196 / 1096
S183
ESTRO 36
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differences in scatter conditions around the tumour.
Initial analysis showed a high proportion of plans where
PTV coverage was compromised. Plan quality metrics were
therefore developed which were independent of PTV
coverage. These metrics are defined in eqn1 and eqn2:
where V
100%
and V
50%
are the volumes covered by 100% and
50% of the prescription dose (the dose intended to cover
the target) respectively. The mean, median and standard
deviation are reported for both metrics, split into PTV
V
100%
volume ranges of 0-20cc, 20-40cc and >40cc.
Results
38 lung and 77 non-lung (lymph node, liver, adrenal and
bone) plans were reviewed, produced for treatment using
Cyberknife (29), Tomotherapy (7), VMAT (71), fixed gantry
angle IMRT (5) or 3D conformal (3) modalities. 11% of lung
patients and 29% of non-lung patients had significantly
compromised PTV coverage (PTV V
100%
< 90%). The spillage
results for lung and non-lung sites were similar. Modified
Gradient Index (MGI) values were higher for lung than non-
lung sites and decreased with increased treated volume
(see table 1). No clinically significant differences were
seen between treatment platform or modality.
Table 1. The mean, median and standard deviation of the
“Spillage” and “Modified Gradient Index” plan quality
metrics for lung and non-lung oligometastatic SBRT plans.
Conclusion
The high proportion of non-lung patient plans with
compromised target coverage suggests that future
guidance documents should use plan quality metrics which
are independent of coverage, such as those proposed
here. The similar spillage results for lung and non-lung
sites suggest that for this metric, site specific tolerances
are not required. The MGI is higher for lung plans, as
expected with the increased scatter in low density
surroundings. MGI lung and non-lung results are similar in
absolute terms and so equivalent planning tolerances
could be applied to both groups. These data provide
evidence of what plan quality is achievable across multiple
treatment platforms, modalities and clinical sites. These
are particularly useful for non-lung oligometastatic SBRT
plans where there is currently a lack of data in the
literature.
OC-0347 Key factors for SBRT planning of spinal
metastasis: Indications from a large scale multicentre
study
M. Esposito
1
, L. Masi
2
, M. Zani
3
, R. Doro
2
, D. Fedele
3
, S.
Clemente
4
, C. Fiandra
5
, F.R. Giglioli
6
, C. Marino
7
, S.
Russo
1
, M. Stasi
8
, L. Strigari
9
, E. Villaggi
10
, P. Mancosu
11
1
Azienda Sanitaria USL centro, S.C. Fisica Sanitaria,
Firenze, Italy
2
Centro CyberKnife IFCA, Medical Physics, Firenze, Italy
3
Casa di cura San Rossore, Radioterapia, Pisa, Italy
4
Azienda Ospedaliera Universitaria Federico II, Medical
Physics, Napoli, Italy
5
Università degli Studi di Torino, Medical Physìcs,
Torino, Italy
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