Radiation sensitive films and gels

Radiographic film is the dosimeter of choice for a wide range of therapeutic and diagnostic applications in radiation-therapy departments. It is particularly useful for qualitative measurements of ionising radiation beams (e.g. high and low energy x-rays and beams of electrons and protons) and the verification of clinical radiation treatments both prior to and during therapy. For many years, film has been used to verify light-field and radiation-field coincidence and beam homogeneity of a linear accelerator. Films are also often used to verify the correct set-up of a megavoltage therapeutic beam with respect to the patient by means of a portal image of the beam as it exits the patient (see Section 40.1). These applications all benefit from the primary strengths of film that include high contrast (image quality), high spatial resolution and high sensitivity. The limitations of radiographic film include a variation of response as a function of photon energy, a high sensitivity to processing conditions*, and the fact that film is only useable in the form of thin two-dimensional (2D) sheets that do not easily lend themselves to the examination of complex dose-distributions in 3D. The advent of multi-detector arrays (Jursinic and Nelms 2002; Letourneau et al. 2004) which can be convenient, are near-real time, do not require processing, and can directly interface to computer read-out, has led to a decline in the use of film in many clinics. Recently, this trend has somewhat reversed as many clinics implement Intensity Modulated Radiation Therapy where it is common practice to compare 2D computed dose distributions with film measurements in the same conditions (Dogan et al. 2002; Bucciolini et al. 2004). Ideally, IMRT would be verified through a comprehensive measurement of the distribution in 3D, and this has led to the development of radiation sensitive gels and plastics that respond and can be imaged in 3D, giving more complete information for complex distributions. Geldosimetry is a relatively new development to the field of dosimetry and is discussed further in the second part of this chapter (see Section 17.3).

Duke Scholars

Author Mark Oldham Radiation Oncology