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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.
Results
Thirteen patients had their RT plan adapted at least once
during treatment. In the first adaptation (13 pts), the
median decrease in the CTV receiving 95% of prescribed
dose (V95%) and the planning target volume (PTV) was
0.2% [0-6.4%] and 5.2% [0-11.7%], respectively, see Fig2.
The largest underdosage was related to interfractional
baseline shifts in the diaphragm position (6 pts), with a
median decrease in CTV V95% of 0.6% [0-6.4%] and PTV
V95% of 8.6% [1.7-11.7%]. Target deviations registered as
changes in SS (8 pts), were typically caused by swelling of
the target area (7 pts), shrinkage or swelling of the
mediastinum (4 pts) and/or compression or stretching of
the target due to changed diaphragm position (1 pt).
Changes in SS were pooled and showed a median decrease
in CTV V95% of 0.2% [0-0.6%] and PTV V95% of 5.0% [0.5-
6.9%].