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.

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet: linearity, signal-to-noise ratio, and energy response characteristics.

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

Reduction of corneal astigmatism at cataract surgery.

We studied the effect at three months of four transverse astigmatic keratotomy incisions (TAK) performed prior to phacoemulsification in 61 eyes on corneal astigmatism. The optical zone was varied in each case depending upon the magnitude of preoperative astigmatism. The eyes were compared to 105 control eyes in which no astigmatic incisions were performed to assess the estimated effect of the TAK incisions. Keratometry readings were taken preoperatively and three months postoperatively. Surgically induced astigmatism was measured using vector corrected astigmatism. Improvement in astigmatism was reported in all three optical zone groups, but the astigmatism was undercorrected in each. No complications affecting vision were reported, indicating that TAK may be a safe way to reduce postoperative astigmatism when combined with phacoemulsification.