Abstract
BackgroundThe purpose of the present study is to compare finite size pencil beam (fsPB) and Monte Carlo (MC) based optimization of lung intensity-modulated stereotactic radiotherapy (lung IMSRT).Materials and methodsA fsPB and a MC algorithm as implemented in a biological IMRT planning system were validated by film measurements in a static lung phantom. Then, they were applied for static lung IMSRT planning based on three different geometrical patient models (one phase static CT, density overwrite one phase static CT, average CT) of the same patient. Both 6 and 15 MV beam energies were used. The resulting treatment plans were compared by how well they fulfilled the prescribed optimization constraints both for the dose distributions calculated on the static patient models and for the accumulated dose, recalculated with MC on each of 8 CTs of a 4DCT set.ResultsIn the phantom measurements, the MC dose engine showed discrepancies < 2%, while the fsPB dose engine showed discrepancies of up to 8% in the presence of lateral electron disequilibrium in the target. In the patient plan optimization, this translates into violations of organ at risk constraints and unpredictable target doses for the fsPB optimized plans. For the 4D MC recalculated dose distribution, MC optimized plans always underestimate the target doses, but the organ at risk doses were comparable. The results depend on the static patient model, and the smallest discrepancy was found for the MC optimized plan on the density overwrite one phase static CT model.ConclusionsIt is feasible to employ the MC dose engine for optimization of lung IMSRT and the plans are superior to fsPB. Use of static patient models introduces a bias in the MC dose distribution compared to the 4D MC recalculated dose, but this bias is predictable and therefore MC based optimization on static patient models is considered safe.
Highlights
The purpose of the present study is to compare finite size pencil beam and Monte Carlo (MC) based optimization of lung intensity-modulated stereotactic radiotherapy
Use of static patient models introduces a bias in the MC dose distribution compared to the 4D MC recalculated dose, but this bias is predictable and MC based optimization on static patient models is considered safe
It has been well documented that conventional (Pencil beam (PB), superposition/convolution) algorithms fail to some degree when calculating dose to lung [1,2,3,4,5,6,7,8,9,10,11,12], most treatment planning systems (TPSs) use these algorithms both for optimization of lung IMSRT and for final dose calculation, and MC only as a benchmarking tool
Summary
The purpose of the present study is to compare finite size pencil beam (fsPB) and Monte Carlo (MC) based optimization of lung intensity-modulated stereotactic radiotherapy (lung IMSRT). It has been well documented that conventional (Pencil beam (PB), superposition/convolution) algorithms fail to some degree when calculating dose to lung [1,2,3,4,5,6,7,8,9,10,11,12], most treatment planning systems (TPSs) use these algorithms both for optimization of lung IMSRT and for final dose calculation, and MC only as a benchmarking tool. Fraass et al [13] have stated that the use of MC dose calculation algorithms for clinical planning improves the dose accuracy in heterogeneous regions of lung and bony anatomy, in particular when applying very small field sizes which exhibit lateral electron disequilibrium effects. A TPS comprises a MC dose engine, since an inaccurate dose algorithm will introduce dose errors, but will lead to wrongly optimized treatment plans
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