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ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
Superiority of a newly developed secondary Monte Carlo Dose calculation system over measurements.
Matthias Kowatsch, Austria
PO-1390

Abstract

Superiority of a newly developed secondary Monte Carlo Dose calculation system over measurements.
Authors: Markus Alber.(Scientific-RT, Scientific-RT, Munich, Germany), Markus Alber.(Universitätsklinikum Heidelberg, Klinik für RadioOnkologie und Strahlentherapie, Heidelberg, Germany), Matthias Kowatsch.(LKH Feldkirch, Institut of Medical Physics, Feldkirch, Austria), Thomas Künzler.(LKH Feldkirch, Institut of Medical Physics, Feldkirch, Austria), Matthias Söhn.(Scientific-RT, Scientific-RT, Munich, Germany), Philipp Szeverinski.(LKH Feldkirch, Institut of Medical Physics, Feldkirch, Austria), Philipp Szeverinski.(Private Universität im Fürstentum Liechtenstein, Private Universität im Fürstentum Liechtenstein, Triesen, Liechtenstein)
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Purpose or Objective

The accuracy of secondary dose calculation algorithms is more and more crucial because of the increase of highly modulated treatment plans. Kry et. al [1] shows that independent recalculation outperforms traditional measurements to detect unacceptable plans in IMRT QA. This study performs  a dosimetric review of the secondary dose calculation algorithm SciMoCa  (Scientific-RT, München),  Monaco TPS (V5.11.02, Elekta, Crawly) and an ionization chamber PTW-Semiflex 0.125 (PTW, Freiburg). Both dose calculation systems have their own base data set and are fundamentally different in MC head modelling.

Material and Methods

In total 118 treatment plans (H&N, Breast, Brain, Thorax, SRT) were recalculated in Monaco and SciMOCa within the IBA Body Phantom (IBA-Dosimetry, Schwarzenbruck). All measurements were performed in isocenter with an Elekta Synergy linac and Agility MLC (Elekta, Crawly). A PTW Semiflex 0.125cc ionization chamber was set as reference, typically applied in IMRT/VMAT verification. The point dose values of the two calculation systems were derived from the mean dose of a structure with a volume of 0.125cc.

Results

SciMoCa and Monaco TPS showed a mean deviation of -0.1% ± 1.6% and +0.1% ± 1.6% respectively compared to the ionization chamber measurement (p<0.01). The largest deviation compared to measurements showed a very complex and high modulated stereotactic brain case with 5.7% for SciMoCa and 4.7% for the Monaco respectively. In 21 cases the difference between measurement and calculation for both systems (mean 4.1% ± 3%, respectively 3.4% ± 1.5%) exceeded ±2%. Comparing the calculated dose for this 21 cases the difference between SciMoCa and the TPS was 0.4% ± 0.9%.

Conclusion

The secondary dose calculation system and the treatment planning system show good agreement to the measurement results. High deviations in single cases can be connected to the size of the target volume (stereotactic fields) or to the point of measurement in steep dose gradients of VMAT plans. Further challenges in dosimetry such as secondary electron equilibrium as well as the effective point of measurement in thimble chambers need to be addressed. In Conclusion, relying on just one measurement point and the lack of a reasonable interpretation of deviations lead to arguments from Kry et. al [1] and show the advantages of a totally independent secondary dose calculation system. 

[1] Idependent recalculation outperforms traditional measurement-based IMRT QA methodes in detecting unacceptable plans. Stephen F. Kry, Mallory C. Glenn, Christine B. Peterson, Daniela Branco, Hunter Mehrens, Angela Steinmann, David S. Followill, Med Phys. 2019 Aug;46(8):3700-3708. doi: 10.1002/mp.13638. Epub 2019 Jun 23