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

Purpose: To investigate whether the standardized helmet technique adequatel y covers the cribriform plate. Methods and Materials: For II patients with acute leukemia or primary intra cerebral neoplasms undergoing irradiation with the standardized helmet tech nique, 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 th e cribriform plate by the 95% isodose (according to the ICRU 50 report) wit h a IO-mm safety margin allowing for deviations during treatment planning a nd delivery, the eye block had to be moved in the ventrocaudal direction wi th an average vector length of 13.6 mm. Consequently, the mean dose receive d 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 i rradiation of the frontobasis by the standardized helmet technique. (C) 200 1 Elsevier Science Inc.