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dose distributions, larger volumes of the surrounding
tissues receive a more or less pronounced low dose bath.
Clinical results demonstrating a detrimental effect of the
low-dose bath, related to volume and dose levels, with
respect to the two-tangential beam dose delivery, or the
associated risk of secondary cancer induction, are
currently not available. In the absence of such data, a
good approach is to drive the inverse VMAT optimization
processes to decrease the dose to all the critical
structures as much as possible, and to maximize the target
dose homogeneity. This can be primarily achieved by
means of adequate beam arrangement, and by highly
restrictive planning objectives, more restrictive than the
clinical need. Scope of this work is to evaluate the possible
trade-offs in breast VMAT planning, exploring the degree
of achievable dosimetric sparing of different organs at risk
(OAR) by using two quite similar VMAT plan settings.
Material and Methods
CT scans of 20 patients presenting left sided breast
cancer, in deep inspiration breath hold, were studied.
VMAT plans were optimized for the RapidArc technique in
the Eclipse treatment planning system (Varian) using the
PO algorithm and were calculated with Acuros, to deliver
40.05Gy to mean target dose in 15 fractions. Two partial
arcs were arranged for 6MV, Millennium MLC from a
TrueBeam linac (Varian). Two plans per patient were
optimized: RA_full, where the optimizer used the entire
partial arc trajectory, and RA_avoid, where sectors (set
from ~0 to ~105 degree) of MU=0 were set. Common dose
objectives included a stringent dose homogeneity, mean
dose to heart <5Gy, ipsilateral lung <8Gy, contralateral
lung <2Gy, contralateral breast <3Gy.
Results
RA_full showed a better dose conformity, lower high dose
volumes in healthy tissue and lower skin dose. The NTHD
(normal tissue high dose, defined as the uninvolved tissue
receiving 90% of the dose prescription relative to the
volume of the target) resulted in 18% and 31% for RA_full
and RA_avoid, respectively. RA_avoid presented a
reduction of the mean doses for all critical structures: 51%
to heart, 12% to ipsilateral lung, 81% to contralateral lung,
73% to contralateral breast. All differences were
significant with p<0.0001.
Conclusion
The adaptation of VMAT options to planning objectives
reduced significantly the healthy tissue dose levels at the
price of some high dose spillage. Evaluation of the trade-
offs to apply to the different critical structures should
drive in improving the usage of the VMAT technique for
breast cancer treatment, as the choice of the trade-offs
would affect the possible future late toxicity and
secondary cancer induction risk.
EP-1561 Comparison of heart, lung doses, and skin
toxicity from different breast cancer RT techniques.
M. Lizondo
1
, N. Jornet
2
, M.J. Fuentes-Raspall
3
, P.
Carrasco
2
, A. Latorre-Musoll
2
, A. Ruiz-Martinez
2
, T.
Eudaldo
2
, P. Delgado-Tapia
2
, C. Cases
2
, M. Ribas
2
1
Institut de Recerca Hospital de la Santa Creu i Sant Pau,
Servei de Radiofíosica i Radioprotecció, Barcelona, Spain
2
Hospital de la Santa Creu i Sant Pau, Servei de
Radiofísica i Radioprotecció, Barcelona, Spain
3
Hospital de la Santa Creu i Sant Pau, Servei d'Oncologia
Radioteràpica, Barcelona, Spain
Purpose or Objective
The aim of this study was to assess whether there are
significant differences in lung and heart doses for
different breast cancer radiotherapy techniques. This
study is based on a plan comparison from the dosimetric
and patient data prospectively collected in a breast RT
treatment database. Patient and treatment risk factors
for acute skin toxicity were analysed.
Material and Methods
Patient, treatment variables and treatment-related
outcomes were abstracted for 469 breast cancer patients
who completed radiotherapy treatment from 2013 to
2016. We selected patients with similar nominal dose to
the whole breast (50 Gy) and to the boost (EQD2 66 Gy),
patients with no electron boost were excluded. We also
included
patients
with
lymph
nodes(LN),
irradiation(50Gy). The available techniques were 3DCRT
and IMRT and from April 2016 onwards, deep inspiration
breath hold (DIBH) is used for all left breast patients.
Following our technique decision criteria, all boost areas
deeper than the range of our highest e- energy, are
treated with integrated boost using IMRT. And till 2016,
left breast cancer patient not fulfilling heart and/or lung
dose restrictions with 3DCRT were moved to IMRT. For
each patient, the technique was chosen in order to
maximize PTV coverage and homogeneity while keeping
doses to OAR as low as possible.
Heart dose (D
mean
,V
25
) and the ipsilateral lung dose (V
20
)
values were compared separately as a function of
laterality for patients undergoing 3DCRT (with and without
DIBH) or IMRT. Correlation tests were made between
maximum acute skin toxicity and technique, season, skin
photo-type, breast volume and smoking habits.
Results
Table I shows mean values and significance results. For
right-sided breast patients, heart D
mean
was significantly
lower for 3DCRT techniques than for IMRT techniques,
while heart V
25
and lung V
20
are not significantly different.
For left-sided breast patients heart D
mean
was lower but
heart V
25
was higher with the 3DCRT techniques than with
IMRT techniques when LN were not included, and heart
D
mean
also when LN were included. Lung V
20
was not
significantly different. For those patients treated with
DIBH, heart D
mean
and heart V
25
were considerably lower.
Skin toxicity shows a significant correlation with breast
volume (p=0.01) but not with technique, smoking habits,
skin photo type or period of the year in which the patient
was treated.