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S834
ESTRO 36 2017
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of cardiac and pulmonary radiation dose with moderate
deep inspiration breath hold (DIBH) technique compared
with free breathing (FB) for irradiation of left-sided breast
including IMN.
Material and Methods
Ten patients underwent CT simulation scans during FB and
DIBH, which was performed with the SpiroDyn’RX (Dyn’R)
spirometer. The clinical target volumes (CTV) included the
breast, the ipsilateral IMN, the supraclavicular lymph
nodes area and the tumor bed site. Contouring was
performed by the same physician, following ESTRO
consensus guidelines, on both CT scans. Prescribed doses
were 50 Gy in 25 fractions to the breast, the IMN and the
supraclavicular area, followed by 16 Gy to the tumor bed
site. Treatment plans were calculated by the same
physicist with Pinnacle 9.10 (Philips) TPS for both CT
scans. Three isocentric beams were used for the 50 Gy
volumes: two wide Step and Shoot (S&S) tangents for
irradiation of the breast and IMC, and an anterior
conformal beam for treatment of the supraclavicular
nodes. Three oblique S&S beams were used for tumor bed
boost. The resulting averaged dose-volume histograms
(DVH) were generated and compared. Mean dose to the
heart (D
mean, heart
) and heart volume receiving 25 Gy or more
(V
25Gy, heart
), mean left lung dose (D
mean, lung
) and lung volume
receiving 20 Gy or more (V
20Gy, lung
) were evaluated and
compared.
Results
The average DVHs for FB and DIBH are shown on figure 1.
With similar target volumes coverage, average D
mean, heart
was reduced from 9.7 +/- 2.1 Gy [range: from 6.1 to 12.7
Gy] to 5.1 +/- 2 Gy [range: from 3.2 to 8.9 Gy] in DIBH
plans compared to FB. Averaged V
25Gy, heart
was reduced
from 13.5 +/- 4.3% in FB plans to 4.7 +/- 3.6% in DIBH
plans. Figure 2 shows a systematic reduction of V
25Gy, heart
over the 10 patients, superior to 50% for 8 patients.
Averaged mean D
mean, lung
was reduced from 18.5 +/- 3.2 Gy
in FB plans to 17.5 +/- 2.8 Gy in DIBH plans and V
20Gy, lung
by 7.8% in DIBH plans, but were not systematically
inferior.
Conclusion
Without compromising target coverage, DIBH treatment
plans provided an averaged D
mean, heart
reduction of 4.6 Gy,
and a 1.0 Gy reduction of D
mean, lung
on average. Related to
the reduction of mean cardiac dose and the demonstrated
decrease of cardiovascular toxicities, DIBH may be the
preferable treatment technique when radiotherapy of the
internal mammary lymph nodes is required for left sided
breast cancer.
EP-1569 A comparison of SRS plan quality when using
VMAT vs non-coplanar static conformal fields.
R. Brass
1
, L. Howard
1
, M. Gilmore
1
1
The Clatterbridge Cancer Centre, Physics, Bebington,
United Kingdom
Purpose or Objective
To produce a VMAT multi-arc solution in Eclipse for SRS
patients with at least equivalent plan quality to previously
used static conformal field (SCF) technique. To establish
a plan quality tool based on acceptable plan quality
metrics for SRS patients.
Material and Methods
10 clinical SRS plans were chosen to create a cohort with
a variation of indications, target volumes, shapes and
positions within the brain (see Table 1). Patients with
multiple targets were excluded from the study. The plan
quality parameters used were the Paddick conformity
index (CI)
[Paddick 2000]
, Paddick gradient index (GI)
[Paddick & Lippitz 2006]
and normal tissue overdose factor
(NTOF: ratio of volume of normal tissue receiving
prescription isodose to volume of prescription isodose)
along with various dose-volumes (e.g. V5Gy); baseline
values were calculated for the SCF plans.
Several arc configurations were considered, ranging from
1 full arc at 0° couch angle to 1 full arc plus 3 half arcs at
couch angles of 45
o
, 90
o
& 315
o
. One of the more complex
cases was used to develop a VMAT planning solution by
increasing the number of half-arcs used until gains in plan
quality became negligible. The
Normal Tissue Objective
(NTO) parameters in Eclipse were then optimised to
further reduce the dose to OARs and normal brain tissue.
The rest of the cohort was planned using this final solution
and plan quality measures calculated and compared to SCF
baselines. A final VMAT solution was decided upon
consisting of 1 full arc at couch zero plus 2 half arcs at 45
o
and 315
o
.
The NTO
fall off
parameter was tweaked while fixing the
distance from target
,
start dose
and
end dose
parameters
at values suitable for SRS. The final
fall off
value for the
VMAT solution was set to 0.50.
Fall off
dictates how
quickly the dose should decrease outside the target
volume, with a higher value indicating a more rapid
decrease. The 0.50 value used here compares to 0.15 used
at our centre for VMAT plans.
Results