ESTRO 36 Abstract Book

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S145 ESTRO 36
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T2W MRI with 2mm slice thickness for treatment planning.
Dose rates were measured using a fiber-coupled Al2O3:C
luminescent crystal placed in a dedicated needle in the
prostate.
The dose measurements were analysed retrospectively.
The total accumulated dose was compared to the
predicted dose. Secondly, the measured dose rate
originating from each dwell position in a needle was
compared to the predicted dose rate obtained from the
dose planning system. The discrepancies between
measured and predicted dose rates were assumed to be
caused by geometrical offsets of the needles relative to
the dosimeter from the treatment plan. An algorithm
shifted each treatment needle virtually in radial and
longitudinal directions relative to the dosimeter until
optimal agreement between the predicted and measured
dose rates was achieved.
Results
Table 1 shows the relative difference between the
measured and predicted accumulated dose and the
average radial and longitudinal shifts of 337 needles in 22
treatments. The average shifts are expected to
correspond to systematic uncertainties in dosimeter
positions, and the standard deviations reflect the shift of
needles relative to the dosimeter. Two treatments were
not further analysed because of dosimeter drift by
>15mm.
The longitudinal and radial shifts of each needle are
plotted in Fig. 1. The relative needle-dosimeter geometry
was determined with sub-millimetre precision for 98% of
the treatment needles (error bars in Fig. 1). More than 90%
of the needles were shifted less than 4mm longitudinally
and 2mm radially, which is consistent with typical
uncertainties in needle and dosimeter reconstructions and
needle movements between MR-scan and treatment.
There was no relation between deviations in measured
dose and shifts of needles. E.g. patient 6 and patient 7
have similar shifts but very different accumulated dose
deviations. This illustrates how a small shift in a nearby
needle can lead to significant changes in the measured
dose, making it hard to use the accumulated dose for
treatment verification.
Conclusion
Accumulated dose and dose rate have been measured in
real-time for 22 treatments. We have used real-time in-
vivo dosimetry to determine the rela tive geometry
between needles and dosimeter with high precision. This
could potentially lead to real-time treatment verification
in BT.
OC-0280 Benefit of repeat CT in high-dose rate
brachytherapy as radical treatment for rectal cancer
R.P.J. Van den Ende1, E.C . Rijkmans1, E.M. Kerkhof1,
R.A. Nout1, M. Ketelaars1, M.S. Laman1, C.A.M .
Marijnen1, U.A. Van der Heide1
1Leiden Univers ity Medical Center, Department of
Radiation Oncology, Leiden, The Netherlands
Purpose or Objective
High-dose rate endorectal brachytherapy (HDR-BT) for
rectal cancer can be used to increase the dose to the
tumor while sparing surrounding organs due to a smaller
treated volume and the steep dose gradient.
Conventionally, one treatment plan is derived from a
planning CT with applicator in situ prior to the start of
treatment, which is then used for all further applications
(non-adaptive approach). An adaptive approach would be
to acquire a repeat CT scan at each application for
treatment planning. The purpose of this study was to
evaluate the difference in dose conformity and clinical
target volume (CTV) coverage between the non-adaptive
and the adaptive approach.
Material and Methods
Eleven patients included in a dose-escalation study were
included in this study. Patients received a radical
treatment consisting of 13x3 Gy external beam
radiotherapy (EBRT) followed by three weekly applications
HDR-BT of 5-8 Gy. A planning CT with applicator in situ
was acquired at application one and repeat CT scans with
applicator in situ were acquired at application two and
three. The CTV was defined as residual macroscopic tumor
or scarring after EBRT. The CTV, rectal wall without CTV,
mesorectum and anus were delineated by an expert