Film dosimetry of megavoltage photon beams: A practical method of isodensity‐to‐isodose curve conversion

Abstract

The central problems of photonbeam film dosimetry are the dependence of film response upon photon energy, processing conditions and film plane orientation. The depth-dependent sensitometric curve of Kodak XV-2 film (exposed parallel to beam axis) was accurately fitted to the equation OD(D,d) = ODs(1.sbd.exp {.sbd..alpha.o[1 + .beta.(d.sbd.dm)]D}, where OD(D,d) was the optical density for dose D at depth d, [dm was depth of maximum electron buildup and .beta. was slope of the line of best fit]. ODs, .alpha.o, and .beta. were constants characteristic of the film and beam energy, but were independent of field size. Only central axis depth dose for a single field was required to determine their values. A computer program based upon this equation was written, which successfully generated single field isodose curves from film for a variety of field sizes (including wedged fields) with an accuracy of .+-. 3%. 60Co and 4 and 10 MV X-rays are demonstrated.