Does the standardized helmet technique lead to adequate coverage of the cribriform plate? An analysis of current practice with respect to the ICRU 50 report

Purpose: To investigate whether the standardized helmet technique adequately covers the cribriform plate. Methods and Materials: For 11 patients with acute leukemia or primary intracerebral neoplasms undergoing irradiation with the standardized helmet technique, three-dimensional isodose distributions were evaluated with special respect to the dose to the cribriform plate and the ocular lenses. Results: The average dose received by 95% of the cribriform plate with the standardized helmet technique was 85% of the prescribed dose. To enclose the cribriform plate by the 95% isodose (according to the ICRU 50 report) with a 10-mm safety margin allowing for deviations during treatment planning and delivery, the eye block had to be moved in the ventrocaudal direction with an average vector length of 13.6 mm. Consequently, the mean dose received by 5% of the lenses rose from 18% to 91% of the prescribed total dose. Conclusion: Sufficient lens shielding is usually not compatible with safe irradiation of the frontobasis by the standardized helmet technique