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lungs, intestine and stomach; smaller than 5Gy for scalp
and kidneys (Figure 2). Moreover, protons provide the
smallest non-PTV integral doses (V1Gy: 53% 3D-CRT, 69%
photons MT, 15% PBS; V5Gy: 23% 3D-CRT, 43% photons MT,
12% PBS). A considerable variation in PTV and OAR
dosimetry was observed within a certain technique.
Conclusion
Modern radiotherapy techniques demonstrate superior
conformity and homogeneity, and reduced mean dose the
OARs compared to 3D-CRT.
PBS produced the case with the lowest mean dose for each
OAR and integral doses. However, the variability among
centres using the same technique means it is not possible
to clearly identify the best technique from this
data. Efforts should be made to improve inter-centre
consistency for each technique.
OC-0346 Multicentre audit of SBRT oligometastases
plan quality
J. Lee
1
, R. Patel
1
, C. Dean
2
, G. Webster
3
, D.J. Eaton
1
1
Mount Vernon Cancer Centre, National Radiotherapy
Trials QA RTTQA Group, Northwood, United Kingdom
2
Barts Health NHS Trust, Radiotherapy Physics, London,
United Kingdom
3
Worcestershire Oncology Centre, Radiotherapy Physics,
Worcester, United Kingdom
Purpose or Objective
SBRT for oligometastases is currently being used to treat
patients at 17 centres in England, as part of the NHS
England “Commissioning through Evaluation” programme.
The national trials QA group conducted QA for the
programme, which included establishing appropriate
clinical plan quality metrics for auditing submitted SBRT
plans. The purpose of the audit was to inform future
guidance on plan quality metric tolerances and help
centres determine whether a given plan is optimal.
Material and Methods
Plans included were either benchmark plans using pre-
delineated CT images planned by all cen tres prior to
patient recruitment; or plans of initial patients reviewed
prior to treatment. VODCA software (Medical Software
Solutions) was used for independent plan review. Lung
plans were analysed separately due to the inherent
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