Localization errors in mantle-field irradiation for Hodgkin's disease.

The frequency and magnitude of localization errors detected by verification films, during mantle-field irradiation for Hodgkin's disease, were assessed. A total of 216 treatment set-ups was studied. The first verification film, at the start of treatment, showed a localization error of over 1 cm in 13% of cases, leading to a critical margin between the shielding block and the tumor bearing area in 9% of the treatment set-ups. After the first correction, an adequate treatment set-up was obtained in 60% of cases, whereas after two corrections the adequacy rate was 84%. Verification films are therefore very useful in detecting clinically important localization errors and in monitoring and checking subsequent corrections. An additional advantage exists because the first verification film can be used as a final check of the simulation plan itself.