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ESTRO 36 2017
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patient is treated on outpatient basis on 10 fractions of
3.4 Gy over 5 days.The CT scans of these patients, taken
before each treatment were separately imported in to the
treatment planning system and registered with the initial
CT scan respective to the applicator. Three radio channel
markers of the applicator are used as reference points to
conduct landmark registration on each CT scan. Difference
in Max Dose received by skin, ribs and PTV(Planning target
volume) during each treatment is recorded.
Results
Contours of any of the OARs were not exactly similar when
CT images were fused on each other. Deduction in volumes
of PTV and cavity was noticed. There was always a
difference between received doses by the OARs and PTVs
between treatments. Variations in received dose by Skin
and ribs were statistically significant for 3 treatments and
2 treatments respectively under 5% level. Variations were
statistically significant for 4 more fractions for both organs
under 10% level. Some data indicates, few times patients
received more than 145% of prescribed dose that breach
the specific guidelines of APBI.
Conclusion
The difference recorded in volumes of OARs and iso-doses
near the OARs between treatments indicate that the
received doses to OARs differ from the prescribed dose in
the initial treatment plan. Similarly PTV receiving a lesser
dose than the prescribed dose affects the quality of the
treatment. It appears that taking a CT scan before each
treatment and re-planning is important at this stage to
minimize the risk of delivering different doses than the
prescribed.
Electronic Poster: Brachytherapy: Prostate
EP-1768 What is the proper dose in single fraction
HDR brachytherapy as monotherapy for prostate
cancer?
S. Ruiz Arrebola
1
, A.M. Tornero-López
2
, J.M. De la Vega
3
,
P.J. Prada
1
, D. Guirado
3
1
Hospital Universitario Marqués de Valdecilla,
Department of Radiation Oncology, Santander, Spain
2
Hospital Universitario Dr. Negrín, Unidad de Radiofísica,
Las Palmas de Gran Canaria, Spain
3
Complejo Hospitalario de Granada, Unidad de
Radiofísica, Granada, Spain
Purpose or Objective
High dose rate brachytherapy (HDRBT) as monotherapy for
prostate cancer is applied with several fractionation
schedules. Usually the linear-quadratic (LQ) model is used
to establish their equivalence. However, using the
currently accepted value of α/β for prostate cancer, a
significant discrepancy between the LQ model predictions
and clinical outcomes is found for the only single fraction
schedule in use with long-term results [1].
We aim to determine the value of the absorbed dose for
an extreme hypofractionation regime of one fraction in
HDRBT monotherapy leading to a biochemical failure rate
similar to that of most widely used regimes.
Material and Methods
We used available published data from biochemical
control at 5 years, for prostate cancer patients of low and
intermediate risk treated with exclusive HDRBT: 7 clinical
studies with 9 fractionation schedules, from 1 fraction of
19 Gy to 9 fractions of 6 Gy per fraction.
To compare the different schedules, we used the
equivalent total dose in 2 Gy fractions, and to describe the
biochemical control (BC), we used the LQ model together
with the logistics probability function:
being D the total dose, d the dose per fraction, α/β the
LQ parameter that allows to quantify the sensitivity to the
fractionation of prostate cancer, γ the maximum
normalized dose-response gradient and D
50
the total dose
needed to achieve 50% BC.
To fit the model parameters, and to obtain its
uncertainties, we used Monte Carlo methods.
Results
Firstly, the value of EQD
2
of each program was calculated.
Figure 1 shows the value of BC versus EQD
2
if the currently
accepted value for α/β=1.5 Gy is used in equation (1). The
black square corresponds to the single fraction schedule
[1].
The fit of equation (2) to clinical data produces the results
in figure 2. The confidence intervals of the parameters
correspond to 95%. Figure 2 also shows BC versus the new
EQD
2
values, calculated with α/β=8.7Gy.
From these results, the value of the absorbed dose for an
extreme hypofractionation regime with one fraction in
HDRBT monotherapy, allows us to obtaining a BC=90% at 5
years, is 21.9 [19.6,26.3] Gy.
Conclusion
The α/β value obtained for a range of dose per fraction
between 6 and 19 Gy is much greater than the one
currently estimated for prostate cancer.
The absorbed dose in HDRBT monotherapy for a regime
with one fraction would allow us to obtain a BC=90% is 22
Gy.
The value of α/β obtained here explains well the clinical
data in the region of the doses per fraction considered.
Nonetheless it is important to take into account some of
the limitations of the model, which may be overcame by