ESTRO 36 Abstract Book

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S450 ESTRO 36
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PO-0835 PTV margin for pelvic lymph nodes in IGRT
guided prostate radiotherapy
H.R. Jensen1, C.R. Hansen1,2, S.N. Agergaard1, E.L.
Lorenzen 1,2, L. Johnsen1, S. Hansen2,3, L. Dysager3, C.
Brink1,2
1 Odense University Hospital, Laboratory of Radiation
Physics, Odense, Denmark
2 University of Southern Denmark, Faculty of Health
Sciences, Odense, Denmark
3 Odense University Hospital, Department of Oncology,
Odense, Denmark
Purpose or Objective
In recent years irradiation of the pelvic lymph nodes for
high risk prostate cancer has received strong interest, as
a potential way to increase locale control probability.
However the prostate and the pelvic lymph nodes move
independently of each other. The purpose of this study is
to calculate the additional PTV margin needed for
covering the pelvic lymph node region, when performing a
registration and setup on the prostate with implanted gold
fiducials.
Material and Methods
All 40 prostate patients treated at the same accelerator in
2015 were included in the study. The majority of the
patients had stage T3 disease. All patients had three gold
fiducials implanted into the prostate 2-3 weeks before CT
simulation, which were used in the daily online IGRT. A
total of 1284 cone beam CT scans were analyzed. An
automatic gold seed algorithm (used as a surrogate for the
prostate) and bone algorithm covering the upper pelvic
and lower spine area (used as a surrogate for the lymph
nodes) were performed. The deviation between the two
registrations was calculated and the population based
random and systematic setup error was calculated. To
estimate the PTV margin needed the Van Herk margin
formula was used M = 2.5 * Σsystematic + 0.7 * σrandom
Results
The setup margin needed for the lymph node region of this
patient cohort is 2.1, 6.9 and 6.6 mm for the LR, CC, AP
directions, respectively (see table). This margin does not
incl. any other uncertainties. The minimum deviation
between prostate seed and pelvic bone match is shown as
a cumulative histogram in the figure for the individual
directions. More than 15.4% of the fractions have a
deviation of more than 5 mm, and 5% of the fractions have
a larger deviation than 7mm. The largest deviations are
seen in the CC and AP direction, and a small deviation in
the LR direction. The systematic and random errors are
shown in the table.
Conclusion
Even though the analyzed IGRT protocol focuses entirely
on the gold seed of the prostate the needed margins for
the lymph nodes are only slightly larger than 5 mm which
in many centers are used as a standard PTV margin. Thus,
the additional margin needed to include the lymph nodes
is actually somewhat modest. However, the optimal
balance between dose coverage of tumor and lymph nodes
both in regard to local control and toxicity is still unclear,
and needs further investigation.
PO-0836 Impact of Deep Inspiration Breath Hold on
Left Anterior Coronary dose in Left Breast irradiation.
F. Azoury1, S. Achkar1, N. Farah1, D. Nasr1, C. El Khoury1,
N. Khater1, J. Barouky1, R. Sayah1, E. Nasr1
1Hotel Dieu de France Hospital - Saint Joseph University,
Radiation Oncology, Beirut, Lebanon
Purpose or Objective
Irradiation of Left breast cancer exposes women to higher
doses to the heart and LAD coronary. Blocking the heart in
the tangent fields will inevitably cause under dosage in
proximity to the tumor bed. Here we evaluate the effect
of deep inspiration breath hold (DIBH) on the coverage of
the whole breast and the reduction of heart and LAD
doses.
Material and Methods
We performed a dosimetric study on 25 patients treated
with DIBH for left breast cancer utilizing RPM (Varian
Medical Systems). Treatment plans were generated in Free
Breathing (FB) and DIBH. Optimization was done with 3D
Field-in-Field technique utilizing two-tangent setup. Care
was taken to cover the whole breast volume. Prescription
dose was 50Gy in 25 fractions. Planning objectives were:
near minimum dose (D98) > 90% (45Gy), near maximum
dose (D2) <105% and a median dose of 50Gy. Doses to the
heart, LAD and left lung were compiled, left breast
coverage was evaluated, and statistical analysis was
performed using Student T-test with a 95% Interval of
confidence.