One obstacle in applying the very practical techniques of automatic calculation of multifield dose distributions has been the lack of a simple yet exact and workable mathematical description of the per cent depth dose distribution within a single beam. Past work by ourselves (Sterling, Perry and Bahr, 1961; Sterling, Perry and Weinkham, 1963b) and others (Tsien, 1955;1958) has been based on procedures in which the isodose curves, obtained by actual measurements, were digitised by hand for different field sizes and stored on cards or tape. Simple convergent dose distributions were then obtained by systematic summing procedures. For non-convergent and other more complex multifield plans per cent depth dose values were obtained by interpolative techniques (Sterling et al., 1963a, b). While the interpolation procedures developed by us gave relatively accurate results even for complex treatment plans, they did not lend themselves to rotations or more sophisticated optimisations. Finally, despite their simplicity, all hand digitising methods limit calculations to those portal sizes for which digitised values exist.It was our aim to find a simple yet reasonably accurate mathematical expression for the depth dose distribution isodose curves of a beam coming through a portal of any length and width. The work was guided by two suppositions which turned out to be justified:(1) there should exist a simple relationship between the per cent depth doses at points away from the central axis and those on the central axis at the same depth; and (2) that the decrement of dose along the major axis may be expressed as a function of field configuration and depth below the surface. The mathematical expressions developed in the following pages apply to 60 Co ionising radiation sources at 80 cm SSD. Similar work is under way now for other sources of energy. Per cent dose on the major axis as a function of portal size and depth below surfacePfalzner (1960) has recently pointed out that tumour-air ratios for 60 Co radiation plotted on full logarithmic paper against field area, with depth as parameter, are straight lines. The fact that the relation is linear in logarithms is actually beside the point. It is important that axial depth dose may be expressed as some function of depth and field size. A similar relation should hold between per cent depth dose and field size. However, while Pfalzner considered the relation between field area and dose satisfactory, experimental evidence indicates that fields of equal areas but different shapes give rise to different per cent depth doses on the central axis. But if field area is corrected for perimeter of the field, the relation between per cent depth dose and (field area/field perimeter) is almost perfect. It is possible, therefore, to express per cent depth dose on the central axis for any depth as a function of (area/perimeter) of the field such that log C t =hi + m t log (AjP) (1) whereCi is the per cent depth dose on the central axis of the field at distance T>% from the skin. AjP is the r...

Background and study aims Dietary restrictions are integral to colonoscopy preparation and impact patient satisfaction. Utilizing split-dose, lower-volume polyethylene glycol 3350-electrolyte solution (PEG-ELS), this study compared colon preparation adequacy of a low-residue diet to clear liquids using a validated grading scale. Patients and methods This was a prospective, randomized, singleblinded, single-center non-inferiority study evaluating diet the day prior to outpatient colonoscopy. Subjects were randomized to a Low-Residue diet for breakfast and lunch, or Clears only. All subjects received split dose PEG-ELS. The primary endpoint was preparation adequacy using the Boston Bowel Preparation Scale (BBPS), with adequate defined as a score > 5. Secondary endpoints included mean BBPS scores for the entire colon and individual segments, satisfaction, adverse events, polyp and adenoma detection rates, and impact on sleep and daily activities.

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