Figure 1 shows the results normalized to TPS d ose. All
measurements suffer an approach to TP S dose after
applying the mathematical models. The ave rage value of
percentage difference between TPS dose and direct
measured dose was 15% while the average percentage
difference after applying the mathematical models
decreases to 9% without any point exceeding 20%.
Estimated global uncertainty associated to these
corrected measurements were into the range 3.7-4.3%.
Conclusion
The application of mathematical models describing the
significant dependences of MOSFET TN-502RDM on their
measurements results in an accuracy increase besides an
improvement in precision. However, IVD implementation
in HDR prostate BT treatments as a possibility of real-time
decision making related to an error detection needs a
retrospective evaluation of a larger sample data to define
correctly these error detection thresholds.
PO-0944 Dosimetric influence produced by the
presence of an air gap between the skin and the
freiburg flap
M. Fernandez Montes 1, S. Ruíz Arrebola1, R. Fabregat
orrás 1, E. Rodríguez Serafín1, J.A. Vázquez Rodríguez1,
M.T. Pacheco Baldor1, N. Ferreiros Vázquez1, M.A.
Mendiguren Santiago1, J.I. Raba Díez1, M.M. Fernández
Macho1, J.T. Anchuelo Latorre2, M. Ferri Molina2, A.
García Blanco2, I. Díaz de Cerio2, M.A. Cobo Belmonte2,
A. Kannemann2, J. Andreescu Yagüe2, M. Arangüena
Peñacoba2, N. Sierrasesumaga Martín2, D. Guirado
llorente3, I. Bernat Piña4, P.J. Prada Gómez2
1Hospital Universitario Marqués de Valdecilla,
RADIOPHYSICS, Santander, Spain
2Hospital Universitario Marqués de Valdecilla, Radiation
ONCOLOGY, Santander, Spain
3Hospital Universitario San Cecilio, Radiophysics,
Granada, Spain
4Hospital Universitario Marqués de Valdecilla, Medical
Oncology, Santander, Spain
Purpose or Objective
Surface applicators were proposed as a way to treat
superficial lesions with HDR brachytherapy. The Freiburg
Flap (FF) is an applicator used in this type of treatment
that has limited flexibility, so that in certain situations it
is not perfectly adapted to the surface treatment. The
purpose of this study is to quantify the discrepancy in the
TPS dose calculation produced by unsuitable positioning of
the applicator, as opposed to the ideal situation, when the
applicator is perfectly adapted to the patient's skin leaving
no air gap.
Material and Methods
Nucletron FF, is an applicator comprising of silicone
spheres attached to each other, 1 cm in diameter,
arranged in parallel rows, capable of adapting to the
surface to be treated.
The TPS Brachy Nucletron Oncentra (Elekta, v-4.5.2) was
used for dose calculation using an 192Ir radiation source and
radiochromic film (Gafchromic EBT3) have been used for
dose measures which were subsequently analyzed with
ImageJ
To quantify the discrepancy between the TPS dose
calculation and the real administrated dose when
adaptation to the surface is not suitable, the experimental
setup designed shown in figure 1 was made, where we can