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

The central problems of photon beam film dosimetry are the dependence of film response upon photon energy, processing conditions, and film plane orientation. We have overcome these problems by accurately fitting the depthâ€Â�dependent sensitometric curve of Kodak XVâ€Â�2 film (exposed parallel to beam axis) to the equation OD(D,d) =OD_s(1 − exp {−α₀[1+β(d−d_m)]D}) where OD(D,d) is the optical density for dose D at depth d. OD_s, α₀, and β are constants characteristic of the film and beam energy but are independent of field size. Only central axis depth dose data for a single field are required to determine their values. A computer program based upon this equation has been written which successfully generates single field isodose curves from film data for a variety of field sizes (including wedged fields) with an accuracy of ±3%. Data are presented for ⁶⁰Co, and 4 and 10 MV x rays.