ESTRO 36 Abstract Book

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S454 ESTRO 36
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Department of Oncology and Hemato-oncology, MIlan,
Italy
Purpose or Objective
The aim of the study (partially supported by a research
grant from Accuray Inc. entitled “Data collection and
analysis of Tomotherapy and CyberKnife breast clinical
studies, breast physics studies and prostate study”) is to
assess the dosimetric benefit of intensity-modulated
radiotherapy (IMRT) in postmastectomy patients with
implant–based immediate breast reconstruction (IBR),
candidates to locoregional radiotherapy with
hypofractionation.
Material and Methods
Data of the first 121 consecutive post-mastectomy
locoregional patients treated with Helical Tomotherapy
between May 2012 and May 2015 with a hypofractionated
scheme (2.67Gy/fr, 15 fractions) have been prospectively
collected. At the time of surgery, all patients underwent
IBR using either temporary tissue expander or permanent
prosthesis.
The impact of immediate breast reconstruction on the
planning was analyzed. Treatment plans were scored in
terms of coverage of the PTVs (chest wall and
supraclavicular region) and sparing of organs at risk
(heart, lungs and contralateral breast). The coverage of
chest wall and supraclavicular region was evaluated
according to the amount of volume receiving the 90% of
the prescribed dose (V90%) while the sparing of each OAR
was evaluated according to the number of satisfied
constrains (Tab.1). A plan with optimal coverage of both
PTVs had 2 PTV points, while a plan with optimal sparing
of all OARs had 4 OARs points. An overall score was
assigned to each plan.
Results
71.1% (86/121) of the 121 post-mastectomy radiotherapy
plans had high total scores (total score=6 points) as a
result of an optimal coverage of both chest wall and
supraclavicular region and optimal sparing of all OARs. The
remaining 28.9% (35/121) of plans had a compromised
distribution of dose (total score<6 points). In particular,
13.2% (16/121) of plans fully satisfied all the OAR
constraint but at a cost of moderate coverage of chest wall
(7/121 plans) or supraclavicular region (9/121 plans)
target volumes. On the other hand, 13.2% (16/121) of
plans fully satisfied coverage of both PTVs compromising
the sparing of OARs (heart, ipsilateral lung, or
contralateral breast). The residual 2.5% of plans (3/121)
had both coverage of PTVs and sparing of OARs
compromised.
Conclusion
In patients having implant-based IBR, IMRT allows optimal
treatment plans in more than 2/3 of cases. Superior
dosimetric results are even more important when
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. Lafond1,2,3, J. N'Guessan2, G. Dréan 1,3, N. Perichon2, N.
Delaby2, O. Acosta1,3, A. Simon 1,3, R. De Crevoisier1,3,4
1University Rennes 1, LTSI, Rennes, France
2Centre Eugène Marquis, medical physics department,
Rennes CEDEX, France
3INSERM, U1099, Rennes, France
4Centre 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