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Radiation therapy in the thoracic region is difficult due to the presence of many dose-limiting structures and the large density differences that affect the dose distribution. Conventional irradiation techniques use low-energy photon beams to avoid build-up effects superficially in the tumour and increased lateral scattering of the beams. For deep-seated tumours higher beam energies could have lung-sparing properties that would enable dose escalation. A comparison was made for a conventional low photon energy (6 MV) and 50 MV photons for the treatment of a lung tumour. A representative patient geometry was selected, consisting of a small tumour semi-enclosed in lung tissue. Treatment plans were designed using a commercial 3D-pencil beam treatment planning system. The treatment beams designed in the TPS were simulated with the Monte Carlo code EGS4/BEAM and the dose distribution in the phantom created from the patients CT-data was calculated using MCDOSE with identical beam geometry for both energies. The intrinsic difference between the two photon energies implies a sparing effect of lung that can be utilized for dose escalation. For a treatment with two beams the mean total dose to the tumour could be increased by 5.3% for 50 MV, corresponding to 3.2 Gy for a prescription dose of 60 Gy, with the same complication probability for the treated lung as for 6 MV. In conclusion, high-energy beams have qualities that can be taken advantage of for irradiation of lung tumours. Optimum solutions would probably require the use of both high- and low-energy beams.

This paper describes how a conventional treatment head can be modified for use of multileaf collimated electron beams. Automatic and dynamic beam delivery are possible for both electrons and photons by using the computer controlled multileaf collimator (MLC) for both photon and electron beams. Thereby, the electron beams can be mixed more freely into the treatment to take advantage of the specific depth modulation characteristics of electrons. The investigation was based on Monte Carlo calculations using the software package BEAM. The physical parameters used in this optimization were the beam penumbra and the virtual/effective point source position. These parameters are essential for shaping beams, beam matching and for dosimetry calculations. The optimization was carried out by modifying a number of parameters: replacing the air atmosphere in the treatment head with helium, adding a helium bag below the MLC, changing the position of the scattering foils, modifying the monitor chamber, and adjusting the position of the MLC. The beam characteristics for some of these designs were found to fulfil our criteria for clinically useful beams down to at least 9 MeV. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00610-0].