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hypofractionation is used and they are expected to
translate into lower late toxicity and improved aesthetic
outcome.
PO-0841 Feasibility of dose decrease in a rectal sub-
region predictive of bleeding in prostate radiotherapy
C. Lafond
1,2,3
, J. N'Guessan
2
, G. Dréan
1,3
, N. Perichon
2
, N.
Delaby
2
, O. Acosta
1,3
, A. Simon
1,3
, R. De Crevoisier
1,3,4
1
University Rennes 1, LTSI, Rennes, France
2
Centre Eugène Marquis, medical physics department,
Rennes CEDEX, France
3
INSERM, U1099, Rennes, France
4
Centre Eugène Marquis, radiation oncology department,
Rennes CEDEX, France
Purpose or Objective
The inferior–anterior hemi anorectum has been found as
highly predictive of rectal bleeding in case of prostate
cancer radiotherapy, shown in Figure 1
(Dréan et al.,
Radiother Oncol 2016)
. The aim of this dosimetric study
was to evaluate the feasibility of decreasing the dose in
this rectal sub-region (SRR), while keeping a high PTV
coverage. Two new and simple strategies were used:
identifying the SRR during inverse planning and/or using a
recent dosimetric model. This model was used allowing to
better define the achievable mean dose to the rectal
structures at the inverse planning step of IMRT
(Moore et
al., Int. J. Radiation Oncology Biol. 2011)
. This model
integrates the overlap volume between the OAR and the
PTV.
Material and Methods
60 patients data already treated for prostate cancer to a
total dose of 78 Gy were used. For each patient, 4 VMAT
plans were generated with Pinnacle v9.10 (Philips): one
standard plan corresponding to the current practice
(“Standard”), one plan adding specific objectives to the
SRR (“SRR”), one plan using the Moore model applied to
the rectal wall only (“model”) and one plan using the
Moore model applied to both the rectal wall and the SRR
(“model+SRR”). The plans were compared regarding dose
distribution, indexes of conformity and homogeneity, risk
of 3-year Grade > 1 RB using the Lyman–Kutcher–Burman
NTCP model, and efficiency (Monitor Units and complexity
indexes).
Results
Figure 2 shows the mean DVH of the 60 patients for each
of the 4 plans. “Model + SRR” plans showed the most
important SRR dose sparing, with mean dose decreases of
4.7 Gy, 5.3 Gy and 7.7 Gy relatively to the “Model”, “SRR”
and “Standard” plans respectively. Mean NTCP values
were 0.22 for “Standard” plans, 0.19 for “SRR” plans, 0.18
for “Model” plans and 0.17 for “Model + SRR” plans.
Plans “Model + SRR” showed slightly less dose
homogeneity: mean homogeneity indexes varied from
0.077 for “standard” plans to 0.101 for “Model + SRR”
plans. Dose conformity was very similar for all plans: the
conformal index varied of 1% in average. “Model + SRR”
plans required an increase of 22% in the number of MU
compared to the “Standard” plan. The irregularity and
modulation indexes increased of 58% and 10%,
respectively.
Conclusion
Compared to standard prostate VMAT plans, applying
specific dose constrains to the SRR and rectal wall using
the “Moore method” should decrease of around 8 Gy the
mean dose to the SRR and decrease relatively of 23% the
risk of rectal bleeding.
PO-0842 Choosing the best heart sparing technique for
breast and internal mammary chain radiotherapy
A. Ranger
1
, A. Dunlop
1
, K. Hutchinson
2
, M. Maclennan
3
,
H. Convery
4
, H. Chantler
2
, C. Rose
2
, N. Twyman
2
, E.
Donovan
1
, E. Harris
5
, C. Coles
2
, A. Kirby
6
1
The Royal Marsden NHS Trust and The Institute of
Cancer Research, Physics, London, United Kingdom
2
Cambridge University Hospitals NHS Trust, Physics,
Cambridge, United Kingdom
3
Lothian and Dumfries NHS Trust, Oncology, Edinburgh,
United Kingdom
4
The Royal Marsden NHS Trust, Physics, London, United
Kingdom
5
The Institiute of Cancer Research, Physics, London,
United Kingdom
6
The Royal Marsden NHS Trust and The Institute of
Cancer Research, Clinical Oncology, London, United
Kingdom
Purpose or Objective
Published data demonstrate a 4.4% overall survival benefit
t associated with inclusion of the internal mamm ary chain
(IMC) in the radiotherapy (RT) target volume in patients
with breast cancer. Survival gains will be maximised by
minimising radiation doses to heart and lungs. This
dosimetry study compares the ability of breath-hold
techniques in 3D conformal radiotherapy, arc therapy and
protons to achieve target volume constraints whilst
minimising dose to heart and lungs with a view to defining
implementable class solutions for irradiating the IMC.
Material and Methods
Breast tissue, level I-IV axillary and IMC lymph nodes were
outlined using ESTRO consensus guidelines in 14 patients
scanned in both free breathing (FB) and breath hold (BH).
Seventy two locoregional RT plans, prescribed to 40Gy/15