title: Portal films for relative exit dose determinations.
abstract: Portal films are radiographic images taken regularly in external photon beam radiotherapy in order to verify geometrical set-up. However, they also contain quantitative dose information. Absolute dosimetry with film is possible but the desired accuracy of +/- 5% is difficult to achieve. However, relative dosimetry with film would be useful in combination with a point determination of absolute dose, though a dose response linearisation is required for relative dosimetry over the range of doses given to portal films in routine clinical practice. For this purpose, a technique for the rapid dose response linearisation of portal films is investigated in simulated treatment situations and evaluated in terms of the ability to yield relative exit dose distributions from the portal films. A combination of such distributions with a single determination of radiation transmission with diodes should generate full two-dimensional transmission information. Distributions obtained from two anthropomorphic phantoms provided results consistent with distributions obtained from a treatment planning system to generally within +/- 5%, and most often within +/- 2%. It is concluded that the extraction of exit dose information from portal films would be useful for mantle, compensated, or other unusual treatment situations.
authors.affiliation: Department of Physics, University of Newcastle, NSW.
authors.forename: M
authors.initials: M
authors.lastname: Ebert
authors.forename: T
authors.initials: T
authors.lastname: Kron
mesh_terms: D002138:Calibration; D004307:Dose-Response Relationship, Radiation; D005373:Film Dosimetry; D006801:Humans; D008961:Models, Structural; D011879:Radiotherapy Dosage