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ESTRO 36
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OC-0349 Prediction of GTV median dose differences
benefit Monte Carlo re-prescription in lung SBRT
D. Dechambre
1
, Z.L. Janvary
1
, N. Jansen
1
, C. Mievis
1
, P.
Berkovic
1
, S. Cucchiaro
1
, V. Baart
1
, C. Ernst
1
, P. Coucke
1
,
A. Gulyban
1
1
C.H.U. - Sart Tilman, Radiotherapy department, Liège,
Belgium
Purpose or Objective
The use of Monte Carlo (MC) dose calculation algorithm for
lung patients treated with stereotactic body radiotherapy
(SBRT) can be challenging. Prescription in low density
media and time-consuming optimization conducted
CyberKnife centers to propose an equivalent path length
(EPL)-to-Monte Carlo re-prescription method, for example
on GTV median dose (Lacornerie T, et al. Radiat Oncol
2014;9:223). The aim of this study was to evaluate the
differences between the two calculation algorithms and
their impact on organs at risk (OAR) and to create a
predictive model for the re-prescription.
Material and Methods
One hundred and twenty seven patients (with 149 lesions)
were treated with CyberKnife (CK; Accuray, Sunnyvale,
US) between 2010 and 2012. A high-resolution grid (512³)
was used for the EPL and MC calculations (2% variance).
All re-calculation from EPL to MC maintained the number
of beams and their monitor units. Relative differences in
GTV D50 between the two algorithms were assessed and
uni/multivariate linear regression was performed using
prescription dose (Gy), tracking (ITV concept if not
available), location (peripheral or central) and volume (in
cc) of the lesion as input parameters. Statistical
significance was determined using F-test at p-value<0.05.
OARs volumetric dose constraints were applied from
Timmerman RD et al. (Semin Radiat Oncol 2008;18:215-
22). As tolerance limits were defined based on simple
heterogeneity correction algorithm (e.g. EPL), correlation
between EPL and MC OARs dose values was assessed
following the work from the Rotterdam team (van der
Voort van Zyp NC , et al. Radiother Oncol 2010;96:55–60).
Results
The observed difference (MC compared to EPL) varied
from 0 % to 48% (median = 10%, standard deviation = 9%).
The uni- and multi-variate analysis showed statistical
significance for all parameters except lesion location
(table 1).
The high coefficient associated to the peripheral
character is probably due to the EPL un-modelled lateral
electron equilibrium caused by the prevalent presence of
low density lung tissues surrounding the peripheral lesion,
thus greatly impacting dose calculation differences. Based
on the multivariate analysis predictive nomogram was
generated (R²=0.58, Figure 1).
Dose to OARs calculated with EPL and MC showed strong
linear correlation (R²=0.99-1.00). The dose constraints
decreased by 1% in the heart (D10cc), great vessels
(D10cc) and spinal cord (D0.25cc), 2% in the oesophagus
(D5cc), 5% in the ribs (D5cc) and 16% in the trachea (D4cc).
.
Conclusion
The differences between MC and EPL are significantly
impacted by dose, tracking, location and the volume of
the lesion. Predictive nomogram helps to estimate the
differences on GTV D50. EPL to MC OAR dose tolerance
limit proved to have a strong linear correlation with
conversion factors ranging from 0.84 to 0.99. Based on our
model, re-prescription value can be estimated and, if
required, used to further restrict the constraints on the
OARs during EPL optimization.
OC-0350 ExacTrac®-based Fractionated Radiosurgery
(fSRS) of Choroidal Melanoma (CM)
M. Wösle
1
, P. Goldschmidt
1
, G. Lohm
1
, L. Grajewski
2
, L.
Krause
2
, I. Ciernik
1
1
Dessau City Hospital, Radiation Oncology, Dessau,
Germany
2
Dessau City Hospital, Ophthalmology, Dessau, Germany
Purpose or Objective
Proton therapy (PT) has been a standard for
treating choroidal melanomas for the last three
decades. However, PT is not easily available for the
majority of patients. Advances in photon therapy allow
highly conformal dose delivery while sparing normal
tissue. However, fSRS for small moving target volumes,
such as CM, has remained a challenge.
Material and Methods
Since December 2014, we treated 40 patients with central
choroidal tumors with fSRS. Radiotherapy plans were
obtained with iPlan® RT (Version 4.5.3, Brainlab,
Feldkirchen, D) after placement of 4 Tantalum clips
(Altomed Ltd., U.K.) a week prior the planning CT/MRT.
Thirteen cases have been reanalysed: ten treated with 10
Gy x 5 for CM, one case of haemangioma treated with 14,5
Gy in one fraction, and 2 cases of breast cancer metastasis
treated with 6 Gy x 5 and 4. iPlan® RT uses HybridArc™ as
field configuration with three to six dynamic conformal
arcs complemented with five to seven dynamic IMRT