The Study of Beam Quality of Low Monitor Unit Delivery for Intensity Modulated Radiation Therapy

IMRT is currently widely used in the treatment of cancer patients; however in order to make the treatment plan more applicable and render less damage to normal tissues, the beams in a field and the monitor units should be made smaller; this study aims to find the changes and difference between lower and higher monitor units in terms of dosage, flatness and symmetry; the measurement is made by using ELEKTA linear accelerator in 0.6cc Farmer ion chamber and setting the monitor unit at 1-15, 50, 100; the dosage in the field of such sizes as 5×5 cm^2、10×10 cm^2、25×25 cm^2 is measured under each of the abovementioned MU; the flatness and symmetry is measured by using PTW 2-D Array ion chamber, where MUs are set at 3-15, 50, 100, 200 and the field size ranges from 5×5 cm^2, 10×10 cm^2 to 25×25 cm^2. The result shows that the error in dosage under 5 MU ranges between 0.3% and 0.5%, which is particularly more significant in the small field of 5×5 cm^2 under low MU and that the small field is much more unstable than the large field when 10MV high energy is input; however, in terms of flatness and symmetry, the output value is a high and unstable number under low MU while it shows a stable status above 7 MU. It is recommended that in clinical practices, the production of small field with high energy should be avoided as much as possible and the lowest limit of a small field be between 5 and 7 MU, because the lighting of the beams is prone to become stable in this range and make the clinical treatment results more ideal.