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ESTRO 36
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Material and Methods
The planning CT and delineated structures of four rectum
cancer patients were selected. For each patient, a MRL
treatment plan was generated with Monaco using a 7-
beam IMRT technique (25 x 2.0 Gy) including all MRL-
specific properties (7MV, 1.5 T magnetic field, collimator
90°, FFF, SAD: 143.5 cm). Patient setup errors of 1.0 cm
and 2.0 cm in the CC and LR directions were simulated by
shifting the planning CT with respect to the isocenter
position. For each setup error, the initial plan was adapted
by first adjusting the leaves of each segment to
approximate the shift and second re-optimize the weight
of each segment. Also, a reference plan was generated by
adapting the initial plan with a 0.0 cm shift, as the second
phase of plan adaptation was observed to introduce dose
changes even for a 0.0 cm shift. All plans were rescaled
(PTV V
95%
= 99%). The reference and adapted plans were
irradiated on the MRL on a slab phantom with a 2D
detector array (PTW Octavius 1500
MR
) inserted parallel to
the couch at the center position of the PTV. For each plan,
the phantom position was changed according to the
introduced shift. Patient setup errors in the AP direction
cannot be evaluated using this measurement setup. The
measured 2D dose distribution of the reference plan was
rigidly registered to the measured 2D dose distribution of
the adapted plans in order to assess the positional
accuracy of the simple dose shift. After alignment, the
similarity between the 2D dose distributions of the
reference plan and the adapted plans was evaluated using
a 3%/3mm γ analysis (local dose, 20% low dose threshold).
Results
For all adapted plans, the measured positional accuracy
was within 0.1 cm. The γ analysis between the dose
distributions of the reference plan and the adapted plans
resulted in an average pass rate (γ≤1) of
96.2% (range: 83.3% – 99.9%). Smaller values of γ
mean
were
observed for dose shifts in the CC direction compared to
the LR direction as well as for 1.0 cm dose shifts compared
to 2.0 cm dose shifts (Table 1). Figure 1 shows an example
of a 2D γ distribution. High γ values are measured in the
low dose area mainly. A simple dose shift to correct for a
1.0 cm setup error in the CC direction resulted in limited
dose differences. Various γ hotspots were observed for
the 2.0 cm setup error in the LR direction.
Conclusion
The use of plan adaptations to correct patient setup errors
was experimentally validated on the MRL for the first
time. The reference dose distribution was reproduced at
the shifted location for rectum cancer patients. In
addition, high gamma pass rates were measured.
Acknowledgements:
The autors like to thank Robert Spaninks (Elekta).
PO-0879 Differences between planned and delivered
maximum spinal cord dose in Head &Neck cancer
patients
D. Noble
1,2
, P. Yeap
3
, K. Harrison
3
, S. Thomas
1,4
, M.
Parker
3
, N. Burnet
1,2
1
VoxTox Research Group - University of Cambridge.,
Oncology, Cambridge, United Kingdom
2
Addenbrooke's Hospital - Oncology Centre University of
Cambridge, Oncology, Cambridge, United Kingdom
3
VoxTox Research Group - University of Cambridge.,
Cavendish Laboratory- Department of High Energy
Physics, Cambridge, United Kingdo
4
Addenbrooke's Hospital - Oncology Centre University of
Cambridge, Medical Physics, Cambridge, United Kingdom
Purpose or Objective
Adaptive radiotherapy (ART) for head and neck cancer
remains resource intensive, and there is little consensus
on which patients will benefit most from having it done.
Concerns regarding maximum spinal cord dose sometimes
trigger re-planning at our centre, and we sought to
compute and model differences between planned and
delivered maximum dose to the spinal cord in patients
undergoing IMRT with daily image-guidance (IG) on the
TomoTherapy system.
Material and Methods
We drew planning kVCT, IG MVCT and planned dose
datasets from archive for 33 patients who were treated
for head & neck cancer (HNC) on TomoTherapy units at
our centre. All patients underwent daily IG, with matching
to high dose PTV (close to the spinal cord), or cervical
spine vertebrae. To automatically contour the spinal cord,
we developed an intensity based deformable image
registration (DIR) algorithm using the open source Elastix
toolkit to propagate manual contours from the planning
CT. Using ‘gold standard’ contours of an expert observer,
the algorithm was optimised on 30 MVCT datasets (567
slices) and validated on a further 90 (2203). Conformity
was measured with Jaccard conformity index (JCI) and
distance between centres (DBC), and compared with
results from intra- and inter-observer studies.
Using in-house dose recalculation software (CheckTomo),
TomoTherapy sinograms, MVCT datasets and algorithm