The value of frequent treatment verification films in reducing localization error in the irradiation of complex fields.

Using treatment verification film to detect geometric miss or localization error (LE), a follow-up analysis of LE for patients treated with extended mantle fields was made for the period 1973-1974. There were 451 treatment verification films for 19 patients reviewed and 67 errors were detected for an error rate of 15%. This rate represented a continued decrease compared to the previous rate of 35% (330 errors for 902 films) during 1969-73. There was a continued reciprocal increase in number of films per patient per course of treatment which was thought, in part, to account for the improvement in technical precision. Reanalysis of localization error by site within the field showed a decrease in rate of error for the axillae from 1969-73 to 1973-1974 and a relative increase in rate of error for the upper abdominal nodes. The decrease in error rate for the axillae was ascribed to increased attention to leaving adequate margin between pulmonary shield and axilla and the increase in error rate for upper abdominal nodes was ascribed to inadequate margin between shield and spine. The relative increase in errors for the upper abdominal nodes correlated with the physicians who made the original decision about margin between spine and shield. Propagation of LE by site has been noted and can be prevented by monitoring treatment fields with verification film and correcting errors as they are noted.