DOSIMETRY OF HIGH-ENERGY PHOTON AND ELECTRON-BEAMS WITH CEA FILMS

With all the advantages of film dosimetry in the megavoltage energy ra nge, the use of film as a dosimeter is still limited due to the variou s difficulties associated with films such as energy dependence, film o rientation, and sensitometric nonlinearity. Recently, therapy verifica tion and localization films (CEA TVS and TLF films) from a Swedish man ufacturer have become available in vacuum-sealed water-proof packaging in the US. The packaging renders the CEA films useful in a water phan tom and ideal for photon and electron dosimetry. A systematic study ha s been carried out to investigate the potential of dosimetric applicat ion of the new films for high energy photon and electron beams. For th e TVS films, the characteristic curve is generally energy independent but appears to be dependent on the source of the radiation, i.e., whet her it is gamma rays or bremsstrahlung x rays. Compared to Kodak Ready pack XV films, the CEA TVS film is linear in optical density over a mu ch larger range of radiation dose. The inter- and intra-variation of t he TVS films is less than 2%. For electrons, the characteristic curve is linear over a similar density range as photons but exhibit a slight energy dependence. TVS film is slightly directional dependent on the incident radiation for both photons and electrons. The perpendicular o rientation results in higher optical density than the parallel orienta tion. The differences are within +/-2% except in the buildup region fo r photons and in the exponential fall-off region of the electron beams where differences up to 4% are noted. For the CEA TLF film which is a bout three times faster than the TVS film, the characteristic curve is reasonably linear over the dose range of 0-15 cGy and energy independ ent within the experimental uncertainty (+/-5%). Percent depth dose an d isodose measurements with the TVS films are in good agreement with i on chamber results. (C) 1996 American Association of Physicists in Med icine.