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ESTRO 36 2017
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Conclusion
The availability of a real-time 4D dose accumulation based
treatment assessment tool allows to assess the quality of
the delivered dose during progress of the treatment course
and to take appropriate actions, as for example, plan
adaptation, in cases of significant deviations. It is foreseen
to extend this study for a full treatment course of a
broader population of patients.
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.