BUILDUP REGION AND DEPTH OF DOSE MAXIMUM OF MEGAVOLTAGE X-RAY-BEAMS

The depth of dose maximum, d(max) of megavoltage x-ray beams was studi ed as a function of beam energy and field size for 6-, 10-, and 18-MV x-ray beams and field sizes ranging from 1 X 1 to 30 X 30 cm2. For a g iven beam energy, d(max) increases rapidly with increasing field size at small fields, reaches a maximum around 5 X 5 cm 2 and then graduall y decreases with increasing field size for large fields. Monte Carlo s imulations combined with measurements verified that the effect observe d at small field sizes is caused by in-phantom scatter, while at large fields the effect is due to scatter contamination of the primary beam from the linac head. A comparison between the d(max) behavior of flat tened beams to that of unflattened beams indicates that the d(max) dec rease at large fields for flattened beams is caused mainly by contamin ation electrons which are produced in the flattening filter and furthe r scattered by collimator jaws and air.