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In 54 cases of stereotactic irradiation (STI) seen at our hospital from April 2000 to March 2001, we examined the deficit in top-of-the-head computed tomography (CT) images and the influence that this deficit had on calculation of the STI dose. Results showed a slice deficit of more than 5 mm in 16 of the 54 cases and a maximum deficit of 25 mm. In most cases, the Gill-Thomas-Cosman (GTC) frame was used. The error in total dose monitor unit (DMU) calculation can be ignored if the top-of-the-head CT image deficit is less than 10 mm. If the deficit is more than 20 mm, it is possible that the total DMU calculation error will exceed 2  . In cases in which the deficit was greater than 30 mm, the average calculation error was 2.00  , and, in one case, the error was more than 7  . The GTC frame tends to produce CT image deficits in top-of-the-head images, whereas such images do not suffer this loss when a Brown-Roberts-Wells (BRW) head ring is used. When the CT image deficit is large, it is necessary to reduce the ratio of the arc that passes the area of the CT image deficit and to decrease the dose weight of the arc.

Influence of Deficit in Top-of-the-Head CT Images on STI Dose Calculation