DETERMINATION OF % DEPTH DOSE AT CENTRAL AXIS OF THE IRREGULARLY SHAPED (BLOCKED) FIELDS USING CO-60 PHOTON BEAMS

There are several methods to calculate the % depth dose in the irregular fields, which are produced by shielding blocks that are used to protect the normal tissues within the radiotherapy fields. The negative area equivalent-sguares method is one of these methods which is commonly used in daily practice. This study has been done to evaluate whether this method is feasible or not. The % depth doses at the central axis of the six irregular fields are measured by using water phantom and an ion chamber of 0.60 cc. These measured values were compared to the field which is found by subtracting the blocked field from the total field (collimator field). In conclusion, if blocked field is smaller then 25% of the total field, it is feasible to use the values for total field instead of the values of negative area equivalent-squares. If the blocked field is above 25% of total field, it is better to use the values for negative area equivalent-squares.