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S15
ESTRO 36 2017
_______________________________________________________________________________________________
1
Karolinska University Hospital, Medical Radiation
Physics and Nuclear Medicine, Stockholm, Sweden
2
San Raffaele Scientific Institute, Medical Physics,
Milano, Italy
3
Fondazione IRCCS Istituto Nazionale dei Tumori,
Prostate Cancer Program, Milano, Italy
4
San Raffaele Scientific Institute, Radiotherapy, Milano,
Italy
5
Ospedale Bellaria, Radiotherapy, Bologna, Italy
6
Istituto di Candiolo- Fondazione del Piemonte per
l'Oncologia IRCCS, Radiotherapy, Candiolo, Italy
7
Università degli Studi di Milano, Oncology and Hemato-
Oncology, Milano, Italy
Purpose or Objective
To explore which features of the dose distribution in the
ano-rectal wall determine the risk of late rectal bleeding
and late faecal incontinence following prostate cancer
radiotherapy (RT).
Material and Methods
Patients from the DUE-01 study with available 3D dose
distributions and follow-up data at 24 months after RT
were included in this study. Patients with pre-treatment
symptoms were excluded. An incidence of 22 in 152 was
observed for a maximum grade ≥ 2 rectal bleeding, while
12 patients in 110 experienced a mean grade > 1 faecal
incontinence, calculated from at least 3 occasions from 6
to 24 months after RT.
Dose surface maps were extracted and converted to EQD2;
structures considered were the rectum, anal canal and the
combination of the two. For each endpoint, the mean of
the dose surface maps in the group of patients with and
without toxicity respectively were calculated. A t-test was
performed on the mean values of each pixel to identify
regions where the dose differed between patients with
and without toxicity (i.e. with low p-value).
The lateral and longitudinal extent, and eccentricity, of
EQD2 isodoses from 5 to 73 Gy were extracted from the
dose maps. Univariate NTCP models using each parameter
were fitted to the outcome data and the performance
evaluated using AUC.
Results
The patients who experienced rectal bleeding received
higher dose to the posterior part of the rectal wall (see
figure; dose map unfolded along anterior axis); the
greatest difference was found when aligning all dose maps
at the inferior border of the rectum. Patients with faecal
incontinence had a higher dose in the posterior wall of the
anal canal compared to patients without; here a greater
difference was found when aligning the dose maps
according to the centre of mass of the dose maps.
For rectal bleeding, the highest AUC was found for the
lateral extent of the 31-Gy isodose; this is in agreement
with the difference in dose to the posterior wall in the
toxicity vs. non-toxicity groups. For faecel incontinence,
on the other hand, the model based on the lateral extent
of the highest isodose (73 Gy) had the highest AUC.
Conclusion
The dose received by the posterior part of the rectal wall
is related to the risk of late rectal bleeding, and the
lateral extent of the 31-Gy isodose is the best spatial dose-
parameter to include in an NTCP model. The risk of
causing late faecal incontinence is related to the dose to
the anal canal.
OC-0039 Unique sparing of spatial memory in mice
after whole brain irradiation with dose rates above
100Gy/s
K. Petersson
1
, P. Montay-Gruel
2
, M. Jaccard
1
, G. Boivin
2
,
J. Germond
1
, B. Petit
2
, F. Bochud
1
, C. Bailat
1
, J.
Bourhis
2
, M. Vozenin
2
1
Lausanne University Hospital, Institute of Radiation
Physics IRA, Lausanne, Switzerland
2
Lausanne University Hospital, Department of Radiation
Oncology, Lausanne, Switzerland
Purpose or Objective
Radiotherapy at ultra high dose rate (Flas h-RT) has been
suggested to increase the differential response between
normal and tumor tissue compared to conventional
radiotherapy. In order to further explore Flash-RT and to
validate its protective effect on normal tissues, we
decided to investigate brain response to Flash-RT as it is
a well-defined and robust model in radiobiology.
Material and Methods
10 Gy was used as the prescription dose for the whole
brain irradiations (WBI). The irradiation settings,
corresponding to the prescription dose, were defined
according to film (Gafchromic™ EBT3), TLD (LiF-100),
Alanine pellets, and ion-chamber (Advanced Markus,
corrected for ion recombination) measurements at the
surface of a solid water phantom, positioned behind a 1.7
cm in diameter aperture of a graphite applicator. The
measurements and the subsequent mice WBI were
performed for different dose rates, ranging from a
conventional radiotherapy dose rate of 0.1 Gy/s to 10 Gy
delivered in a single 1.8 µs electron pulse. TLD were
positioned inside the skull of a sacrificed mouse to
validate the dose delivered to the brain during WBI for the
highest and lowest dose rate setting. 75 Female C57BL/6J
mice were used in the study. Dose rate effect on
neuroprotection was evaluated by 'Novel Object
Recognition test” two months post-irradiation. All the
experiments were video-recorded. Analysis was
performed blindly and the time the mice spent
investigating each object was measured in order to
calculate the Recognition Ratio (RR) such as: RR= (time
spent investigating the novel object / time spent
investigating the two objects).
Results