The purpose of this study was to implement the Monte Carlo method for clinical radiotherapy dose calculations. We used the EGS4/BEAM code to obtain the phase-space data for 6-20 MeV electron beams and 4, 6, and 15 MV photon beams for Varian Clinac 1800, 2100C, and 2300CD accelerators. A multiple-source model was used to reconstruct the phase-space data for both electron and photon beams, which retained the accuracy of the Monte Carlo beam data. The multiple-source model reduced the phase-space data storage requirement by a factor of 1000 and the accelerator simulation time by a factor of 10 or more. Agreement within 2% was achieved between the Monte Carlo calculations and measurements of the dose distributions in homogeneous and heterogeneous phantoms for various field sizes, source-surface distances, and beam modulations. The Monte Carlo calculated electron output factors were within 2% of the measured values for various treatment fields while the heterogeneity correction factors for various lung and bone phantoms were within 1% for photon beams and within 2% for electron beams. The EGS4/DOSXYZ Monte Carlo code was used for phantom and patient dose calculations. The results were compared to the dose distributions produced by a conventional treatment planning system and an intensity-modulated radiotherapy inverse-planning system. Significant differences (>5% in dose and >5 mm shift in isodose lines) were found between Monte Carlo calculations and the analytical calculations implemented in the commercial systems. Treatment sites showing the largest dose differences were for head and neck, lung, and breast cases.

No abstract

A hybrid approach for commissioning electron beam Monte Carlo treatment planning systems has been studied. The approach is based on the assumption that accelerators of the same type have very similar electron beam characteristics and the major difference comes from the on-site tuning of the electron incident energy at the exit window. For one type of accelerator, a reference machine can be selected and simulated with the Monte Carlo method. A multiple source model can be built on the full Monte Carlo simulation of the reference beam. When commissioning electron beams from other accelerators of the same type, the energy spectra in the source model are tuned to match the measured dose distributions. A Varian Clinac 2100C accelerator was chosen as the reference machine and a four-source beam model was established based on the Monte Carlo simulations. This simplified beam model can be used to generate Monte Carlo dose distributions accurately (within 2%/2 mm compared to those calculated with full phase space data) for electron beams from the reference machine with various nominal energies, applicator sizes, and SSDs. Three electron beams were commissioned by adjusting the energy spectra in the source model. The dose distributions calculated with the adjusted source model were compared with the dose distributions calculated using the phase space data for these beams. The agreement is within 1% in most of cases and 2% in all situations. This preliminary study has shown the capability of the commissioning approach for handling large variation in the electron incident energy. The possibility of making the approach more versatile is also discussed.

We have investigated the tongue-and-groove effect on the IMRT dose distributions for a Varian MLC. We have compared the dose distributions calculated using the intensity maps with and without the tongue-and-groove effect. Our results showed that, for one intensity-modulated treatment field, the maximum tongue-and-groove effect could be up to 10% of the maximum dose in the dose distributions. For an IMRT treatment with multiple gantry angles (> or = 5), the difference between the dose distributions with and without the tongue-and-groove effect was hardly visible, less than 1.6% for the two typical clinical cases studied. After considering the patient setup errors, the dose distributions were smoothed with reduced and insignificant differences between plans with and without the tongue-and-groove effect. Therefore, for a multiple-field IMRT plan (> or = 5), the tongue-and-groove effect on the IMRT dose distributions will be generally clinically insignificant due to the smearing effect of individual fields. The tongue-and-groove effect on an IMRT plan with small number of fields (< 5) will vary depending on the number of fields in a plan (coplanar or non-coplanar), the MLC leaf sequences and the patient setup uncertainty, and may be significant (> 5% of maximum dose) in some cases, especially when the patient setup uncertainty is small (< or = 2 mm).

Modulated electron radiation therapy (MERT) has been proposed as a means of delivering conformal dose to shallow tumors while sparing distal structures and surrounding tissues. Conventional systems for electron beam collimation are labor and time intensive in their construction and are therefore inadequate for use in the sequential delivery of multiple complex fields required by MERT. This study investigates two proposed methods of electron beam collimation: the use of existing photon multileaf collimators (MLC) in a helium atmosphere to reduce in-air electron scatter, and a MLC specifically designed for electron beam collimation. Monte Carlo simulations of a Varian Clinac 2100C were performed using the EGS4/BEAM system and dose calculations performed with the MCDOSE code. Dose penumbras from fields collimated by photon MLCs both with air and with helium at 6, 12, and 20 MeV at a range of SSDs from 70 to 90 cm were examined. Significant improvements were observed for the helium based system. Simulations were also performed on an electron specific MLC located at the level of the last scraper of a 25x25 cm2 applicator. A number of leaf materials, thicknesses, end shapes, and widths were simulated to determine optimal construction parameters. The results demonstrated that tungsten leaves 15 mm thick and 5 mm wide with unfocused ends would provide sufficient collimation for MERT fields. A prototype electron MLC was constructed and comparisons between film measurements and simulation demonstrate the validity of the Monte Carlo model. Further simulations of dose penumbras demonstrate that such an electron MLC would provide improvements over the helium filled photon MLC at all energies, and improvements in the 90-10 penumbra of 12% to 45% at 20 MeV and 6 MeV, respectively. These improvements were also seen in isodose curves when a complex field shape was simulated. It is thus concluded that an MLC specific for electron beam collimation is required for MERT.