ESTRO 36 Abstract Book

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S254 ESTRO 36 2017
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binning in 10 phases, Siemens Somatom Definition AS, 2
mm slices).
Results
Based on r0 , 10 patients showed an intra-fractional
baseline drift of more than 5 mm, in one or more
directions, in 50% or more fractions. For all patients, intra-
fractional differences in amplitude were not statistically
significant (p > 0,05). An example of intra-fractional
variability is shown in Fig 1.
In terms of long-term variability, for 11 patients the
difference in mean amplitude between at least 2 fractions
or a fraction and 4DCT is statistically significant (p < 0,05).
Fig 2 shows the mean amplitude per fraction per patient,
together with the amplitude based on 4DCT.
Both intra- and inter-fractional changes in hysteresis were
smaller than 5 mm for all patients. No correlation could
be found between long or short-term variability and
tumour size or location.
Conclusion
Based on this select group of patients, it can be concluded
that breathing-induced tumour motion can vary
significantly over the entire course of treatment, not
rarely exceeding common safety margins of 5 mm.
A previous study indicates that a single 4DCT is sufficient
to evaluate breathing motion, however, long- term
variability was never investigated. The results in this study
indicate that a single 4DCT, a snapshot in time, can not
accurately predict the motion amplitude during all
fractions, which is especially cumbersome when used for
ITV treatment planning.
OC-0485 How many plans are needed for an optimal
plan library in ART for locally advanced cervical
cancer?
E. Novakova1, S.T. Heijkoop1, S. Quint1, A.G. Zolnay1,
J.W.M. Mens 1, J. Godart1 , B.J.M. Heijmen1, M.S.
Hoogeman1
1 Erasmus MC Cancer Institute, Physics, Rotterdam, The
Netherlands
Purpose or Objective
In our institute, locally advanced cervical cancer patients
are treated with online, Plan-of-the-Day ART. For patients
with a tip-of-uterus displacement >2.5 cm, as measured in
pretreatment full and empty bladder CTs, the plan library
contains 2 plans to cope with daily anatomy variations.
Increasing the number of plans for larger motion could be
beneficial for tissue sparing, but is burdensome and the
dosimetric benefit is yet to be proven at an individual
patient level. We investigated an easy-to-use metric to
individualize the number of plans in the library, balancing
gain in organs-at-risk (OAR) sparing and clinical feasibility.
Material and Methods
Data of 14 previously treated patients were analyzed. For
each patient, plan libraries were created containing either
1 plan, based on the full-motion-range ITV as derived from
the pre-treatment full and empty bladder CT-scans, or 2,
3 or 4 plans based on sub-motion-range ITVs. To create
PTVs, the nodal CTV was expanded by 7-mm and the ITVs
by 10-mm. For all PTVs, VMAT plans were created to
deliver 46 Gy in 23 fractions. Daily CBCT scans were used
for plan selection by calculating the bladder volume.
For the dosimetric evaluations, the cervix-uterus and the
OAR (bladder, rectum, and bowel) were contoured on
daily CBCT scans. For each plan library, the OAR V40Gy were
recorded for all CBCT scans. As the gain in sparing varied
over the OAR during the fractionated treatment, a
composite volume was calculated by summing up the DVH
parameters. Pearson correlation was estimated between
DVH parameters and a maximum pretreatment extent of
uterus motion defined as the Hausdorff distance (HD)
(99%-ile) (MaxUtHD). A Wilcoxon signed-rank test was used
to assess statistical differences among strategies.
Results
Strong correlations were found between MaxUtHD and the
total volume of spared normal tissue (composite gain of
DVH parameters for bowel, bladder and rectum, see
Fig.1). 3 patients were identified as outliers having more
than 30% of the fractions an emptier or a fuller bladder
than on the planning CT. They should be re-planned during
treatment and were excluded from further analysis. For 2
plan (R= 0.8), 3 plan (R= 0.9) and 4 plan libra ries (R= 0.6),
p˂ 0.01. For patients with MaxUtHD >35 mm, adding a 3 rd
plan would significantly reduce composite V4 0Gy by 18 ± 6
cc on average (from that bowel 10 ± 6 cc, blad der 6 ± 2
cc and rectum 2 ± 1 cc). For patient >50 mm MaxUtHD,
additional 12 cc would be spared by adding a 4th plan to
the library.
Conclusion
Our results indicate that an extension of the plan library
would have the most impact on sparing of bowel cavity.
Patients with large MaxUtHD (>35 mm) would benefit from
adding a 3rd plan to the library and patients with the
extremely large MaxUtHD (>50 mm) would benefit from
adding a 4th plan to the library. This study provides an
easy-to-implement criteria to select patients who would
benefit the most from additional plans in a plan library
approach.
OC-0486 Multi-criterial patient positioning based on
dose recalculation on scatter-corrected CBCT images
J. Hofmaier1,2, J. Haehnle3, C. Kurz1,2, G. Landry2, C.
Maihöfer1, P. Süss3, K. Teichert 3, N. Traulsen4, C.
Brachmann5, F. Weiler5, C. Thieke1, K.H. Küfer 3, C.