IS THERE A NEED FOR A REVISED TABLE OF EQUIVALENT SQUARE FIELDS FOR THE DETERMINATION OF PHANTOM SCATTER CORRECTION FACTORS

The use of the British Journal of Radiology (BJR) (supplement 17) tabl es of equivalent square fields for dose calculations is widespread. A revised version of the supplement was published recently, with a more elaborate discussion, but without changes in data given in these table s (Br. J. Radiol. suppl 25). The tables were generated for use in dose calculations, with relative beam data such as PDD, BSF, PSF, all with d(max) as the reference depth. However, the current philosophy in dos e calculational methods is based on quantities defined at a reference depth, d(ref) = 10 cm, on a separation of phantom and head scatter, an d on the use of the relative depth-dose or tissue-phantom ratios norma lized at d(ref). By using these quantities asa starting point, problem s at shallow depths related to the influence of contaminating electron s in the beam can be eliminated. Recently, a comprehensive set of phan tom scatter factor data with d(ref) = 10 cm has been published for a s et of square held sizes and a wide range of photon beam energies, show ing that phantom scatter is a smoothly varying function of field size and quality index. It is not a priori evident that the conventional co ncept of equivalent squares for rectangular fields is also fully appli cable for phantom scatter factors and phantom scatter related quantiti es at a depth of 10 cm. It was questioned whether or not new tables of equivalent square fields are needed for this purpose. In this paper, new tables have been constructed for four photon beam energies in the range of Co-60 to 25 MV (quality index from 0.572 to 0.783). The small differences between the outcome of these new tables allowed the const ruction of one averaged table of equivalent square fields. Phantom sca tter factors were calculated for rectangular fields based on the use o f the BJR table and on the use of the newly constructed tables and the differences were quantified. For Co-60 no improvements could be shown when using the new averaged table, but for beam energies of 6 to 10 M V small improvements of the order of 0.5 to 1.0% were found. For a hig her beam energy of 25 MV the improvement is smaller. Deviations result ing from the BJR table are within the limits of accuracy as stated by the authors. Therefore, for clinical use, the continued use of the BJR table of equivalent squares for phantom scatter factors and phantom s catter related quantities of rectangular fields is justified, irrespec tive of photon beam energy.