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couch. The gamma passing rates of both cases were
compared to determine the overall impact of including the
couch in the calculations.
Results
The rED densities that best reproduced the measured
values were 0.5ρ
e,w
and 0.1 ρ
e,w
for both CF and Foam,
respectively (see Figure 1). The agreement between
measured and calculated attenuation is good, mainly in
the posterior angles, with differences under 1%. At 110
and 250 gantry degrees we measured differences up to
4.2% and 2.4% for both 6 and 15 MV, respectively. These
angles match the lateral edges of the couch, where there
are many different components, which make this
simplified model less accurate.
For the 36 VMAT plans we compared the 3D global gamma
passing rates with (W) and without (W/O) the couch, and
the average results were (detailed in Figure 2):
γ(3%,3mm): W/O: 98.9%, W: 99.3%; γ (2%2mm): W/O:
92.5%, W: 93.8%. The median improvement is 0.3% for
γ(3,3) and 1.1% for γ(2,2).
Conclusion
Relying on the results, it is advisable to implement the
treatment couch in the TPS, especially using techniques
such as VMAT. In this work we present a successful model
of the iBEAM® evo Couchtop in Monaco for both 6 and 15
MV. We plan to implement it for all our both 3D and
IMRT/VMAT plans.
EP-1454 Comparison of Treatment Planning
Algorithms and Monte Carlo Simulations in Oesophageal
Radiotherapy
D. Johns
1
, E. Spezi
2
, P. Downes
1
, D. Lewis
1
1
Velindre Cancer Centre, Medical Physics, Cardiff,
United Kingdom
2
Cardiff University, School Of Engineering, Cardiff,
United Kingdom
Purpose or Objective
Many workers have published comparisons of dose
distributions generated by conventional radiotherapy
planning systems and those produced by various Monte
Carlo (MC) dose calculation packages [1]. However, due
to the large computational load involved in producing each
MC plan, the number of cases cited in each study is
relatively small and hence the statistical power of these
studies is low. In this work, distributed computing
resources have been used to simulate over fifty
oesophageal radiotherapy treatment plans, to provide a
more statistically powerful comparison of analytical dose
calculations and MC dose calculations.
Material and Methods
Radical oesophageal radiotherapy plans are routinely
produced in our centre according to a protocol originally
developed for the national SCOPE trial [2]. Plans were
produced using the Pencil Beam Enhanced (PBE)
algorithm, and re-calculated using the Collapsed Cone
Enhanced (CCE) algorithm, of Oncentra MasterPlan (OMP)
v4.3. MC simulations were performed using the BEAMnrc
package.
Results
An initial sample of 12 oesophageal radiotherapy
treatment plans were simulated using the RTGrid
system. The differences between the dose calculation
methods, and the variance in the twelve cases, were used
to calculate the sample size needed to detect a 3%
difference in the proportion of the Planning Target Volume
receiving 95% of the prescription dose (PTV V95%) with 90%
power, following the method of Altman [3]. The required
sample size was determine to be 38, so a further 40
oesophageal cases were subsequently simulated. The
volumetric dose data for the different dose calculation
methods was used to calculate the Tumour Control
Probability (TCP) using the method of Geh [4].
Conclusion
A statistically significant decrease in the PTV V95% when
changing from CCE to MC dose calculations has been
demonstrated, in a sample of 40 cases. A statistically
significant decrease in the values of TCP calculated based
on the CCE and MC dose calculations was also found,
suggesting that the differences in physical dose would
have clinical significance.
References
1. Rogers DWO.
Fifty years of Monte Carlo
simulations for medical physics
. Physics in Medicine and
Biology. 2006,
51
(13), R287-301.
2. Wills L, Millin A, Paterson J, Crosby T, Staffurth J.
The effect of planning algorithms in oesophageal
radiotherapy in the context of the SCOPE 1 trial.
Radiotherapy and Oncology 2009,
93
(3), 462-7.
3. Altman DG
Practical Statistics for Medical Research
1999 (Boca Raton: Chapman & Hall/CRC) 455-461
4. Geh, J.I. et al,
Systematic overview of
preoperative (neoadjuvant) chemoradiotherapy trials in
oesophageal cancer: Evidence of a radiation and
chemotherapy dose response.
Radiotherapy and
Oncology, 2006,
78
(3), pp.236–244..
EP-1455 IGRT kV-CBCT dose calculations using Virtual
Source Models and validated in phantoms using OSL
G. Boissonnat
1
, H. Chesneau
1
, J. Garcia-Hernandez
1
, D.
Lazaro
1
1
CEA-LIST, DM2I/LM2S, GIF SUR YVETTE, France
Purpose or Objective
With the growing use of X-ray imaging equipment in
Image-Guided RadioTherapy (IGRT), the need to evaluate
the dose-to-organs delivered by kV-CBCT imaging
acquisition increases. This study aims to propose accurate
Monte Carlo (MC) calculations of the patient dose-to-
organs delivered by two commercially available kV-CBCT
systems: the XVI from Elekta’s VERSA HD accelerator and