ESTRO 36 Abstract Book

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S111 ESTRO 36 2017
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SP-0224 Brachytherapy for bladder/prostate
rhabdomyosarcoma: clinical outcome and functional
results
C. Chargari1, H. Martelli2, F. Guérin2, R. Mazeron1, V.
Minard-Colin 3, E. Deutsch1, C. Haie-Meder4
1 Institut Gustave Roussy, Brachytherapy Unit, Villejuif,
France
2 Kremlin Bicêtre University Hospital, Pediatric Surgery,
Kremlin Bicêtre, France
3 Institut Gustave Roussy, Pediatric oncology, Villejuif,
France
4 Institut Gustave Roussy, Radiotherapy Department,
Villejuif, France
Historically, the standard treatment of children with
bladder and/or prostate rhabdomyosarcoma (BP RMS) was
based on total cystectomy or cysto-prostatectomy. The
severe urinary and sexual sequelae of this radical surgical
approach have prompted collaborative groups to look at
alternative strategies, based on a multimodal
conservative approach combining chemotherapeutic
agents with radiation therapy. Although the probability of
long-term survival with bladder preservation has improved
with multimodal approaches, the risk of late
gastrointestinal and genitourinary toxicities remains a
major issue in children undergoing pelvic external beam
radiotherapy (EBRT). Brachytherapy has been used in our
center as part of the multimodal treatment of patients
with BP RMS, in an effort to minimize sequelae. We report
the results of an original conservative strategy based on
surgery combined with brachytherapy. The outcome of
children treated in our department between 1991 and
2015 for a BP RMS and undergoing a multimodal approach
combining a conservative surgery (partial cystectomy
and/or partial prostatectomy) and a perioperative
interstitial low-dose rate or pulse-dose rate brachytherapy
was prospectively documented. Prior to brachytherapy,
children had received chemotherapy with modalities
depending on their risk group of treatment. A total of 100
patients were treated, median age of 28 months (5.6
months-14 years). According to the Intergroup
Rhabdomyosarcoma Study (IRS) Group group, 84 were IRS-
III and 12 were IRS-IV tumors. Four patients were treated
at relapse. Median number of chemotherapy cycles before
local therapy was 6 (4–13). After surgery, 63 patients had
a macroscopical tumor residuum. Five patients underwent
a brachytherapy boost before pelvic external beam
radiotherapy (EBRT) because of nodal involvement and 95
had exclusive brachytherapy. Median follow-up was 64
months (6 months-24.5 years). Five year disease-free and
overall survival rates were 84% (95%CI: 80–88%) and 91%
(95%CI: 87–95%), respectively. At last follow-up, most
survivors presented with only mild to moderate genito-
urinary sequelae and a normal diurnal urinary continence.
Five patients required a secondary total cystectomy: 3 for
a nonfunctional bladder and 2 for relapse. A specific
analysis of the urinary outcome of patients treated
according to this strategy showed that 75% of long-term
male survivors considered they had a normal quality of life
after the combined conservative local treatment of their
BP RMS. Therefore, brachytherapy is effective as part of a
conservative strategy on BP RMS, with a relatively low
delayed toxicity as compared with previously published
studies using EBRT. Longer follow-up is required to ensure
that the functional results are maintained over time.
SP-0225 Intraoperative HDR brachytherapy for
pediatric cancers
S. Wolden1
1 Memorial Sloan Kettering Cancer Center, New York- NY,
USA
Brachytherapy remains the most conformal technique for
delivering therapeutic radiation, making it an ideal option
for pediatric patients. The normal tissue sparing is even
superior to proton therapy. I will discuss high dose rate
(HDR) intraoperative brachytherapy (IORT) as a specific
technique for the treatment of pediatric tumors. IORT
may be used as an adjunct or even in place of external
beam radiotherapy to maximize local control while
minimizing normal tissue complications. We will review
various IORT techniques as well as specific
indications. Long term outcomes of relatively large series
of children with neuroblastoma and pediatric sarcomas
will be reviewed. A brief overview of other unique forms
of brachytherapy for rare childhood tumors will also be
presented.
Proffered Papers: Dose measurement and dose
calculations
OC-0226 Towards consistency of TPS dose
calculations: converting dose to medium to dose to
water
N. Reynaert1, F. Crop1, E. Sterpin 2, H. Palmans 3
1Centre Oscar Lambret, PHYSIQUE MEDICALE, Lille,
France
2Katholieke Universiteit Leuven, Department of
Oncology- Laboratory of Experimental Radiotherapy,
Leuven, Belgium
3National Physical Laboratory, Radiation Dosimetry,
Teddington, United Kingdom
Purpose or Objective
The aim of the current work is to demonstrate that
conversion factors between dose to medium and dose to
water calculated by different treatment planning systems
for photon beams should be based on mass energy
attenuation coefficients and that stopping power ratios
should not be considered.
Material and Methods
A theoretical explanation is introduced establishing the
inadequacy of stopping power values when converting
dose to medium to dose to water (when considering TPS
dose calculations). Monte Carlo calculations (EGSnrc) are
performed in a simple bone phantom, validating the
theoretical model. A bone slab is modeled and calculations
are performed in bone and in water having the same
electron density as bone. Special attention was paid on
the importance and the range of the interface effects.
Calculations were performed for 6 and 20 MV photons
beams and 6-18 MeV electron beams are also considered.
Results
The Monte Carlo simulations clearly confirm the
theoretical model. In the framework of TPS
reporting/prescription, the dose to medium to dose to
water conversion problem cannot be considered as a
cavity problem as the composition of all voxels is modified
simultaneously, leading to large electron fluence
differences. For photon beams, the secondary electron
fluence is modified by two effects. On one hand, fewer
electrons are generated in bone because of the lower
attenuation coefficients compared to water with the same
electron density, which tends to increase the secondary
electron fluence in water compared to bone. On the other
hand, the range of these secondary electrons is larger in
bone than in water with bone density which leads to an
inverse effect. The first effect is defined by the ratio of
mass attenuation coefficients; while the second by the
ratio of stopping powers which is compensating the
stopping power ratio present in the formal equation of the
ratio of dose to water to dose to medium. Only at