Variation of relative transit dose profiles with patient-detector distance

Background and purpose: In view of using portal images for exit dosimetry, an experimental study is performed of relative transit dose profiles at dif ferent distances behind patients (and phantoms) and of their relation to th e exit dose profile. Materials and methods: Irregular, homogenous polystyrene phantoms with a va riable thickness to simulate head and neck (H&N) treatments (6-MV photon be am) are investigated by ionization chamber measurements performed close to the exit surface and at various distances behind the phantom (10, 20 and 30 cm). Similar measurements are performed for a rectangular phantom with lar ge inhomogeneities (Al and air). For one irregular homogenous phantom and a n irregular phantom containing an Al inhomogeneity, ionization chamber meas urements are performed at the exit surface, and a portal film image is take n at 30 cm behind the phantom. Portal films of a patient treated for a head and neck malignancy are evaluated for different air gaps behind the patien t. Results: For the irregular phantoms, deviations up to 15% and more are obse rved between the exit dose profile (along the shaped surface of the phantom ) and the transit profile close to the phantom (perpendicular to the beam a xis). There is, however, a good agreement - within 3% - between the exit pr ofile and the transit profile at 30 cm. For the rectangular, inhomogenous p hantom, the deviation between the exit profile and the transit dose profile at 30 cm does not exceed 5%; transit dose profiles overestimate the exit d ose for the air cavity and underestimate the dose for the Al inhomogeneity. Measurements on portal films of a H&N patient for different air gaps confi rm the order of magnitude of the difference observed between transit dose p rofiles close to the patient and transit dose profiles at some distance beh ind the patient. Conclusions: For 6-MV photon beam treatments with significant thickness var iations (H&N), large variations (>10%) are observed in transit dose profile s as a function of the air gap between the patient and the portal film. For this energy, a good agreement is found between the exit profile and the tr ansit profile at about 30 cm behind the patient. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.