A measurement and analysis of buildup region dose for open field photon beams (cobalt-60 through 24 MV).

The central axis depth dose in the build-up region (surface to dmax) of single open field photon beams (cobalt-60 through 24 MV) has been measured utilizing parallel plate and extrapolation chamber methodology. These data were used to derive, for a prescription dose of 100 cGy, values of surface dose, the maximum value of dose along the central axis (Dmax) and the depth (nearest the surface) at which 90% of the prescription dose occurs (d90). For both single and parallel opposed pair (POP) open field configurations, data are presented at field sizes of 5 x 5, 15 x 15 and 25 x 25 cm2 for prescription depths of 10, 15 and 20 cm (midplane for POP). For the treatment machines, field sizes, and prescription depths studied, it is possible to conclude that: for single field irradiation, surface dose values (as a percentage of the prescription dose) can be either low (< 10%) or comparable to the prescription dose itself; for POP open fields, surface dose values are relatively independent of photon energy and midplane depth, and range between 30% and 70% of prescription dose, being principally dependent on field size; the depth of the initial 90 cGy point for a prescription dose of 100 cGy, d90, was larger for POP fields. For either single or POP open field treatments, d90 was always less than 22 mm, while for 6 MV or less, values of d90 were less than 4 mm; Dmax values can be very large (e.g., above 300 cGy) for certain treatment situations and are reduced significantly for POP treatments; for open field POP treatments, the percent reduction in Dmax with each increment in beam energy above 10 MV is reduced over that seen at 10 MV or less and, possibly, this further reduction may be clinically insignificant; for open field POP treatments, changes in surface dose, d90 and Dmax with beam energy above 10 MV do not suggest, with regard to these specific build-up curve parameters, any obvious advantage for treatment with beam energies greater than 10 MV for the specific machines and situations studied.