The financial support by the Federal Ministry of Science, Research and Economy and the National Foundation for Research, Technology and Development is gratefully acknowledged

http://www.meduniwien.ac.at/hp/radonc

1Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Austria

2Department of Radiotherapy, Comprehensive Cancer Center, Medical University of Vienna / AKH Vienna, Austria

Commissioning of the new Monte Carlo algorithm SciMoCa for a VersaHD LINAC

Wolfgang Lechner1,2*, Hermann Fuchs1,2, Dietmar Georg1,2

Purpose To validate the dose calculation accuracy of the Monte Carlo

algorithm SciMoCa (ScientificRT GmbH, Munich, Germany) for a

VersaHD (Elekta AB, Stockholm, Sweden) linear accelerator.

SciMoCa is a recently developed Server/Client based Monte

Carlo algorithm, which provides fast and accurate dose

calculation for various applications, e.g. independent dose

assessment of 3D-CRT, IMRT and VMAT treatment plans or

general research purposes.

Materials and Methods A beam model of a 6 MV flattened beam provided by a

VersaHD was used to calculate the dose distribution of square

fields in a virtual 40 x 40 x 40 cm³ water block. The

investigated field sizes ranged from 1 x 1 cm² to 40 x 40 cm².

For the acquisition of percentage depth dose profiles (PDDs)

and for output factor measurements, a PTW Semiflex 31010

was used for field sizes down to 3 x 3 cm² and a PTW DiodeE

as well as a PTW microDiamond were used for field sizes

ranging from 1 x 1 cm² to 10 x 10 cm². The measured output

factors were corrected for small field effects where necessary.

The lateral profiles of all fields were acquired using a PTW

DiodeP at depths of dmax, 5 cm, 10 cm, 20 cm and 30 cm,

respectively. A calculation grid size of 2 mm and a Monte Carlo

variance of 0.5% were used for the calculations. PDDs and

lateral profiles were extracted from the calculated dose cube.

These calculated dose profiles were re-sampled to a grid size

of 1 mm and compared to previously measured depth dose and

lateral profiles using gamma index analysis with a 1 mm/1%

acceptance criteria. The mean values of g indices (gmean

) as well

as the relative difference of measured output factors (OFmeas

)

and calculated output factors (OFcalc

) were used for the

evaluation of the calculation accuracy.

Results Table 1 summarizes the results of the gamma analysis of each

investigated field as mean and standard deviation for each

field. The mean values of gmean

and the standard deviation of

the mean increased with increasing field size. Figure 1 depicts

the distribution of gmean

values with respect to profile type, field

size and measurement depth. The majority of gmean

values were

well below 1. The highest gmean

values were found for the 40 x

40 cm² field and for larger measurement depths. The high gmean

of the 40 x 40 cm² field were attributed to the size of the

digital water phantom. The gmean

values of the all PDDs were

below 0.5 for all field sizes. The calculated and measured

output factors agreed within 1% for field sizes larger and 1 x 1

cm². For the 1 x 1 cm² the difference between measured and

calculated output factors was 1.5%.

Conclusion The investigated beam model showed excellent agreement

with measured data over a wide range of field sizes and

measurement depths with improved agreement for small field

sizes. These commissioning results are a solid basis for

ongoing investigations focusing on more complex treatment

types such as IMRT and VMAT and heterogeneous phantoms.

Figure 1 gmean

values for all investigated profile types, field sizes and

measurement depths.

Field size mean(gmean

) std(gmean

) OFcalc

OFmeas

difference

cm²

1 x 1 0.28 0.09 0.675 0.665 1.5%

2 x 2 0.31 0.09 0.807 0.801 0.8%

3 x 3 0.30 0.11 0.839 0.844 -0.6%

4 x 4 0.29 0.07 0.880 0.879 0.1%

5 x 5 0.32 0.10 0.908 0.905 0.3%

8 x 8 0.39 0.08 0.967 0.970 -0.3%

10 x 10 0.40 0.15 1.000 1.000 0.0%

15 x 15 0.47 0.18 1.059 1.058 0.1%

20 x 20 0.49 0.28 1.095 1.097 -0.2%

30 x 30 0.39 0.12 1.140 1.144 -0.4%

40 x 40 0.77 0.68 1.172 1.164 0.8%

Table 1 Summary of gmean

values and output factors for each investigated

field size. For each field size, the mean values and standard deviations of

gmean

were calculated.

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