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Conclusion
The rectum dose maps and the connected NTCP models
used in this study identified clinically relevant changes in
rectum dose distributions caused by organ motion during
the course of therapy. This model validation study showed
that these maps and models are useful tools to evaluate
the risk of normal tissue reactions in the rectum.
EP-1611 Dose-response relationships for radiation-
induced urgency syndrome after gynecological
radiotherapy
E. Alevronta
1
, V. Skokic
1
, U. Wilderäng
1
, G. Dunberger
2
,
F. Sjöberg
1
, C. Bull
1
, K. Bergmark
1,3,4
, G. Steineck
1,4
1
Institute of Clinical Sciences- Sahlgrenska Academy at
the University of Gothenburg, Department of Oncology-
Division of Clinical Cancer Epidemiology, Gothenburg,
Sweden
2
Ersta Sköndal College University, Department of Health
Care Sciences, Stockholm, Sweden
3
Sahlgrenska University Hospital, Department of
Oncology, Gothenburg, Sweden
4
Karolinska Institutet, Department of Oncology and
Pathology- Division of Clinical Cancer Epidemiology,
Stockholm, Sweden
Purpose or Objective
To find out what organ and doses are most relevant for
‘radiation-induced urgency syndrome’ in order to derive
the corresponding dose-response relationships as an aid
for avoiding the syndrome in the future.
Material and Methods
Of the 99 survivors treated with radiation therapy for
gynecological cancer, 24 developed ‘radiation-induced
urgency syndrome’. The survivors included in the study
had not received brachytherapy, but other treatment
combinations of external radiation therapy, surgery, and
chemotherapy at the Karolinska University Hospital,
Stockholm or the Sahlgrenska University Hospital,
Gothenburg during the period 1991 to 2003. The rectum,
the sigmoid and small intestines were delineated and the
dose-volume histograms were exported for each patient.
‘Radiation-induced urgency syndrome’ consists of
different self-reported bowel symptoms. To combine the
symptoms, factor analyses was used. Dose-response
relationships were estimated fitting the data to the Probit
model. ROC curve analyses were used to identify which
organ at risk is correlated the most with the clinical
outcome.
Results
The maximum likelihood estimates of the dose-response
parameters for Probit model, the three organs at risk and
‘radiation induced urgency syndrome’, the Log Likelihood
(LL) value and the AUC are:
D
50
= 51.3 (48.3-54.6),
γ
50
=
1.19 (0.98-1.42), α/β=0.59 (0.034-1.56), LL = -50.2 and
AUC=0.63 for rectum,
D
50
= 51.6 (48.7-54.9) Gy,
γ
50
= 1.20
(0.98- 1.44),
α/β
=2.02 (0.85-4.73), LL = -51.0 and
AUC=0.66 for the sigmoid and
D
50
= 61.4 (56.4-67.5) Gy,
γ
50
= 0.90 (0.73- 1.08),
α/β
= 10.3 (2.1- 1e+06 Gy), LL = -
51.4
and
AUC=0.60
for
small
intestines.
Conclusion
For the studied organs at risk, the dose to the sigmoid is
the best predictor of ‘radiation-induced urgency
syndrome’ among gyneocological cancer survivors. Dose
planners having the ambition to eliminate the syndrome
may consider to delineate the sigmoid as well as rectum
in order to incorporate the dose-response results.
EP-1612 Estimates of the α/β ratio for prostate using
data from recent hypofractionated RT trials.
S. Gulliford
1
, C. Griffin
2
, A. Tree
3
, J. Murray
4
, U. Oelfke
1
,
I. Syndikus
5
, E. Hall
2
, D. Dearnaley
3
1
The Institute of Cancer Research and The Royal Marsden
NHS Foundation Trust, Joint Department of Physics,
London, United Kingdom
2
The Institute of Cancer Research, Clinical Trials and
Statistics Unit, London, United Kingdom
3
Royal Marsden NHS Foundation Trust, Academic Urology
Unit, London, United Kingdom
4
Guy’s & St Thomas' NHS Foundation Trust, Department
of Clinical Oncology, London, United Kingdom
5
Clatterbridge Cancer Centre, Department of Clinical
Oncology, Wirral, United Kingdom
Purpose or Objective
The α/β ratio for prostate cancer has been widely studied
with growing evidence for a value significantly lower than
the standard value for tumours of 10 Gy. Previous studies
have also indicated that there may be a time factor
whereby tumour repopulation should be considered during
the course of radiotherapy[1]. Recent reporting from 4
separate phase III clinical trials comparing
hypofractionated schedules with standard schedules for
prostate radiotherapy allow for further exploration of the
α/β value.
Material and Methods
The α/β ratio for prostate was derived independently for
each of the CHHiP[2], HYPRO[3], PROFIT [4]and RTOG
0415 studies[5] by comparing the outcomes in the
standard and hypofractionated trial arms. This approach