Biomedical subjects

M Miften

Publications and source records attributed to M Miften.

4 recordsLinked to original sources

PubMed2001

Comparison of RTP dose distributions in heterogeneous phantoms with the BEAM Monte Carlo simulation system.

Therapeutic treatment plan evaluation is often based on examining the radiotherapy treatment planning (RTP) system dose distributions in the target and surrounding normal structures. To study the effects of tissue inhomogeneities on photon dose distributions, we compared FOCUS RTP system dose distributions from the measurement-based Clarkson and model-based MultiGrid Superposition (MGS) algorithms with those from the BEAM Monte Carlo code system in a set of heterogeneous phantoms. The phantom inhomogeneities mimic relevant clinical treatment sites, which include lung slab, lung-bone slab, bone-lung slab, mediastinum, and tumor geometries. The benchmark comparisons were performed in lung densities of 0.20 and 0.31 g/cm3, and a bone density of 2.40 g/cm3 for 5x5 cm2 and 10x10 cm2, 6- and 15-MV photon beams. Benchmark comparison results show that the MGS model and BEAM doses match better than 3% or 3 mm, and the MGS model is more accurate than the Clarkson model in all phantoms. The MGS model, unlike the Clarkson model, predicts the build-down and build-up of dose near tissue interfaces and penumbra broadening in lung associated with high energy beams. The Clarkson model overestimates the dose in lung by a maximum of 10% compared to BEAM. Dose comparisons suggest turning-off the effective path length inhomogeneity correction in the Clarkson model for lung treatments.

Algorithms↗

PubMed2000

Implementation of enhanced dynamic wedge in the focus rtp system.

The FOCUS RTP system implementation of Varian's enhanced dynamic wedge (EDW) is presented. Calculations of both dose distributions and wedge factors (WFs) are based on segmented treatment tables (STTs). Calculating dose requires a "transmission matrix" derived from an STT to model the modified fluence from the source. The dose calculation is then performed using either the Clarkson or convolution/superposition algorithms. An initial "primary dose/monitor unit (MU) fraction" WF estimate at the weight point of symmetric and asymmetric fields is calculated from the STT as the ratio of MU delivered on the axis of the weight point divided by total MU delivered for the treatment field. In our approach, we go beyond this initial estimate with a "scatter dose" correction. This requires measured 60 degrees WFs for 5 fields. Scatter corrections derived from measured WFs are interpolated for other wedge angles and field sizes in much the same way as arbitrary wedge angle STTs are derived from a "golden STT" using the "ratio of tangents" formalism. Dose comparisons with measured distributions show good agreement to within 3% or 3 mm for 6-MV beams and all EDW angles. Agreement with measurements to within 1% is obtained for WFs in all symmetric and asymmetric fields for 6- and 10-MV beams. For large wedge angles and field sizes, this represents a significant improvement over the 3% to 4% errors often observed using the MU fraction model alone.

Humans↗

PubMed2000

Implementation of FFT convolution and multigrid superposition models in the FOCUS RTP system.

In radiotherapy treatment planning, convolution/superposition algorithms currently represent the best practical approach for accurate photon dose calculation in heterogeneous tissues. In this work, the implementation, accuracy and performance of the FFT convolution (FFTC) and multigrid superposition (MGS) algorithms are presented. The FFTC and MGS models use the same 'TERMA' calculation and are commissioned using the same parameters. Both models use the same spectra, incorporate the same off-axis softening and base incident lateral fluence on the same measurements. In addition, corrections are explicitly applied to the polyenergetic and parallel kernel approximations, and electron contamination is modelled. Spectra generated by Monte Carlo (MC) modelling of treatment heads are used. Calculations using the MC spectra were in excellent agreement with measurements for many linear accelerator types. To speed up the calculations, a number of calculation techniques were implemented, including separate primary and scatter dose calculation, the FFT technique which assumes kernel invariance for the convolution calculation and a multigrid (MG) acceleration technique for the superposition calculation. Timing results show that the FFTC model is faster than MGS by a factor of 4 and 8 for small and large field sizes, respectively. Comparisons with measured data and BEAM MC results for a wide range of clinical beam setups show that (a) FFTC and MGS doses match measurements to better than 2% or 2 mm in homogeneous media; (b) MGS is more accurate than FFTC in lung phantoms where MGS doses are within 3% or 3 mm of BEAM results and (c) FFTC overestimates the dose in lung by a maximum of 9% compared to BEAM.

Algorithms↗

PubMed2000

Implementation and verification of virtual wedge in a three-dimensional radiotherapy planning system.

Virtual Wedge (VW) is a Siemens treatment modality which generates wedge-shaped dose distributions by moving a collimator jaw from closed to open at a constant speed while varying the dose rate in every 2 mm jaw position. In this work, the implementation and verification of VW in a radiotherapy treatment planning (RTP) system is presented. The VW implementation models the dose delivered by VW using the Siemens monitor units (MU) analytic formalism which determines the number of MU required to generate a wedge-fluence profile at points across the VW beam. For any set of treatment parameters, the VW algorithm generates an "intensity map" that is used to model the modification of fluence emanating from the collimator. The intensity map is calculated as the ratio of MU delivered on an axis point, divided by the monitor units delivered on the central-axis MU(0). The dose calculation is then performed using either the Clarkson or Convolution/ Superposition algorithms. The VW implementation also models the operational constraints for the delivery of VW due to dose rate and jaw speed limits. Dose verifications with measured profiles were performed using both the Clarkson and Convolution/Superposition algorithms for three photon beams; Siemens Primus 6 and 23 MV, and Mevatron MD 15 MV. Agreement within 2% or 2 mm was found between calculated and measured doses, over a large set of test cases, for 15, 30, 45, and 60 degree symmetric and asymmetric VW fields, using the manufacturer's supplied mu and c values for each beam.

Algorithms↗