IMRT dose reconstruction on conebeam computed tomography CBCT: A platform for headandneck adaptive t

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IMRT dose reconstruction on cone-beam computed
tomography (CBCT) A platform for head-and-neck
adaptive therapy
Louis Lee, Quynh-Thu Le, Trang La, Lei
Xing Department of Radiation Oncology, Stanford
University School of Medicine, Stanford, CA 94305
MLC log-file retrieval and processing The MLC
log-files were retrieved from the MLC Workstation
after the treatment session the leaf positions
and delivered fractional MUs were extracted and
converted to leaf sequence files by an in-house
software. The leaf sequence files were then
imported back into the treatment planning system
(TPS) to re-generate the delivered fluence maps
for dose reconstruction (Fig. 1).
Procedures for dose reconstruction The HN CBCT
was imported into the TPS contours were
initially mapped onto the CBCT from the pCT
through image co-registration. These contours
were edited accordingly to cater for the changes
in the patients anatomy. The original plan was
copied to the CBCT and placed at the same
isocenter of the pCT. The derived leaf sequence
files were attached to the corresponding fields
(Fig. 3). The dose reconstruction was
calculated on the CBCT-derived patient model.
The resultant dose distributions, dosimetric
endpoints, and DVHs of the target and critical
organs were compared to those of the original
plan and the other two CBCTs.
The comparisons of the dose distributions and the
DVHs between the pCT- and CBCT-based plans for
two of the patients are shown in fig. 5 and fig.
6 respectively.
Fig. 1. The retrieval and processing of the MLC
log-file.
Phantom study Planning CT (pCT) and CBCT were
acquired for the Catphan-600 phantom. A 5-field
IMRT plan was applied to two image sets of the
phantom to validate the use of CBCT for dose
calculation. The resultant dose distributions,
orthogonal dose profiles, and DVHs of a
hypothetical target from the pCT- and CBCT-based
calculations were compared. Patient CBCT
scheduling and acquisition Five HN IMRT patients
were chosen. Three CBCTs of 2 weeks apart
starting from the first fraction in a 30-fraction
treatment course (Fig. 2) were taken for the
patients.
Phantom study For the IMRT dose calculation, the
dose distributions on the same axial slice from
the CBCT- and pCT-based dose calculations were
essentially identical, no difference in dose
maximum was noted (Fig. 4a and b). The
horizontal and vertical dose profiles for the
CBCT- and pCT-based calculations agreed to each
other to within 1 (Fig. 4c). The DVHs of the
hypothetical target from the two calculations
agreed to each other to within 2 (Fig. 4d).
Patient study For most treatment sessions, the
CBCT-based dose reconstructions yielded DVHs of
the targets very close (within 3) to that of the
original treatment plans. However, dosimetric
changes (within 10) were observed for the
critical organs due to the anatomic variations
caused by setup inaccuracy, organ deformation,
tumour shrinkage or weight loss or both.

• To develop a platform for reconstructing the dose
  delivered to a HN patient using on-treatment CBCT
  and the delivered fluence maps derived from the
  MLC log-files recorded during the treatment.
• To study the potential dosimetric impact on the
  intended treatment plan taking into account the
  patients geometric changes over time, residual
  setup errors, and the inherent delivery errors
  associated with the MLC.

In this work, we have established a methodology
and procedures to reconstruct the IMRT dose
delivered based on a serial of on-treatment CBCTs
and the MLC log-files taking into account the
geometric changes of the patients anatomy over
time and the potential inaccuracy in the IMRT
delivery in a pragmatic way. The dose
reconstruction is valuable to examine the actual
dose delivered to the patient at a particular
fraction. This maneuver affords an objective
dosimetric basis for the clinical decision on
whether a re-planning or re-optimization is
necessary during the course of treatment, and
provides a valuable platform for heading toward
adaptive radiation therapy in future.
CBCT image acquisition The CBCT images in this
work were acquired by the on-board imager (OBI)
integrated on a Trilogy medical linear
accelerator. The half-fan mode of CBCT was
used to give a field-of-view (FOV) of 45 cm.
this work has been accepted for publication in
IJROBP Journal