The dosimetric parameters of small diameter photon beams of a commercially available radiosurgery system were measured in a water phantom using a p-type Si photon diode. Tissue maximum ratios, total scattering factor and beam profiles at 5 and 10 cm depth were measured for 17 circular beams ranging from 5 mm to 50 mm in diameter, in 1.5 mm steps. The total scattering factor decreased slowly from 0.947 to 0.888 for the cones with diameter between 50 mm and 12.5 mm (7% variation) whereas for cones with diameter between 10 mm and 5 mm the factor decreased more steeply, from 0.854 to 0.666 (28% variation). These tissue maximum ratio data are consistent with the data published by other authors. The beam profiles measured in the x and y directions were aproximately 0.2 mm between the two directions for all cone sizes. The full widths at half maximum were within 1 mm of the nominal cone sizes.

Keywords: Diode detector, Radiosurgery, Photon beam dosimetry

Human mistakes are an important source of errors in radiotherapy and may occur at every step of the radiotherapeutic planning and treatment. To reduce this level of uncertainties, several specialized organizations have recommended a comprehensive quality assurance program. In Brazil, the requirement for these programs has been strongly stressed, and most radiotherapy services have pursued this goal regarding radiation units and dosimetry equipment, as well as the verification of the calculations of the patient's dose and the revision of the plan charts. As a contribution to the improvement of quality control, we present some recommendations to avoid failure of treatment due to error in the delivered dose, such as redundant check of the manual or computer calculations, weekly check of the total dose for each patient, and prevention of inadvertent access to any safety system of the equipment by any staff member that is only supposed to operate the machine. Moreover, the use of a computerized treatment record and verification system should be considered in order to eliminate errors due to incorrect selection of the treatment parameters, in a daily basis. We report four radioactive incidents with patient injuries occurred throughout the world and some gross errors of dose.

Keywords: Radiotherapy. Treatment. Dose errors. Quality assurance.