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S260
ESTRO 36
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OC-0489 Variation in bladder volume and associated
spatial dose metrics in prostate and pelvic
radiotherapy
O. Casares-Magaz
1
, V. Moiseenko
2
, A. Hopper
2
, N.J.
Pettersson
2
, M. Thor
3
, R. Knopp
2
, J.O. Deasy
3
, L.P.
Muren
1
, J. Einck
2
1
Aarhus University Hospital - Aarhus University, Medical
Physics, Aarhus, Denmark
2
University of California San Diego, Radiation Medicine
and Applied Sciences, San Diego, USA
3
Memorial Sloan Kettering Cancer Center, Medical
Physics, New York, USA
Purpose or Objective
The bladder displays considerable inter-fractional changes
during a course of radiotherapy (RT) which leads to
differences between delivered and planned dose/volume
metrics. The aim of this study was to compare planned
with actually delivered spatial bladder dose distributions
for patients receiving RT for prostate cancer with a full
bladder/empty rectum protocol, by using daily on-board
cone-beam CT (CBCT) and to assess impact of
concomitantly treating the pelvic lymph nodes.
Material and Methods
Twenty-five prostate cancer patients (fifteen cases
receiving local prostate irradiation and ten cases also
receiving pelvic node irradiation) received daily CBCT-
based image-guidance RT (81 Gy in 45 fractions) adhering
to full bladder and empty rectum protocol. For each
patient, 8-9 CBCTs were registered to the planning CT
using the clinically applied patient set-up (translations).
Bladder was segmented on each CBCT and approved by a
radiation oncologist. Bladder shells were extracted using
a 3mm inner margin, and bladder shell quadrants were
created using axial and coronal planes drawn through the
center of mass of the bladder. Dose/volume histograms
(DVHs) were extracted for bladder, bladder shell (BS), as
well as anterior (A), posterior (P), superior (S), inferior (I),
A/I, A/S, P/I, P/S sectors of the BS in each planning CT
and CBCT. Differences in DVH metric between the planned
and the delivered were calculated, and the association
between DVH metrics and bladder volume was evaluated
using the Spearman rank correlation coefficient (r
s
). DVH
metrics per fraction (D
x
, absolute V
x
and relative V
x
; x:5-
100% in 5% steps) were calculated for all bladder sectors
and compared between the two groups of patients.
Results
Bladder volumes varied considerably during RT, with a
coefficient of variation ranged between 14% to 54% across
treatment. Lower bladder volumes were found for
patients receiving pelvic RT compared to patients treated
locally (population mean±SD: 173±94cm
3
vs. 217±119 cm
3
;
p<0.01). At the anterior and superior part of the bladder,
positive associations were found between DVH metrics and
bladder volume for pelvic node irradiation fractions, while
negative associations were found for prostate alone
fractions, 25% and 75% r
s
percentiles: (0.74, 0.93) and
(0.78, 0.96) of S and A/S sectors for pelvic RT vs. (-0.79, -
0.43) and (-0.80, -0.40) of S and A/S sectors for prostate
RT across all Vx metrics (Fig. 2). Similar trend was found
for the BS 25% and 75% r
s
percentiles: 0.91-1.00 vs. 0.09-
0.61; however, for the whole bladder, differences were
smaller between 25% and 75% r
s
percentiles: (0.93, 1.00)
vs. (0.23, 0.71) for pelvic and prostate RT, respectively.
Conclusion
CBCT-based bladder analysis exhibits significant volume
changes along RT course even under full bladder daily
image-guided RT protocol. Larger bladder volumes meant
higher delivered doses to the superior and anterior
bladder subsectors in pelvic node irradiation, but reduced
overall delivered doses for prostate treatment.
OC-0490 A robust and fast planning approach for
adaptive MR-guided treatment of pancreatic cancer
O. Bohoudi
1
, A. Bruynzeel
1
, B. Slotman
1
, S. Senan
1
, F.
Lagerwaard
1
, M.A. Palacios
1
1
VUMC, Radiotherapy, Amsterdam, The Netherlands
Purpose or Objective
In May 2016, we implemented stereotactic MR-guided
adaptive radiation therapy (SMART) using the MRIdian
system (Viewray) for locally advanced pancreatic cancer.
Interfractional changes in the anatomy of adjacent organs-
at-risk (OARs) make daily online plan adaptation
desirable. The main challenge of online plan adaptation is
the requirement that it must be performed fast while the
patient remains in treatment position. We evaluated an
in-house developed re-planning strategy, which is
currently in clinical use.
Material and Methods
Before use of SMART, robust baseline IMRT plans for online
re-optimization are first produced with the MRIdian
planning system (ViewRay). The same planning software is
available at the treatment console for plan adaption. The
target structure used for optimization is defined as PTV
opt
(GTV+3mm minus OARs). OAR contours are then spatially
partitioned in separate OAR portions located within 1, 2
and 3cm from the PTV
OPT
surface, thereby allowing direct
control over the spatial dose distribution (Fig. 1). The
optimization process relies on a model which predicts OAR
dose as a function of distance from PTV
OPT
, and generates
optimization objectives to achieve a robust baseline plan
for daily adaption. For daily SMART, physicians only re-