Localization errors frequently arise in the portal fl1m technique due to over- or underdevelop films, for high energy radiotherapy machines. To guarantee the accuracy and reproducibility of the localization film technique portal film charts were introduced into the routine planning for the Mevatron 6MV linear accelerator of the Beilinson Radiotherapy Oept. Maintaining reproducible fl1m processor conditions, two fl1ms were used: medicai X-ray ftlm Curix RPI AGFA and localization fl1m X-OMAT TL Kodak, combined with front and rear copper screens of 0.5 mm thickness each. The sensitometric curves for each fl1m-screen combination were obtained (optical density as function of cassette dose). The least square fitted 0.0. curve was used to obtain the cassette dose which produces an 0.0. of 1.6. A theoretical equation for the calculation of the dose at the cassette positions, beyond a water phantom, as funetion of field size, SSO, patient thickness and patient to cassette separation, was experimentally checked with a 0.6 cc Farmer ionization chamber coupled to a 2570 Farmer dosimeter. As a result portal film charts (expressed in MU) were constructed as a funetion of FS, air-gap and patient-thichness, and the best partition dose for the "double-exposure" technique was also established. The patient dose in the double exposure technique was compared to that delivered in diagnostic radiology procedures.

Localization errors frequently arise in the portal fl1m technique due to over- or underdevelop films, for high energy radiotherapy machines. To guarantee the accuracy and reproducibility of the localization film technique portal film charts were introduced into the routine planning for the Mevatron 6MV linear accelerator of the Beilinson Radiotherapy Oept. Maintaining reproducible fl1m processor conditions, two fl1ms were used: medicai X-ray ftlm Curix RPI AGFA and localization fl1m X-OMAT TL Kodak, combined with front and rear copper screens of 0.5 mm thickness each. The sensitometric curves for each fl1m-screen combination were obtained (optical density as function of cassette dose). The least square fitted 0.0. curve was used to obtain the cassette dose which produces an 0.0. of 1.6. A theoretical equation for the calculation of the dose at the cassette positions, beyond a water phantom, as funetion of field size, SSO, patient thickness and patient to cassette separation, was experimentally checked with a 0.6 cc Farmer ionization chamber coupled to a 2570 Farmer dosimeter. As a result portal film charts (expressed in MU) were constructed as a funetion of FS, air-gap and patient-thichness, and the best partition dose for the "double-exposure" technique was also established. The patient dose in the double exposure technique was compared to that delivered in diagnostic radiology procedures.