489 / 1096
S474
ESTRO 36
_______________________________________________________________________________________________
(∆R
o
) was determined at the distal and proximal edge of
the tumor respectively. Secondly, for each angle and
patient, the 90th percentile of ∆R
u
and ∆R
o
were
calculated. Finally, median and inter-quartile ranges for
these beam-specific range uncertainties were evaluated.
Results
Figure 1 shows the median and inter-quartile range of ∆R
u
and ∆R
o
for the 13 different angles. For both ∆R
u
and ∆R
o,
the range errors of the lateral beams (around 90
⁰
) are
significant lower (paired T-test, p < 0.05) than the anterior
and posterior beams. Moreover, there is considerable
inter-patient differences in range uncertainties.
Figure 1. Median and inter-quartile range of the range
error ∆R in overshoot and undershoot at the beam’s
proximal and distal edge of the tumor respectively. The
angle of 90
⁰
is the lateral beam to the ipsilateral lung.
Conclusion
Variation in anatomy during the course of irradiation
causes variation in range, possibly leading to tumor under-
coverage and high dose in normal tissue. These range
uncertainties depend on patient and beam angle, and are
smaller for the lateral beams. Taking these beam-specific
range uncertainties into account, could improve the
robustness of proton treatment plan against anatomical
variations.
PO-0870 DIBH produces a meaningful reduction in lung
dose for some women with right-sided breast cancer
J.L. Conway
1
, L. Conroy
1
, L. Harper
1
, M. Scheifele
1
, W.
Smith
1
, T. Graham
1
, T. Phan
1
, H. Li
1
, I.A. Olivotto
1
1
Tom Baker Cancer Centre, Radiation Oncology, Calgary-
Alberta, Canada
Purpose or Objective
To determine whether deep inspiration breath hold (DIBH)
produced a clinically meaningful reduction in pulmonary
dose in comparison to free breathing (FB) during adjuvant
loco-regional radiation (RT) for right-sided breast cancer.
Subsequently, to prospectively evaluate DIBH in right-
sided breast cancer cases with a FB V20Gy ≥30%.
Material and Methods
Thirty consecutive women with breast cancer treated with
tangent pair RT following breast conserving surgery were
included. ESTRO guidelines were used to contour right-
sided IMC nodes on DIBH and FB scans, with care taken to
ensure comparability between scans. A four-field,
modified-wide tangent plan was developed on each scan
to include the right breast and full regional nodes with a
minimum dose of 80% to the IMC CTV. The junction
between the supraclavicular and tangent fields was at the
inferior extent of the ossified medial clavicle. Treatment
plans were calculated in Eclipse using Acuros algorithm
version 11. FB and DIBH plan metrics were compared using
Wilcoxon-signed rank testing. Commencing in March 2016,
as per a new institutional policy based on the above
results, all right-sided breast cancer patients with a FB
ipsilateral lung V20 ≥30% had a DIBH treatment plan
developed prior to compromising on IMC coverage. If the
absolute difference in lung V20 was ≥5% between plans,
the DIBH plan was used. The junction was moved
superiorly in only one case.
Results
IMC coverage was equivalent between DIBH and FB plans
(Table 1). Twenty-one patients (70%) had ≥5% reduction in
ipsilateral lung V20 with DIBH (Figure 1). A reduction in
lung metrics was observed with an absolute reduction in
mean ipsilateral lung V20 by 7.8% (range: 0.0-20.0%; Table
1). There was a mean reduction of 42.3cc (range: 0-178.9)
in the volume of liver receiving 50% of the prescription
dose. The differences in cardiac doses were statistically
significant, but unlikely clinically significant (Table 1).
Seven patients with stages IA-IIIC right-sided breast cancer
treated between April to October 2016 received a DIBH
scan as a result of a FB V20Gy ≥30%, treatment volumes
included IMCs in all cases. In this cohort a ≥5% difference
between plans was observed for all patients with a mean
FB V20 of 34.8% (range: 30.0-43.4%), which was reduced
by an absolute value of 11.8% (range: 5.4-19.1%) with
DIBH.
Conclusion
DIBH reduced mean ipsilateral lung V20 by 7.8% and mean
lung dose by 3.4Gy. For some patients, the volume of liver
receiving ≥25Gy can also be reduced with DIBH. DIBH
should be available as a treatment strategy to reduce right
lung V20 without compromising on IMC coverage for
patients with right-sided breast cancer during loco-
regional RT. This strategy can be advantageous when the
ipsilateral FB V20 ≥30%, a value that prompts many
radiation oncologists to exclude IMCs. Within a small
prospectively evaluated cohort reflective of a change in
institutional policy, we have observed an absolute
reduction in mean ipsilateral lung V20 by 11.8% with DIBH.
PO-0871 Study of the effect of heterogeneous setup
random errors in treatment margins
D. Sevillano
1
, A.B. Capuz Suárez
1
, R. Colmenares
Fernández
1
, R. Morís Pablos
1
, M.J. Bejar Navarro
1
, D.
Prieto Moran
1
, F. García-Vicente
1
1
Hospital Ramón y Cajal, Medical Physics, Madrid, Spain