Present status of the treatment planning for d(50)-Be neutron beams and mixed schedules at 'Cyclone'

A brief description of the neutrontherapy facility at CYCLONE, Louvain-la Neuve, is first presented. Neutron beams, produced by 50 MeV deuterons, are routinely used for clinical applications since March 1978. Irregular shaped fields are frequently used; they are obtained by inserting iron shielding blocks just below the collimator. This raises problems of skin sparing which are discussed. Treatment planning is performed with the SIDOS-U system routinely used for photon treatments and which has been rather easily adapted to neutron treatments. The isodose curves for single neutron beams could be generated, a priori, by the SIDOS-U system, using the decrement line technique or a matrix representation. The isodoses computed using the decrement line technique deviate from the measured curves especially near the surface region. This deviation can be related to the variation, with depth, of the neutron beam profiles. With the matrix representation, a good agreement between computed and measured isodose is achieved when 3 profiles, or more, are introduced. For reporting the target absorbed dose, ICRU recommendations (Report 29) are followed; the total absorbed doses Dsub[n+γ] are stated first and then, between brackets, the gamma component Dsub(γ). As the cyclotron is available only three times a week, different schedules have been adopted depending on the types of tumour: 3 neutron fractions/week or mixed schedules combining d(50)-Be neutrons and 18 MV X-rays. For mixed schedules, dose distributions are computed in terms of 'photon equivalent dose', applying a weighting factor of 3 to the neutron absorbed doses ('clinical RBE'). However, as discussed in the text in order to avoid exceeding the tolerance limits, in any case (neutron beams alone or mixed schedules), the photon equivalent doses at the critical organs are assessed using the true (experimentally determined) RBE values and not the 'mean clinical RBE'. (orig.)