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ESTRO 36 2017
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11
San Raffaele Scientific Institute, Medical Physics,
Milan, Italy
Purpose or Objective
The main objective of this work was to fit the Normal
Tissue Complication Probability (NTCP) model for the
prediction of late urinary symptoms at three years after
radical radiotherapy (RT) for prostate cancer. Then, to
evaluate the effect of leading clinical risk factors on the
NTCP.
Material and Methods
Patients were enrolled in a prospective, multicentre,
observational trial in 2010-2014. They were treated at
different prescription doses with conventional (74-80 Gy
at 1.8-2 Gy/fr,) or moderately hypo-fractionated RT (65-
75.2 Gy at 2.2-2.7 Gy/fr) in 5 fractions/week.
Urinary symptoms were evaluated through the
International Prostate Symptom Score (IPSS) questionnaire
filled in by the patients before RT, after RT, and every 6
months until 5 years of follow-up. The incidence of late
toxicity at 3 years was defined as the occurrence of an
IPSS>=15 at least once between 6 and 36 months after RT.
Bladder dose volume histograms were collected for each
patient. They were corrected to the equivalent doses in 2
Gy/fraction (EQD2), with α/β=3Gy, and then reduced to
the Equivalent Uniform Doses (EUD), computed at varying
n values. Several clinical factors were also collected
(comorbidities, drugs, hormone therapies, previous
surgeries, smoking, alcohol, age, and body mass index).
Maximum likelihood estimation (MLE) was employed for
calculating the best-fit NTCP parameters: n (volume
parameter summarizing the organ architecture), EUD
50
(EUD causing 50% toxicity risk) and k (steepness of the
NTCP) [1].
Leading clinical factors associated with urinary toxicity
were evaluated with univariable logistic regression
(p<0.05) and included in the NTCP model as EUD
50
modifying coefficients.
Results
217 patients had complete IPSS questionnaires at 30 or 36
months, at least 3 follow-up evaluations, and did not show
urinary symptoms before RT (baseline IPSS<=12). 35/217
patients (16%) showed late urinary toxicity.
MLE pointed toward EUD values with very low
n=0.01±0.01. Thus indicating a serial behaviour of bladder
for this toxicity. The best-fit parameter for the NTCP
steepness was k=12±4, while EUD
50
was estimated to be
89±5Gy. The results were confirmed in the
hypofractionated population.
The use of antiaggregants was the only clinical risk factor
associated with toxicity (p=0.02, odds ratio = 3.6). Its
inclusion in the NTCP model implied a reduction of the
EUD
50
to 81±5Gy.
Conclusion
An NTCP model for prediction of late urinary toxicity was
determined. Bladder was found to act as a serial organ, so
that toxicity risk highly depends on small bladder volumes
irradiated with high doses. Patients with antiaggregants
exhibit enhanced radiosensitivity. The dose-response
relationship points out that hypofractionation (resulting in
higher EUDs) should be combined with careful
optimization in the bladder-PTV overlap to reduce the risk
of late urinary toxicity.
[1] T.E. Schultheiss, C. G. Orton, R. A. Peck, 'Models in
radiotherapy: volume effects”. Med. Phys. 10 (1983).
PO-0730 The independent benefit deriving from high
doses and WPRT in salvage post-prostatectomy
radiotherapy
C. Cozzarini
1
, B. Noris Chiorda
1
, C. Fiorino
2
, M. Pasetti
1
, A.
Briganti
3
, C.L. Deantoni
1
, A.M. Deli
1
, A. Fodor
1
, N. Fossati
3
,
G. Gandaglia
3
, C. Sini
2
, F. Montorsi
3
, N. Di Muzio
1
1
San Raffaele Scientific Institute, Radiotherapy, Milano,
Italy
2
San Raffaele Scientific Institute, Medical Physics, Milano,
Italy
3
San Raffaele Vita-Salute University, Urology, Milano,
Italy
Purpose or Objective
The success rate after salvage radiotherapy (SRT) for
biochemical recurrence after radical prostatectomy is still
unsatisfactory, possibly owing to inadequate irradiation
doses and fields. The few retrospective studies
investigating the role of whole-pelvis radiotherapy (WPRT)
in this setting have pertained to patients (pts) treated
with conventional (60-68 Gy) SRT doses, a factor that may
have ”diluted” its true effect. The purpose of this analysis
was to evaluate the possible clinical benefit of WPRT in
addition to high-dose salvage radiotherapy (HDSRT), and
to identify the cohort that would benefit most from it.
Material and Methods
From 1998 to 2009, 235 node-negative pts underwent
HDSRT (median 75.6 Gy, range 64.8-77.4). 149 pts were
irradiated to prostatic bed (PB) only, at a median dose of