ESTRO 36 Abstract Book

886 / 1096

S870 ESTRO 36
_______________________________________________________________________________________________
RT since the QUANTEC publications. There is an increased
recognition of intra- and inter-structure specific doses,
and even though rectal bleeding remains the most studied
symptom, there is also a trend towards other non-
aggregated symptoms.
EP-1610 Predictors for morbidity from planned vs.
delivered rectal dose maps in RT of prostate cancer
J. Trane1 , O. Casares Magaz1, L. Bentzen2, K. Busch1, M.
Thor3, L.P. Muren1
1 Aarhus University Hospital - Aarhus University, Medical
Physics, Aarhus, Denmark
2 Aarhus University Hospital, Oncology, Aarhus, Denmark
3 Memorial Sloan-Kettering Cancer Center, Medical
Physics, New York, USA
Purpose or Objective
Patient-reported gastro-intestinal (GI) symptoms
following radiotherapy (RT) for prostate cancer have
recently been associated with metrics derived from rectal
dose surface maps. In a recent study we developed rectum
dose map based normal tissue complication probability
(NCTP) models for three common late GI symptoms (at
least 20% prevalence in the cohort used for modelling). In
the present study we used such dose maps and connected
NTCP models to compare the planned, daily and summed
rectal dose distributions for patients with repeat
volumetric imaging acquired during the course of RT.
Material and Methods
The patients included in this study were treated according
to a national clinical trial for patients with locally
advanced prostate cancer, irradiating concomitantly the
pelvic lymph nodes and seminal vesicles to 55 Gy and the
prostate to 78 Gy using volumetric modulated arc therapy.
The treatment plans were recalculated on weekly repeat
cone-beam (CB) CTs (6-8 CBCTs per patient) following
Hounsfield Unit override to bone and water. Rectal dose
maps were created for the planned dose distribution as
well as for the dose distributions re-calculated on weekly
CBCTs using a method recently developed by our group.
The weekly CBCTs were averaged to provide a measure for
the summed/accumulated dose across the course of RT.
NTCPs were calculated for the planned, weekly and
averaged rectal dose maps using three spatially based
response models (based on areas and extents from the
rectal dose maps) for three patient-reported GI symptoms:
faecal leakage, obstruction and defecation urgency. The
study included four prostate cancer patients, one with and
three free from late Grade 2+ GI symptoms after RT.
Results
Dose differences exceeding +/- 10 Gy (scaled to the full
treatment course) were seen in the dose maps for all
patients and in all scans (Fig. 1). The largest systematic
dose increase in the maps during the course of therapy was
seen in the patient that experienced Grade 2+ GI
symptoms after treatment. This patient also had higher
NTCPs for all three spatial dose metric based models for
the average map across treatment compared to the
planned dose distribution (e.g. 10% vs 6% for faecal
leakage), while smaller differences were seen for the
three other patients (Table 1).
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. Alevronta1, V. Skokic1, U. Wilderäng1, G. Dunberger 2,
F. Sjöberg1, C. Bull1, K. Bergmark1,3,4, G. Steineck1,4
1Institute of Clinical Sciences- Sahlgrenska Academy at
the University of Gothenburg, Department of Oncology-
Division of Clinical Cancer Epidemiology, Gothenburg,
Sweden
2Ersta Sköndal College University, Department of Health
Care Sciences, Stockholm, Sweden
3Sahlgrenska University Hospital, Department of
Oncology, Gothenburg, Sweden
4Karolinska 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.