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S902
ESTRO 36
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EP-1660 Improvement in tumour control probability by
adapting dose to daily OAR DVCs
D. Foley
1
, B. McClean
1
, P. McBride
1
1
St Luke's Reseach Oncology Network, Physics, Dublin,
Ireland
Purpose or Objective
A technique using analysis of on-board CBCT images to
adapt the dose to the target on a fraction-by-fraction basis
was developed. This new approach involves using the
upper limit of dose volume constraints (DVCs) as the
objective to be met at each fraction by tracking and
accumulating dose voxels. The aim was to adapt the dose
per fraction such that it was optimised each day without
any organ at risk (OAR) DVCs being exceeded. The impact
on tumour control probability (TCP) and normal tissue
complication probability (NTCP) was evaluated.
Material and Methods
31 patients who underwent prostate treatment were
retrospectively investigated for this study. Initial VMAT
plans consisting of 2 arcs were designed to deliver 74 Gy
in 37 fractions of 2 Gy each to the target. The patients had
on-board CBCT scans taken prior to treatment for between
9 and 33 fractions (436 in total).
An in-house registration algorithm based on phase
correlation[1] was used to retrospectively register CBCT
images to the planning CT to determine the
transformations and deformations in patients’ anatomy.
This allowed the original plan to be recalculated on the
registered CT image that provided the position of the
target and OARs for that fraction. By tracking individual
voxels throughout treatment, the dose was accumulated
and the DVHs and DVC values were determined for each
fraction.
The DVCs were then used as limits such that the dose that
could be delivered would result in the tightest constraint
being just met. Therefore, the dose for that fraction was
increased or decreased to ensure that the DVC was on the
tolerance limit. The impact of the dose escalation was
then evaluated using TCP and NTCP.
Results
Thirteen of the patients investigated could have received
net higher doses during their treatment without exceeding
their OAR DVCs. In the remaining 18 patients, only 20
fractions out of 257 would allow an increase in dose while
staying below the DVC limits. The rectum was the limiting
structure in 97 % of fractions.
The largest individual increase possible for a given fraction
was 87.4 cGy. If all changes were made, the maximum
accumulated net increase in dose possible for any patient
was 13.58 Gy, assuming the imaged fractions were
representative of the patients’ entire treatment and
scaling to a full treatment. This corresponded to an
increase in TCP and rectal NTCP of 13.7 % and 13.6 %
respectively. Table 1 shows the results for the 13 patients.
Conclusion
Adapting the dose to be delivered to the patient on a
fraction-by-fraction basis has the potential to allow for
significant dose escalation while staying within
institutional DVCs, significantly increasing TCP. This could
be particularly useful in the hypofractionation approach
to
treatments.
[1] Physica Medica, 32(4):618–624, 2016.
EP-1661 Adaptive strategy to accommodate
anatomical changes during RT in oesophageal cancer
patients
T. Nyeng
1
, M. Nordsmark
2
, L. Hoffmann
1
1
Aarhus University Hospital, Medical Physics, Aarhus C,
Denmark
2
Aarhus University Hospital, Department of Oncology,
Aarhus C, Denmark
Purpose or Objective
During chemoradiotherapy (chemoRT) in oesophageal
cancer (EC), some patients show large interfractional
anatomical changes. These changes may affect the dose
distribution adversely, demanding adaptation of the
treatment plan. The aim of this study was to investigate a
decision support system for treatment adaptation based
on daily cone-beam CT (CBCT) scans.
Material and Methods
Twenty consecutive patients treated with chemoRT for
oesophageal and gastro-oesophageal junction cancer were
setup to the spinal cord with a tolerance of 5mm using
daily CBCT scans. On CBCT, mediastinal structures are
barely visible. Therefore, a surrogate structure (SS) was
used to evaluate the actual target position. The SS was
generated by indicating the borders between dense tissue
nearby the clinical target volume (CTV) and lung tissue or
air, see Fig1. Geometrical changes above 3mm in the
tissue defined by the SS were registered by the radiation
therapists (RTTs) for each fraction. Additionally, the RTTs
noted changes of the base line diaphragm position above
5mm, the mediastinum above 5mm, the body contour
above 10mm, and the shoulder blades above 10mm. Three
consecutive registrations in any category triggered an
adaptation of the treatment plan, requiring a new CT-scan
with IV contrast. Targets and organs at risk were re-
delineated, based on deformably propagated contours
from the planning CT-scan. We recalculated the original
treatment plan on the new CT-scan to evaluate the
consequences of the observed anatomical changes.