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re-plans.
Conclusion
Visual detection of anatomical changes on CBCT during
treatment of head and neck cancer, without pre-defined
adaptive radiotherapy protocol, results in re-planning in 1
out of 11 patients.
OC-0356 Adaptive strategy for rectal cancer:
evaluation of plan selection of the first 20 clinical
patients
R. De Jong
1
, N. Van Wieringen
1
, J. Visser
1
, J. Wiersma
1
,
K. Crama
1
, D. Geijsen
1
, L. Lutkenhaus
1
, A. Bel
1
1
Academic Medical Center, Department of radiation
oncology, Amsterdam, The Netherlands
Purpose or Objective
For rectal cancer, sparing the organs at risk with the use
of state-of-the-art planning techniques (IMRT/VMAT) is
compromised by the large margins that are necessary to
compensate for daily shape changes. In our clinic we
implemented a plan selection strategy with multiple plans
made prior to treatment. For each fraction, the best
fitting plan is selected based on daily cone beam CT
(CBCT) scans. The aim of this study is to assess the plan
selection strategy for the first 20 clinical patients with
respect to available plans, selected plans and safety.
Material and Methods
Multiple plans for plan selection were created for each
patient based on a single CT scan. For 20 patients, 3 PTVs
were created with different anterior margins for the upper
mesorectum. Margins could be either 25 mm, 15 mm, 0
mm, or -15 mm, with choice of margins based on the
anatomy as captured on the CT scan (fig. 1). Patients were
treated with either a long or short treatment schedule
(25x2 Gy, and 5x5 Gy, respectively). All plans were
delivered with VMAT. Plan selection was based on daily
CBCT. Selection was performed by 1 trained radiotherapist
(RTT), a physician and a physicist for all fractions of the
first week, and from the second week onwards by 2 RTTs,
one of whom trained in plan selection. Once a week a
post-treatment CBCT scan was acquired to assess the
validation of the selected plan at the end of treatment.
An expert IGRT RTT performed a weekly review,
inspecting all plan selections retrospectively, as well as
consistency between selected plans in the imaging system
and the radiotherapy management system, as the two
systems are not linked.
Results
In total, 10 patients were treated with the long treatment
schedule, and 10 with the short treatment schedule,
resulting in 300 plan selections. Margin sets of 25 mm, 15
mm, 0 mm were created for 6 patients, and margin sets of
15 mm, 0 mm, -15 mm for 13 patients. One patient had a
set of only two margins available (0 mm, 15 mm), due to
insufficient time at treatment planning. Overall, the -15
mm, 0 mm, 15 mm and 25 mm plans were selected in 2%,
45%, 39% and 14% of fractions, respectively. For
distributions per patient, see figure 2. The largest
available margin was always sufficient. Treatment was
delayed a total of 7 times (of which 5 times in 1 patient)
to obtain a more favorable anatomy in case of a very full
rectum, usually caused by gas pockets. Evaluation of the
post-treatment CBCT scans showed for 1 fraction the
selected plan was no longer suitable due to a moving gas
pocket. The weekly review showed that a plan with a
smaller margin could have been selected in 20% of
fractions, and a larger margin in 2% of fractions. No
inconsistencies were found in selected plans between the
imaging system and radiotherapy management system.
Conclusion
A plan selection strategy for rectum cancer patients was
successfully and safely implemented. Next we will
quantify the dosimetric impact of plan selection to the
dose of the organs at risk in this dataset.
Proffered Papers: Physics Dosimetry