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S182
ESTRO 36 2017
_______________________________________________________________________________________________
6
Azienda Ospedaliera Città della Salute e della Scienza,
Medical Physics, Torino, Italy
7
Humanitas Centro Catanese di Oncologia, Medical
Physics, Catania, Italy
8
Ospedale Ordine Mauriziano di Torino- Umberto I,
Medical Physics, Torino, Italy
9
Istituto Regina Elena - Istituti Fisioterapici Ospedalieri,
Medical Physics, Roma, Italy
10
AUSL di Piacenza, Medical Physics, Piacenza, Italy
11
Istituto Clinico Humanitas, Medical Physics, Rozzano,
Italy
Purpose or Objective
SBRT planning for spinal metastases is particularly
challenging due to the high dose required for covering the
PTV complex shape, and to the steep dose gradient
mandatory for sparing the spinal cord. Many combinations
of delivery systems and TPSs are clinically available in
different institutions. Aim of this study was to investigate
the dosimetric variability in planning spine SBRT among a
large number of centers.
Material and Methods
Two spinal cases were planned by 38 centers (48 TPS) with
different technologies (table 1): a single dorsal
metastasis, and double cervical metastases. The required
dose prescription (DP) was 30 Gy in 3 fractions. Ideal PTV
coverage request was: V
DP
>90% (minimum request:
V
DP
>80%). Constraints on the organs at risk (OAR) were:
PRV spinal cord: V
18Gy
<0.35cm
3
, V
21.9Gy
<0.03 cm
3
;
oesophagus: V
17.7Gy
<5cm
3
, V
25.2Gy
<0.03 cm
3
.
As a last option, planners were allowed to downgrade DP
to 27 Gy to fulfil OAR constraints. 3D dose matrixes were
analyzed. DVH were generated and analyzed with MIM 6.5
(MIM Software Inc. Cleveland US). Homogeneity index (HI)
was computed for each PTV as HI= (D
2%
-D
98%
)/DP. Planners
did not meet the protocol constraints or PTV dose
coverage were asked to re-plan the wrong case.
Multivariate statistical analysis was performed to assess
correlations between dosimetric results and planning
parameters.
Table1: Linac , TPS, delivery technique and kind of inverse
optimization used in the intercomparison.
Results
14/96 plans did not meet the protocol requests. After the
re-planning, still 6/96 plans with different technologies
did not respect at least one constraint with differences
>0.5 Gy. For the dorsal case, 3 minimum (<0.5Gy)
deviations (1 VMAT, 1 IMRT, 1 Tomo), and 2 reduced DP (1
VMAT and 1 Tomo) occurred. For the cervical case, 3
minimum deviation (1VMAT 1IMRT 1Tomo), and 2 reduced
DP (1 VMAT and 1Tomo) were observed. Multivariate
analysis showed, for both cases, a significant correlation
(p<0.05) between Homogeneity Index (HI) and both OAR
dose sparing and PTV coverage. Irradiation techniques
correlated with spinal cord sparing; however institutions
using similar/same delivery/TPS techniques produced
quite different dose distributions, highlighting the
influence of the planner experience on the optimization
process (figure 1).
Fig1 Box plot relative to the single metastasis case. 18
plans were computed using VMAT, 8 VMAT FFF (linac Free
of Flattering Filter), 6 Ciberknife, 5 Tomotherapy, 7 IMRT,
1 3dCRT.
Conclusion
At our knowledge, this is the largest non-sponsored
multicentre planning comparison. Differences in DVH
binning among centres could explain minor violations. HI
is a key factor for planning optimization: prescribing to
lower isodose generally leads to better OAR sparing and
higher PTV coverage. Results have a dependence on the
irradiation technique, although the planner's experience
plays a not negligible role. A multicentre analysis as
proposed in this study can have an impact on the
standardization of plan quality for spinal SBRT.
OC-0348 Reducing the dosimetric impact of variable
gas volume in the abdomen during RT of esophageal
cancer
P. Jin
1
, J. Visser
1
, K.F. Crama
1
, N. Van Wieringen
1
, A.
Bel
1
, M.C.C.M. Hulshof
1
, T. Alderliesten
1
1
Academic Medical Center, Radiation Oncology,
Amsterdam, The Netherlands
Purpose or Objective
For middle/distal esophageal tumors, a varying gas volume
in the upper abdomen could induce changes in the
dosimetry of RT. In this study, we investigated the
dosimetric impact of abdominal gas pockets as well as a
density override (DO) strategy to mitigate dosimetric
effects.
Material and Methods
We retrospectively included 1 patient with middle and 8
patients with distal esophageal cancer. For these patients,
it was unclear whether re-planning was needed due to the
varying gas volume during treatment. For each patient, we
measured gas volumes in the planning CT (pCT) and 8–28
(median: 14) CBCTs to assess possible time trends.
Further, we made IMRT and VMAT plans with a prescription