Dosimetric evaluation of a Monte Carlo IMRT treatment planning system incorporating the MIMiC.

The high dose per fraction delivered to lung lesions in stereotactic body radiation therapy (SBRT) demands high dose calculation and delivery accuracy. The inhomogeneous density in the thoracic region along with the small fields used typically in intensity-modulated radiation therapy (IMRT) treatments poses a challenge in the accuracy of dose calculation. In this study we dosimetrically evaluated a pre-release version of a Monte Carlo planning system (PEREGRINE 1.6b, NOMOS Corp., Cranberry Township, PA), which incorporates the modeling of serial tomotherapy IMRT treatments with the binary multileaf intensity modulating collimator (MIMiC). The aim of this study is to show the validation process of PEREGRINE 1.6b since it was used as a benchmark to investigate the accuracy of doses calculated by a finite size pencil beam (FSPB) algorithm for lung lesions treated on the SBRT dose regime via serial tomotherapy in our previous study. Doses calculated by PEREGRINE were compared against measurements in homogeneous and inhomogeneous materials carried out on a Varian 600C with a 6 MV photon beam. Phantom studies simulating various sized lesions were also carried out to explain some of the large dose discrepancies seen in the dose calculations with small lesions. Doses calculated by PEREGRINE agreed to within 2% in water and up to 3% for measurements in an inhomogeneous phantom containing lung, bone and unit density tissue.

Comparison of measured and Monte Carlo calculated dose distributions from the NRC linac.

We have benchmarked photon beam simulations with the EGS4 user code BEAM [Rogers et al., Med. Phys. 22, 503-524 (1995)] by comparing calculated and measured relative ionization distributions in water from the 10 and 20 MV photon beams of the NRC linac. Unlike previous calculations, the incident electron energy is known independently to 1%, the entire extra-focal radiation is simulated, and electron contamination is accounted for. The full Monte Carlo simulation of the linac includes the electron exit window, target, flattening filter, monitor chambers, collimators, as well as the PMMA walls of the water phantom. Dose distributions are calculated using a modified version of the EGS4 user code DOSXYZ which additionally allows scoring of average energy and energy fluence in the phantom. Dose is converted to ionization by accounting for the (L/rho)water(air) variation in the phantom, calculated in an identical geometry for the realistic beams using a new EGS4 user code, SPRXYZ. The variation of (L/rho)water(air) with depth is a 1.25% correction at 10 MV and a 2% correction at 20 MV. At both energies, the calculated and the measured values of ionization on the central axis in the buildup region agree within 1% of maximum ionization relative to the ionization at 10 cm depth. The agreement is well within statistics elsewhere. The electron contamination contributes 0.35(+/- 0.02) to 1.37(+/- 0.03)% of the maximum dose in the buildup region at 10 MV and 0.26(+/- 0.03) to 3.14(+/- 0.07)% of the maximum dose at 20 MV. The penumbrae at 3 depths in each beam (in g/cm2), 1.99 (dmax, 10 MV only), 3.29 (dmax, 20 MV only), 9.79 and 19.79, agree with ionization chamber measurements to better than 1 mm. Possible causes for the discrepancy between calculations and measurements are analyzed and discussed in detail.

A Monte Carlo study of verification imaging in high dose rate brachytherapy.

We have been evaluating the practicality of monitoring the position of an 192Ir source during high dose rate (HDR) brachytherapy treatments using x-ray fluoroscopy. The EGS4 Monte Carlo code has been used to simulate the interactions of 192Ir photons with the patient and the CsI phosphor of an x-ray image intensifier to predict what signals will be generated by these 192Ir photons. The calculations show that it is the 192Ir photons scattered within the patient that are mainly responsible for generating the spurious signals in the x-ray image intensifier that degrade image quality. The scattered 192Ir photons are distributed in the energy range (15-200 keV), which is markedly lower than the average energy of the primaries (360 keV), and therefore interact more efficiently with the CsI phosphor of the x-ray image intensifier. Experimental measurements support these observations, demonstrating that spurious signals produced by the 192Ir source become appreciably larger when the 192Ir source is located within a scattering object rather than air. For a 10 cm airgap, the signal-to-noise ratio (SNR) can decrease by factors ranging between 3 and 10 (no antiscatter grid), depending on the position of a 7 Ci 192Ir source inside a 30 cm thick water phantom. In typical clinical situations, a focused grid (Pb, 12:1, 40 lines/cm) can increase the SNR by about a factor of 2. Furthermore, the SNR rapidly increases with increasing airgap, such that a 20 cm airgap can be as effective as a 12:1 air interspaced grid in eliminating the spurious signals. Our results suggest that use of a high-current x-ray fluoroscopy technique, a large airgap, and a well-designed anti-scatter grid can make the fluoroscopic monitoring of source position in HDR brachytherapy feasible. This, in turn, can improve the quality assurance of such treatments.